Amino acids

Description

Amino acids are known as the building blocks of protein, and are defined as the group of nitrogen-containing organic compounds composing the structure of proteins. They are essential to human metabolism, and to making the human body function properly for good health. Of the 28 amino acids known to exist, eight of them are considered "essential," defined as those that can be obtained only through food. These essential amino acids are tryptophan, lysine, methionine , phenylalaine, threonine, valine, leucine, and isoleucine. The "non–essential" amino acids include arginine , tyrosine, glycine, serine, glutmamic acid, aspartic acid, taurine, cycstine, histidine, proline, alanine, and creatine , which is a combination of arginine, glycine, and methionine.

The human body, minus water, is 75% amino acids. All of the neurotransmitters (proteins) but one are composed of amino acids; and 95% of hormones are amino acids. Amino acids are key to every human bodily function with every chemical reaction that occurs.

Amino acids occur naturally in certain foods, such as dairy products, meats, fish, poultry, nuts, legumes, and eggs. Those sources are considered more complete than vegetable protein, such as beans, peas, and grains, also considered a good—even if not complete—source of amino acids.

Amino acids became popular as dietary supplements by the end of the twentieth century for various uses, including fitness training, weight loss, and certain chronic diseases. Claims exist in holistic medicine that indicate amino acid supplements taken in the proper dosage can aid also in fighting depression, allergies, heart disease , gastrointestinal problems, high cholesterol , muscle weakness, blood sugar problems, arthritis, insomnia , bipolar illness, epilepsy, chronic fatigue syndrome, autism, attention-deficit hyperactivity disorder (ADHD), and mental exhaustion.

Description

Amino acid therapy as a supplemental aid to a healthy diet joined the fitness craze in the United States by the end of the 1990s. According to author Brenda Adderly in Better Nutrition, in September of 1999, "The creation of new protein from amino acids and the breaking down of existing protein into amino acids are ongoing processes in our bodies. If, for example, you are working out and developing certain muscles, amino acids come to the rescue with new protein to build muscle cells," Adderly noted. "Similarly, when you eat a complete protein, such as meat or beans and rice, the body breaks down the amino acids in that food for later use." Understanding the balance of amino acids in the body can be often the first clue to understanding why a person suffers many ailments, ranging from depression to upset stomach to obesity . Deficiencies in the proper balance of amino acids is likely to occur in those with poor diets . Because stress , age, infection, and various other factors including the amount of exercise a person does, can also affect the levels of amino acids, people with healthy, nutritious diets could also find that they also suffer deficiencies. Adderly adds that, "Not only are the symptoms of amino acid deficiencies wide ranging, but there are no RDAs (recommended daily allowances) or other guidelines, to help us tell if we are least covering all the bases. Add to that the complicated matter of keeping track of all 28 some with names most of us have never even heard and the situation begins to seem overwhelming."

Essential amino acids

The amino acids, which are derived only from food and that the body cannot manufacture, perform various functions.

• Tryptophan. This is considered a natural relaxant, helps alleviate insomnia; helps in the treatment of migraine headaches; helps reduce the risk of artery and heart spasms; and works with lysine to reduce cholesterol levels.
• Lysine. Aids in proper absorption of calcium ; helps form collagen for bone cartilage and connective tissues; aids in production of antibodies, hormones, and enzymes. Research has indicated it also might be effective against herpes by creating the balance of nutrients that slows the growth of the virus causing it. A deficiency could result in fatigue , lack of concentration, irritability, bloodshot eyes, retarded growth, hair loss, anemia , and reproductive problems.
• Methionine. Properties include providing the primary source of sulfur that can prevent disorders of the hair, skin, and nails; lowers cholesterol by increasing the liver's production of lecithin ; reduces liver fat; protects kidneys; and promotes hair growth.
• Phenylalaine. This serves the brain by producing norepinephrine, the chemical that is responsible for transmitting the signals between the nerve cells and the brain; can maintain alertness; reduces hunger pains; acts as an antidepressant; and improves memory.
• Threonine. Makes up a substantial portion of the collagen, elastin, and enamel protein; serves the liver by preventing buildup; aids the digestive and intestinal tracts to function better; and acts as a trigger for metabolism.
• Valine. Promotes mental energy; helps with muscle coordination; and serves as a natural tranquilizer.
• Leucine. Works with isoleucine to provide for the manufacture of essential biochemical processes in the body that are used for energy, increasing the stimulants to the upper brain for greater mental alertness.

Roles of certain non–essential amino acids

• Glycine. Facilitates the release of oxygen for the cell–making process; key role in manufacturing of hormones and health of immune system.
• Serine. Source of glucose storage by the liver and muscles; provides antibodies for immune system; synthesizes fatty acid sheath around nerve fibers.
• Glutamic acid. Nature's "brain food" that increases mental prowess; helps speed the healing of ulcers; aids in combatting fatigue.

Creatine in the spotlight

One of the most discussed amino acid supplements available on the market is creatine monohydrate. The body produces small amounts of creatine in the kidneys, liver, and pancreas, making it a non-essential acid. With most diets that include red meat or fish, also come a few grams of creatine. It is stored in muscle cells and is used in activities, such as weight lifting and sprinting, providing the necessary thrust of energy for such activities. But the natural supply of creatine produced by the body is quickly depleted. After approximately 10 seconds, when muscle fatigue becomes apparent, the daily production is used.

According to Timothy Gower, writing for Esquire in February of 1998, "Scientists identified creatine 160– odd years ago, but only in the 1980s did they figure out that muscle cells can be 'loaded' with up to 30% more of the compound than they normally carry. Since then, several studies have shown that weight lifters primed on the supplement tire less easily, allowing them to work out longer." Gower also noted that creatine users find that the weight they add on is fat-free, whether that is lean tissue or some is water weight, no one has yet determined, since muscle cells do fill with water during creatine loading. Additionally, while it can add to the burst of the energy a sprinter needs to perform well, creatine does not do anything for the marathon runner going for several hours.

Commercially available since 1993, the long-term effects still remain unknown. One 2002 study did show that creatine use improved rehabilitation for injured athletes and another has shown that using the supplement does not increase risk of injury. It should be noted that some 20–30% of people researched showed no improvement using creatine. One early report indicated that creatine could be beneficial for some people in spurring metabolism, burning calories and helping in weight loss. Those reports were as yet inconclusive.

General use

Amino acid supplements to a healthy diet are used for various purposes. The most common uses include: sustaining strength in weight training to build muscles; improving heart and circulatory problems or diseases, particularly in the aging ; the treatment of chronic fatigue syndrome; treating depression and anxiety ; treating eating disorders, such as bulimia and/or anorexia, along with overeating; increasing memory; building up and sustaining the body's immune system in fighting bacteria and viruses. It is important to note that, while the necessity and role of all amino acids has been verified in the maintenance of optimum health, research is not extensive enough to provide indisputable verification of the touted benefits of such supplements over the long term.

Nonetheless, some members of the scientific medical community would seem to confirm what amino acid proponents have long believed to be true. One such study from the Journal of the American College of Cardiology brought good news for the millions suffering from chronic heart failure. Dr. Rainer Hambrecht and colleagues from the University of Leipzig, (Germany) tested the amino acid L-arginine on 38 heart-failure patients. Knowing that the human body converted it into nitric oxide, a chemical that relaxes blood vessels, the researchers gave one group 8 g of it daily for four weeks; another group simply did forearm exercises; and a third group combined the supplement with the exercise. The people who took the supplement alone increased their blood-vessel dilation by a factor of four, as did the exercise group. Those who took both the supplement and performed the exercise increased it by six. More recent studies on arginine in 2002 found that the supplement may help reduce risk of postoperative infections . Further, arginine may enhance women's sexual function.

Supplements are recommended by alternative medical practitioners particularly for those who are not getting a proper diet, especially vegetarians who might not be getting a balance of complete protein, as well as athletes, anyone under severe stress, and anyone whose alcohol intake level is moderate to high.

Preparations

Supplements of various amino acids are available primarily in capsule, tablet, or powder form. A common way of taking amino acids is in a "multiple" amino acid gel cap. These contain sources of protein from gelatin, soy, and whey. The market for supplements in wholesale, retail, and internet sales was estimated to reach into the millions of dollars, with literally hundreds available. Internet sales were a fast-growing area particularly with the use of such supplements as creatine powder publicized by well-known Olympic stars and professional athletes. Daily usage of creatine as evident from research indicated that usage should be leveled at 5 g of powder in a glass of orange juice, and could be taken up to four times a day during peak athletic training. Maintenance dosages were recommended at 5 g once a day.

Side effects

Because amino acids are naturally produced substances both in the human body and in the protein derived from animal and dairy products, as well as being present in food combinations such as beans and rice, such supplements are not regulated by the United States Food and Drug Administration (FDA), nor are there any specified daily requirements, and they also do not show up in either drug or urine tests. Amino acid supplements might be classified as having no affect at all. Long-term effects were not yet evident, however, due to the relatively recent phenomenon of use.

Interactions

Interactions of amino acids with drugs has not been sufficiently studied to determine yet if any adverse effects result from using amino acids with medications.

Resources

PERIODICALS

Adderly, Brenda. "Amino Acids." Better Nutrition (September 1999). Available from http://web2.infotrac.galegroup.com.

"Amino acid screening." Everything You Need to Know about Medical Tests, Annual. Springhouse Corporation: 1996. Available from http://web2.infotrac.com.

Antinoro, Linda. "Food and Herbs That Keep Blood Moving, Prevent Circulatory Problems." Environmental Nutrition (February 2000).

"Arginine Seems to Benefit Both Immune and Sexual Response." RN (February 2002): 22.

Austin Nutritional Research. "Amino acids." Reference Guide for Amino Acids. 2000. Available from http://www.realtime.net/anr/aminoacid.html.

Body Trends Fitness Products. "Amino acids." bodytrends.com commercial website. (2000). Available from http://wwwbodytrends.com.

"Creatine Supplementation Speeds Rehabilitation." Health and Medicine Week (January 21, 2002): 6.

Davidson, Tish. "Amino acid disorders screening." Gale Encyclopedia of Medicine. Edition 1. Detroit: 1999. Available from http://web2.infotrac.galegroup.com.

Dolby, Victoria. "Anxiety? Send herbs, 5–HTP, and amino acids to the rescue!" Better Nutrition (June 1998). Available from http://web2.infotrac.galegroup.com.

Gersten, Dennis J., M.D. "Amino Acids: Building Blocks of Life, Building Blocks of Healing." The Gersten Institute for Integrative Medicine. (2000). Available from http://www.imagery.com.

Gower, Timothy. "Eat Powder! Build Muscle! Burn Calories!" Esquire (February 1998). Available from http://www.brittannica.com.

Moyano, D.; Vilaseca, M.AA.; Artuch, R.; and, Lambruschini, N. "Plasma Amino Acids in Anorexia Nervosa." Nutrition Research Newsletter (November 1998). Available from http://web2.infotrac.com.

"Studies Say Creatine is OK." Obesity, Fitness & Wellness Week (January 12, 2002): 12.

Toews, Victoria Dolby. "6 Amino Acids Unleash the Energy." Better Nutrition (June 1999). Available from http://web2.infotrac.com.

Totheroh, Gailon. "Amino Acid Therapy Pays Off." Christian Broadcasting Network (10 May 1999). Available from http://www.cbn.com.

Tuttle, Dave. "Muscle's little helper." Men's Fitness (December 1998). Available from http://web2.infotrac.com.

Wernerman, Jan. "Documentation of clinical benefit of specific amino acid nutrients." The Lancet (5 September 1998). Available from http://web2.infotrac.galegroup.com/itw.

Williams, Stephen. "Passing the Acid Test." Newsweek (27 March 2000).

Jane Spehar

Teresa G. Odle

AMINO ACIDS

CONCEPT

Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins, of which there are countless varieties. Of the 20 amino acids required for manufacturing the proteins the human body needs, the body itself produces only 12, meaning that we have to meet our requirements for the other eight through nutrition. This is just one example of the importance of amino acids in the functioning of life. Another cautionary illustration of amino acids' power is the gamut of diseases (most notably, sickle cell anemia) that impair or claim the lives of those whose amino acids are out of sequence or malfunctioning. Once used in dating objects from the distant past, amino acids have existed on Earth for at least three billion years—long before the appearance of the first true organisms.

HOW IT WORKS

A "Map" of Amino Acids

Amino acids are organic compounds, meaning that they contain carbon and hydrogen bonded to each other. In addition to those two elements, they include nitrogen, oxygen, and, in a few cases, sulfur. The basic structure of an amino-acid molecule consists of a carbon atom bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a fourth group that differs from one amino acid to another and often is referred to as the-R group or the side chain. The-R group, which can vary widely, is responsible for the differences in chemical properties.

This explanation sounds a bit technical and requires a background in chemistry that is beyond the scope of this essay, but let us simplify it somewhat. Imagine that the amino-acid molecule is like the face of a compass, with a carbon atom at the center. Raying out from the center, in the four directions of the compass, are lines representing chemical bonds to other atoms or groups of atoms. These directions are based on models that typically are used to represent amino-acid molecules, though north, south, east, and west, as used in the following illustration, are simply terms to make the molecule easier to visualize.

To the south of the carbon atom (C) is a hydrogen atom (H), which, like all the other atoms or groups, is joined to the carbon center by a chemical bond. To the north of the carbon center is what is known as an amino group (-NH₂). The hyphen at the beginning indicates that such a group does not usually stand alone but normally is attached to some other atom or group. To the east is a carboxyl group, represented as-COOH. In the amino group, two hydrogen atoms are bonded to each other and then to nitrogen, whereas the carboxyl group has two separate oxygen atoms strung between a carbon atom and a hydrogen atom. Hence, they are not represented as O₂.

Finally, off to the west is the R -group, which can vary widely. It is as though the other portions of the amino acid together formed a standard suffix in the English language, such as -tion. To the front of that suffix can be attached all sorts of terms drawn from root words, such as educate orsatisfy or revolt —hence, education, satisfaction, and revolution. The variation in the terms attached to the front end is extremely broad, yet the tail end, -tion, is a single formation. Likewise the carbon, hydrogen, amino group, and carboxyl group in an amino acid are more or less constant.

A FEW ADDITIONAL POINTS.

The name amino acid, in fact, comes from the amino group and the acid group, which are the most chemically reactive parts of the molecule. Each of the common amino acids has, in addition to its chemical name, a more familiar name and a three-letter abbreviation that frequently is used to identify it. In the present context, we are not concerned with these abbreviations. Amino-acid molecules, which contain an amino group and a carboxyl group, do not behave like typical molecules. Instead of melting at temperatures hotter than 392°F (200°C), they simply decompose. They are quite soluble, or capable of being dissolved, in water but are insoluble in nonpolar solvents (oil-and all oil-based products), such as benzene or ether.

RIGHT-HAND AND LEFT-HAND VERSIONS.

All of the amino acids in the human body, except glycine, are either right-hand or left-hand versions of the same molecule, meaning that in some amino acids the positions of the carboxyl group and the R -group are switched. Interestingly, nearly all of the amino acids occurring in nature are the left-hand versions of the molecules, or the L-forms. (There-fore, the model we have described is actually the left-hand model, though the distinctions between "right" and "left"—which involve the direction in which light is polarized—are too complex to discuss here.)

Right-hand versions (D-forms) are not found in the proteins of higher organisms, but they are present in some lower forms of life, such as in the cell walls of bacteria. They also are found in some antibiotics, among them, streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics, several of which are well known to the public at large, can kill bacterial cells by interfering with the formation of proteins necessary for maintaining life and for reproducing.

Amino Acids and Proteins

A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage, between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins, which are the basic building blocks of all living things. The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it. Other aspects of the chemical behavior of protein molecules are due to interactions between the amino and the carboxyl groups or between the various R -groups along the long chains of amino acids in the molecule.

NUMBERS AND COMBINATIONS.

Amino acids function as monomers, or individual units, that join together to form large, chainlike molecules called polymers, which may contain as few as two or as many as 3,000 amino-acid units. Groups of only two amino acids are called dipeptides, whereas three amino acids bonded together are called tripeptides. If there are more than 10 in a chain, they are termed polypeptides, and if there are 50 or more, these are known as proteins.

All the millions of different proteins in living things are formed by the bonding of only 20 amino acids to make up long polymer chains. Like the 26 letters of the alphabet that join together to form different words, depending on which letters are used and in which sequence, the 20 amino acids can join together in different combinations and series to form proteins. But whereas words usually have only about 10 or fewer letters, proteins typically are made from as few as 50 to as many as 3,000 amino acids. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of proteins is truly enormous. There are about two quadrillion different proteins that can exist if each of the 20 amino acids present in humans is used only once. Just as not all sequences of letters make sense, however, not all sequences of amino acids produce functioning proteins. Some other sequences can function and yet cause undesirable effects, as we shall see.

REAL-LIFE APPLICATIONS

DNA (deoxyribonucleic acid), a molecule in all cells that contains genetic codes for inheritance, creates encoded instructions for the synthesis of amino acids. In 1986, American medical scientist Thaddeus R. Dryja (1940-) used amino-acid sequences to identify and isolate the gene for a type of cancer known as retinoblastoma, a fact that illustrates the importance of amino acids in the body.

Amino acids are also present in hormones, chemicals that are essential to life. Among these hormones is insulin, which regulates sugar levels in the blood and without which a person would die. Another is adrenaline, which controls blood pressure and gives animals a sudden jolt of energy needed in a high-stress situation—running from a predator in the grasslands or (to a use a human example) facing a mugger in an alley or a bully on a playground. Biochemical studies of amino-acid sequences in hormones have made it possible for scientists to isolate and produce artificially these and other hormones, including the human growth hormone.

Amino Acids and Nutrition

Just as proteins form when amino acids bond together in long chains, they can be broken down by a reaction called hydrolysis, the reverse of the formation of the peptide bond. That is exactly what happens in the process of digestion, when special digestive enzymes in the stomach enable the breaking down of the peptide linkage. (Enzymes are a type of protein—see Enzymes.) The amino acids, separated once again, are released into the small intestine, from whence they pass into the bloodstream and are carried throughout the organism. Each individual cell of the organism then can use these amino acids to assemble the new and different proteins required for its specific functions. Life thus is an ongoing cycle in which proteins are broken into individual amino-acid units, and new proteins are built up from these amino acids.

ESSENTIAL AMINO ACIDS.

Out of the many thousands of possible amino acids, humans require only 20 different kinds. Two others appear in the bodies of some animal species, and approximately 100 others can be found in plants. Considering the vast numbers of amino acids and possible combinations that exist in nature, the number of amino acids essential to life is extremely small. Yet of the 20 amino acids required by humans for making protein, only 12 can be produced within the body, whereas the other eight—isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must be obtained from the diet. (In addition, adults are capable of synthesizing arginine and histidine, but these amino acids are believed to be essential to growing children, meaning that children cannot produce them on their own.)

A complete protein is one that contains all of the essential amino acids in quantities sufficient for growth and repair of body tissue. Most proteins from animal sources, gelatin being the only exception, contain all the essential amino acids and are therefore considered complete proteins. On the other hand, many plant proteins do not contain all of the essential amino acids. For example, lysine is absent from corn, rice, and wheat, whereas corn also lacks tryptophan and rice lacks threonine. Soybeans are lacking in methionine. Vegans, or vegetarians who consume no animal proteins in their diets (i.e., no eggs, dairy products, or the like) are at risk of malnutrition, because they may fail to assimilate one or more essential amino acid.

Amino Acids, Health, and Disease

Amino acids can be used as treatments for all sorts of medical conditions. For example, tyrosine may be employed in the treatment of Alzheimer's disease, a condition characterized by the onset of dementia, or mental deterioration, as well as for alcohol-withdrawal symptoms. Taurine is administered to control epileptic seizures, treat high blood pressure and diabetes, and support the functioning of the liver. Numerous other amino acids are used in treating a wide array of other diseases. Sometimes the disease itself involves a problem with amino-acid production or functioning. In the essay Vitamins, there is a discussion of pellagra, a disease resulting from a deficiency of the B-group vitamin known as niacin. Pellagra results from a diet heavy in corn, which, as we have noted, lacks lysine and tryptophan. Its symptoms often are described as the "three Ds": diarrhea, dermatitis (or skin inflammation), and dementia. Thanks to a greater understanding of nutrition and health, pellagra has been largely eradicated, but there still exists a condition with almost identical symptoms: Hartnup disease, a genetic disorder named for a British family in the late 1950s who suffered from it.

Hartnup disease is characterized by an inability to transport amino acids from the kidneys to the rest of the body. The symptoms at first seemed to suggest to physicians that the disease, which is present in one of about 26,000 live births, was pellagra. Tests showed that sufferers did not have inadequate tryptophan levels, however, as would have been the case with pellagra. On the other hand, some 14 amino acids have been found in excess within the urine of Hartnup disease sufferers, indicating that rather than properly transporting amino acids, their bodies are simply excreting them. This is a potentially very serious condition, but it can be treated with the B vitamin nicotinamide, also used to treat pellagra. Supplementation of tryptophan in the diet also has shown positive results with some patients.

SICKLE CELL ANEMIA.

It is also possible for small mistakes to occur in the amino-acid sequence within the body. While these mistakes sometimes can be tolerated in nature without serious problems, at other times a single misplaced amino acid in the polymer chain can bring about an extremely serious condition of protein malfunctioning. An example of this is sickle cell anemia, a fatal disease ultimately caused by a single mistake in the amino acid sequence. In the bodies of sickle cell anemia sufferers, who are typically natives of sub-Saharan Africa or their descendants in the United States or elsewhere, glutamic acid is replaced by valine at the sixth position from the end of the protein chain in the hemoglobin molecule. (Hemoglobin is an iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them.) This small difference makes sickle cell hemoglobin molecules extremely sensitive to oxygen deficiencies. As a result, when the red blood cells release their oxygen to the tissues, as all red blood cells do, they fail to re-oxygenate in a normal fashion and instead twist into the shape that gives sickle cell anemia its name. This causes obstruction of the blood vessels. Before the development of a treatment with the drug hydroxyurea in the mid-1990s, the average life expectancy of a person with sickle cell anemia was about 45 years.

Amino Acids and the Distant Past

The Evolution essay discusses several types of dating, a term referring to scientific efforts directed toward finding the age of a particular item or phenomenon. Methods of dating are either relative (i.e., comparative and usually based on rock strata, or layers) or absolute. Whereas relative dating does not involve actual estimates of age in years, absolute dating does. One of the first types of absolute-dating techniques developed was amino-acid racimization, introduced in the 1960s. As noted earlier, there are "left-hand" l-forms and "right-hand" d-forms of all amino acids. Virtually all living organisms (except some microbes) incorporate only the l-forms, but once the organism dies, the l-amino acids gradually convert to the mirror-image d-amino acids.

Numerous factors influence the rate of conversion, and though amino-acid racimization was popular as a form of dating in the 1970s, there are problems with it. For instance, the process occurs at different rates for different amino acids, and the rates are further affected by such factors as moisture and temperature. Because of the uncertainties with amino-acid racimization, it has been largely replaced by other absolute-dating methods, such as the use of radioactive isotopes.

Certainly, amino acids themselves have offered important keys to understanding the planet's distant past. The discovery, in 1967 and 1968, of sedimentary rocks bearing traces of amino acids as much as three billion years old had an enormous impact on the study of Earth's biological history. Here, for the first time, was concrete evidence of life—at least, in a very simple chemical form—existing billions of years before the first true organism. The discovery of these amino-acid samples greatly influenced scientists' thinking about evolution, particularly the very early stages in which the chemical foundations of life were established.

WHERE TO LEARN MORE

"Amino Acids." Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Freie Universität, Berlin (Web site). <http://www.chemie.fu-berlin.de/chemistry/bio/amino-acids_en.html>.

Goodsell, David S. Our Molecular Nature: The Body's Motors, Machines, and Messages. New York: Copernicus, 1996.

"Introduction to Amino Acids." Department of Crystallography, Birbeck College (Web site). <http://www.cryst.bbk.ac.uk/education/AminoAcid/overview.html>.

Michal, Gerhard. Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology. New York: John Wiley and Sons, 1999.

Newstrom, Harvey. Nutrients Catalog: Vitamins, Minerals, Amino Acids, Macronutrients—Beneficial Use, Helpers, Inhibitors, Food Sources, Intake Recommendations, and Symptoms of Over or Under Use. Jefferson, NC: McFarland and Company, 1993.

Ornstein, Robert E., and Charles Swencionis. The Healing Brain: A Scientific Reader. New York: Guilford Press, 1990.

Reference Guide for Amino Acids (Web site). <http://www.realtime.net/anr/aminoacd.html#tryptophn>.

Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. Proteins. Illus. Anne Canevari Green. Brookfield, CT: Millbrook Press, 1992.

Springer Link: Amino Acids (Web site). <http://link.springer.de/link/service/journals/00726/>.

KEY TERMS

AMINO ACIDS:

Organic compounds made of carbon, hydrogen, oxygen, nitro gen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.

AMINO GROUP:

The chemical forma tion NH₂, which is part of all amino acids.

BIOCHEMISTRY:

The area of the bio logical sciences concerned with the chemical substances and processes in organisms.

CARBOXYL GROUP:

The formation COOH, which is common to all amino acids.

COMPOUND:

A substance in which atoms of more than one element are bond ed chemically to one another.

DIPEPTIDE:

A group of only two amino acids.

DNA:

Deoxyribonucleic acid, a molecule in all cells and many viruses containing genetic codes for inheritance.

ENZYME:

A protein material that speeds up chemical reactions in the bodies of plants and animals.

ESSENTIAL AMINO ACIDS:

Amino acids that cannot be manufactured by the body, and which therefore must be obtained from the diet. Proteins that contain essential amino acids are known as complete proteins.

GENE:

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.

HORMONE:

Molecules produced by living cells, which send signals to spots remote from their point of origin and induce specific effects on the activities of other cells.

MOLECULE:

A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen.

PEPTIDE LINKAGE:

A bond between the carboxyl group of one amino acid and the amino group of a second amino acid.

POLYMERS:

Large, chainlike molecules composed of numerous subunits known as monomers.

POLYPEPTIDE:

A group of between 10 and 50 amino acids.

PROTEINS:

Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.

RNA:

Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.

SYNTHESIZE:

To manufacture chemically, as in the body.

TRIPEPTIDE:

A group of three amino acids.

Amino acid

Chemical structure

Bonding

Resources

Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen and, in a few cases, sulfur. The basic structure of an amino acid molecule consists of a carbon atom that is bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a fourth group that differs from one amino acid to another and is often referred to as the –R group or the side chain. The –R group can vary widely and is responsible for the differences in the chemical properties. The name amino acid comes from the amino group and the acid group, which are the most reactive parts of the molecule. The amino acids that are important in the biological world are referred to as a-amino acids because the amino group is bonded to the a-carbon atom; the carbon located immediately adjacent to the carboxyl group.

A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage, between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins. The importance of the amino acids in nature arises from their ability to form proteins, which are the basic building blocks of all living things.

The specific properties of each kind of protein depend on the kind and sequence of the constituent amino acids. Depending on the amino acids present and the sequence that they are arranged in, a protein will assume a certain three-dimensional shape that can be critical to its function. As well, the amino acids themselves are important, since certain amino acids may need to be present for a given protein to actually function. Other chemical behavior of these protein molecules is due to interactions between the amino and the carboxyl groups or between the various –R groups along the long chains of amino acids in the molecule. These chemical interactions confer a three-dimensional configuration on the protein, which is essential to its proper functioning.

Chemical structure

Although most of the known amino acids were identified and isolated (sometimes in impure form) during the nineteenth century, the chemical structures of many of them were not known until much later. Understanding their importance in the formation of proteins, the basis of the structure and function of all cells is of even more recent origin, dating to the first

part of the twentieth century. Only about 20 amino acids are common in humans, with two others present in a few animal species. There are over 100 other lesser known ones that are found mostly in plants.

Although amino acid molecules contain an amino group and a carboxyl group, certain chemical properties are not consistent with this structure. Unlike the behavior of molecules with amino or carboxylic acid functional groups alone, amino acids exist mostly as crystalline solids that decompose rather than melt at temperatures over 392^°F (200^°C). They are quite soluble in water but insoluble in non-polar solvents like benzene or ether. Their acidic and basic properties are exceptionally weak for molecules that contain an acid carboxyl group and a basic amino group.

This problem was resolved when it was realized that amino acids are better represented as dipolar ions, sometimes called zwitterions (from the German, meaning hybrid ions). Although the molecule as a whole does not have a net charge, there is a transfer of an H⁺ ion from the carboxyl group to the amino group; consequently, the amino group is present as an -NH₃⁺ and the carboxyl group is present as a -COO^–. This reaction is an acid-base interaction between two groups in the same molecule and occurs because the -COOH group is a rather strong acid and the -NH ₂ group is a rather strong base. As a result of this structure, amino acids can behave as acids in the presence of strong bases or they can behave as bases in the presence of strong acids.

One other property of amino acids that is important to their chemical behavior is that all of the amino acids except glycine can exist as mirror images of each other; that is, right- or left-handed versions of the molecule. Like the positions of the thumb and fingers of a glove, the right hand being the mirror image of the left hand, the positions of the functional groups on a carbon can be mirror images of each other. Interestingly, nearly all of the amino acids occurring in nature are the left-handed versions of the molecules. Right-handed versions are not found in the proteins of higher organisms but are present in some lower forms of life such as in the cell walls of bacteria. They are also found in some antibiotics such as streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics can kill bacterial cells by interfering with the formation of protein necessary for maintaining life and reproduction.

Bonding

Amino acids are extremely important in nature as the individual units (monomers) will join together in chains. The chains may contain as few as two or as many as 3, 000 amino acid units. Groups of only two amino acids are called dipeptides; three amino acids bonded together are called tripeptides; if there are more than 10 in a chain, they are called polypeptides; and if there are 50 or more, they are known as proteins.

In 1902 the German organic chemist Emil Fischer first proposed that the amino acids in polypeptides are linked together between the carboxyl group of one amino acid and the amino group of the other. This bond forms when the -OH from the carboxyl end of one molecule and the hydrogen from the amino end of another molecule split off and form a small molecule byproduct, H₂ O (water). This type of reaction is called a condensation reaction. The new bond between the carbon atom and the nitrogen atom is called a peptide bond, also known as an amide linkage. Because every amino acid molecule has a carboxyl end and an amino end, each one can join to any other amino acid by the formation of a peptide bond.

All the millions of different proteins in living things are formed by the bonding of only 20 amino acids to form long polymer chains. Like the 26 letters of the alphabet that join together to form different words, depending on which letters are used and in what sequence, the 20 amino acids can join together in different combinations and sequences to form proteins. Whereas words usually have only about 10 or fewer letters per word, proteins are usually made from at least 50 amino acids to more than 3, 000. Because each amino acid can be used many times along the chain, and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of protein is truly enormous.

Order is important in the functioning of a protein. Different arrangements of amino acids produce different peptides. In fact, there are 27 different tripeptides that are possible from these three amino acids. (Each may be used more than once.) There are about two quadrillion different proteins that can exist if each of the 20 amino acids present in humans is used only once. However, just as not all sequences of letters make sense, not all sequences of amino acids make functioning proteins, and other sequences can cause undesirable effects. While small mistakes in the amino acid sequence can sometimes be tolerated in nature without serious problems, at other times malfunctioning proteins can be caused by a single incorrect amino acid in the polymer chain. Sickle cell anemia is a fatal disease caused by a single amino acid, glutamic acid, being replaced by a different one, valine, at the sixth position from the end of the protein chain in the hemoglobin molecule. This small difference causes lower solubility of the sickle cell hemoglobin molecules. They precipitate out as small rods which give the cells the characteristic sickle shape and result in the often fatal disease.

The specific sequence of the amino acids along the protein chain is referred to as the primary structure of the protein. However, these chains are not rigid, but rather they are long and flexible, like string. The strands of protein can twist to form helixes or fold into sheets. They can bend and fold back on themselves to form globs, and several protein molecules sometimes combine into a larger molecule. All of these configurations are caused by interactions both within a single protein strand as well as between two or three separate strands of protein.

Just as proteins are formed when amino acids bond together to form long chains, they can be broken down again into their individual amino acids by a reaction called hydrolysis. This reaction is just the reverse of the formation of the peptide bond. In the process of digestion, proteins are once again broken down into their individual amino acid components. Special digestive enzymes are necessary to cause the peptide linkage to break, and a molecule of water is added when the reaction occurs. The resulting amino acids are released into the small intestine, where they can easily pass into the bloodstream and be carried to every cell of the organism. A cell can use these amino acids to assemble the new and different proteins required for its specific functions. Life goes on by the continual breakdown of protein into the individual amino acid units, followed by the buildup of new protein from these amino acids.

Of the 20 amino acids required by humans for making protein, 12 of them can be made within the body from other nutrients. The other eight, called the essential amino acids, cannot be made by the body and must be obtained from the diet. These are isoleucine, leucine, lysine, methionine, phenlyalanine, threonine, tryptophan, and valine. In addition, arginine and histidine are believed to be essential to growing children but may not be essential to mature adults. An adequate protein is one that contains all of the essential amino

KEY TERMS

Alpha-amino acid— An amino acid in which the -NH₂ (amino) group is bonded to the carbon atom.

Alpha-carbon atom— The carbon atom adjacent to the carboxyl group.

Amino group— An -NH₂ group.

Carboxyl group— A -COOH group; also written -CO₂ H.

Essential amino acid— Amino acids that cannot be synthesized by the body and must be obtained from the diet.

Monomers— Small, individual subunits that join together to form polymers.

Peptide— Substances made up of chains of amino acids, usually fewer than 50.

Peptide bond— The bond formed when the carboxyl group of one amino acid joins with the amino group of a second amino acid and splits off a water molecule.

acids in sufficient quantities for growth and repair of body tissue. Most proteins from animal sources (gelatin being the only exception), contain all the essential amino acids and are considered adequate proteins. Many plant proteins do not contain all of the essential amino acids. Corn, for example, does not contain the essential amino acids lysine and tryptophan. Rice is lacking in lysine and threonine; wheat is lacking in lysine; and soybeans are lacking in methionine. People who are vegetarians and do not consume animal proteins in their diets sometimes suffer from malnutrition because of the lack of one or more amino acids in their diets even though they may consume enough food and plenty of calories. Malnutrition is avoidable by knowing what combinations of plant proteins will supply all the necessary amino acids.

See also Nutrition.

Resources

BOOKS

Lieber, Daniel C. Introduction to Proteomics: Tools for the New Biology. Totowa, NJ: Humana Press, 2006.

Twyman, R. M. Principles of Proteomics. Oxford: BIOS Scientific Publications, 2004.

Whitford, David. Proteins: Structure and Function. New York: John Wiley & Sons, 2005.

Leona B. Bronstein

Amino acid

Amino acids are organic compounds made of carbon , hydrogen , oxygen , nitrogen and, in a few cases, sulfur . The basic structure of an amino acid molecule consists of a carbon atom that is bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom and a fourth group that differs from one amino acid to another and is often referred to as the -R group or the side chain. The -R group can vary widely and is responsible for the differences in the chemical properties. The name, amino acid, comes from the amino group and the acid group which are the most reactive parts of the molecule. The amino acids that are important in the biological world are referred to as a-amino acids because the amino group is bonded to the a-carbon atom, that is, the one adjacent to the carboxyl group.

A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage , between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins . The importance of the amino acids in nature arises from their ability to form proteins, which are the basic building blocks of all living things.

The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it. Other chemical behavior of these protein molecules is due to interactions between the amino and the carboxyl groups or between the various -R groups along the long chains of amino acids in the molecule. These chemical interactions confer a three-dimensional configuration on the protein, which is essential to its proper functioning.

Chemical structure

Although most of the known amino acids were identified and isolated (sometimes in impure form) during the nineteenth century, the chemical structures of many of them were not known until much later. Understanding their importance in the formation of proteins, the basis of the structure and function of all cells is of even more recent origin, dating to the first part of the twentieth century. Only about 20 amino acids are common in humans, with two others present in a few animal species . There are over 100 other lesser known ones that are found mostly in plants.

Each of the common amino acids has, in addition to its chemical name, a more familiar name and a three-letter abbreviation that is frequently used to identify it. They are often grouped by similarities in the chemical properties of the side chains. The side chain of glycine (gly) consists of a single hydrogen atom; alanine (ala), valine (val), leucine (leu), and isoleucine (ile) all have hydrocarbon (containing only hydrogen and carbon) side chains; proline (pro) has a hydrocarbon that is part of a ring structure; serine (ser) and threonine (thr) have an alcohol (-OH) side chain; cysteine (cys) and methionine (met) both have sulfur atoms as part of the -R group; phenlyalanine (phe), tyrosine (tyr), and tryptophan (trp) all contain an aromatic ring (related to the benzene ring) as part of the side chain; aspartic acid (asp) and glutamic acid (glu) have a second carboxylic acid group while asparagine (asn) and glutamine (gln) have a carboxylic acid derivative (a -CONH2) group; and lysine (lys), arginine (arg), and histidine (his) have an -R group that contains a second amino group.

Although amino acid molecules contain an amino group and a carboxyl group, certain chemical properties are not consistent with this structure. Unlike the behavior of molecules with amino or carboxylic acid functional groups alone, amino acids exist mostly as crystalline solids that decompose rather than melt at temperatures over 392°F (200°C). They are quite soluble in water but insoluble in non-polar solvents like benzene or ether . Their acidic and basic properties are exceptionally weak for molecules that contain an acid carboxyl group and a basic amino group.

This problem was resolved when it was realized that amino acids are better represented as dipolar ions, sometimes called zwitterions (from the German, meaning hybrid ions). Although the molecule as a whole does not have a net charge, there is a transfer of an H+ ion from the carboxyl group to the amino group; consequently, the amino group is present as an -NH3⁺ and the carboxyl group is present as a -COO- (Fig. 1). This reaction is an acid-base interaction between two groups in the same molecule and occurs because the -COOH group is a rather strong acid and the -NH2 group is a rather strong base. As a result of this structure, amino acids can behave as acids in the presence of strong bases or they can behave as bases in the presence of strong acids.

One other property of amino acids that is important to their chemical behavior is that all of the amino acids except glycine can exist as mirror images of each other; that is, right- or left-handed versions of the molecule. Like the positions of the thumb and fingers of a glove, the right hand being the mirror image of the left hand, the positions of the functional groups on the a carbon can be mirror images of each other. Interestingly, nearly all of the amino acids occurring in nature are the left-handed versions of the molecules. Right-handed versions are not found in the proteins of higher organisms but are present in some lower forms of life such as in the cell walls of bacteria . They are also found in some antibiotics such as streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics can kill bacterial cells by interfering with the formation of protein necessary for maintaining life and for reproducing.

Bonding

Amino acids are extremely important in nature as the monomers, or individual units, that join together in chains to form copolymers (polymers made of more than one kind of monomer ). The chains may contain as few as two or as many as 3,000 amino acid units. Groups of only two amino acids are called dipeptides; three amino acids bonded together are called tripeptides; if there are more than 10 in a chain, they are called polypeptides; and if there are 50 or more, they are known as proteins.

In 1902 the German organic chemist, Emil Fischer, first proposed that the amino acids in polypeptides are linked together between the carboxyl group of one amino acid and the amino group of the other. This bond forms when the -OH from the carboxyl end of one molecule and the hydrogen from the amino end of another molecule split off and form a small molecule byproduct, H2O or water. This type of reaction is called a condensation reaction. The new bond between the carbon atom and the nitrogen atom is called a peptide bond, also known as an amide linkage. Because every amino acid molecule has a carboxyl end and an amino end, each one can join to any other amino acid by the formation of a peptide bond.

All the millions of different proteins in living things are formed by the bonding of only 20 amino acids to form long polymer chains. Like the 26 letters of the alphabet that join together to form different words, depending on which letters are used and what the sequence is, the 20 amino acids can join together in different combinations and sequences to form proteins. But whereas words usually have only about 10 or fewer letters per word, proteins are usually made from at least 50 amino acids to more than 3,000. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of protein is truly enormous.

The amino acids in polypeptides can be represented in three ways: by writing out the complete chemical formulas; by writing the amino acid sequence using the standard, three-letter abbreviation for each acid as in glyser-ala (which represents glycine, serine and alanine); or by naming the polypeptide as in glycylserylalanine. The name is derived by dropping the -ine or -ic ending of each amino acid along the chain and replacing it with a -yl ending. The last acid of the chain is given its full name. It is common practice to write polypeptides with the free, unbonded amino group on the left and the free carboxylic acid group on the right.

Order is important in the functioning of a protein; gly-ser-ala, gly-ala-ser, and ala-ser-gly, for example, are different peptides. In fact, there are 27 different tripeptides that are possible from these three amino acids. (Each may be used more than once.) There are about two quadrillion different proteins that can exist if each of the 20 amino acids present in humans is used only once. However, just as not all sequences of letters make sense, not all sequences of amino acids make functioning proteins and other sequences can cause undesirable effects. While small mistakes in the amino acid sequence can sometimes be tolerated in nature without serious problems, at other times malfunctioning proteins can be caused by a single incorrect amino acid in the polymer chain. Sickle cell anemia is a fatal disease caused by a single amino acid, glutamic acid being replaced by a different one, valine, at the sixth position from the end of the protein chain in the hemoglobin molecule. This small difference causes lower solubility of the sickle cell hemoglobin molecules. They precipitate out as small rods which give the cells the characteristic sickle shape and result in the often fatal disease.

The specific sequence of the amino acids along the protein chain is referred to as the primary structure of the protein. However, these chains are not rigid, but rather they are long and flexible like string. The strands of protein can twist to form helixes or fold into sheets. They can bend and fold back on themselves to form globs and several protein molecules sometimes combine into a larger molecule. All of these configurations are caused by interactions both within a single protein strand as well as between two or three separate strands of protein.

Just as proteins are formed when amino acids bond together to form long chains, they can be broken down again into their individual amino acids by a reaction called hydrolysis . This reaction is just the reverse of the formation of the peptide bond. In the process of digestion, proteins are once again broken down into their individual amino acid components. Special digestive enzymes are necessary to cause the peptide linkage to break and a molecule of water is added when the reaction occurs. The resulting amino acids are released into the small intestine where they can easily pass into the bloodstream and be carried to every cell of the organism . There, once again, each individual cell can use these amino acids to assemble the new and different proteins required for its specific functions. Life goes on by the continual breakdown of protein into the individual amino acid units followed by the buildup of new protein from these amino acids.

Of the 20 amino acids required by humans for making protein, 12 of them can be made within the body from other nutrients . But the other eight, called the essential amino acids, cannot be made by the body and must be obtained from the diet. These are isoleucine, leucine, lysine, methionine, phenlyalanine, threonine, tryptophan, and valine. In addition, arginine and histidine are believed to be essential to growing children but may not be essential to mature adults. An adequate protein is one that contains all of the essential amino acids in sufficient quantities for growth and repair of body tissue . Most proteins from animal sources (gelatin being the only exception), contain all the essential amino acids and are considered adequate proteins. Many plant proteins do not contain all of the essential amino acids. Corn, for example, does not contain the essential amino acids lysine and tryptophan. Rice is lacking in lysine and threonine, wheat is lacking in lysine, and soy beans are lacking in methionine. People who are vegetarians and do not consume animal proteins in their diets sometimes suffer from malnutrition because of the lack of one or more amino acids in their diets even though they may consume enough food and plenty of calories.

See also Nutrition.

Resources

books

Durbin, Richard, et al. Biological Sequence Analysis: Probabilistic Models of Proteins and Nucleic Acids. Cambridge: Cambridge University Press, 1999.

Lide, D. R., ed. CRC Handbook of Chemistry and Physics Boca Raton: CRC Press, 2001.

Newhouse, Elizabeth L., et al., eds. Inventors and Discoverers:Changing Our World. Washington, DC: National Geographic Society, 1994.

White, James, and Dorothy C. White, eds. Proteins, Peptides, and Amino Acids Sourcebooks. Humana Press; 2002.

periodicals

Bishop, Katherine. "Baby Boomers Fight Aging by Dropping Acid (Amino)." New York Times (10 June 1992): B1(N).

Venter, J.C., et al. "The Sequence of the Human Genome." Science 291 (2001): 1304-1351.

Leona B. Bronstein

KEY TERMS

Amino group

—An -NH2 group.

BETA-amino acid

—An amino acid in which the -NH2 (amino) group is bonded to the carbon atom.

BETA-carbon atom

—The carbon atom adjacent to the carboxyl group.

Carboxyl group

—A -COOH group; also written -CO2H.

Essential amino acid

—Amino acids that cannot be synthesized by the body and must be obtained from the diet.

Monomers

—Small, individual subunits which join together to form polymers.

Peptide

—Substances made up of chains of amino acids, usually fewer than 50.

Peptide bond

—The bond formed when the carboxyl group of one amino acid joins with the amino group of a second amino acid and splits off a water molecule.

Amino Acids

Amino acids are the building blocks of protein . The body has twenty different amino acids that act as these building blocks. Nonessential amino acids are those that the body can synthesize for itself, provided there is enough nitrogen, carbon, hydrogen, and oxygen available. Essential amino acids are those supplied by the diet , since the human body either cannot make them at all or cannot make them in sufficient quantity to meet its needs. Under normal conditions, eleven of the amino acids are nonessential and nine are essential.

Structure

All amino acids have a similar chemical structure—each contains an amino group (NH₂), an acid group (COOH), a hydrogen atom (H), and a distinctive side group that makes proteins more complex than either carbohydrates or lipids . All amino acids are attached to a central carbon atom (C).

The distinctive side group identifies each amino acid and gives it characteristics that attract it to, or repel it from, the surrounding fluids and other amino acids. Some amino acid side groups carry electrical charges that are attracted to water molecules (hydrophilic), while others are neutral and are repelled by water (hydrophobic). Side-group characteristics (shape, size, composition, electrical charge, and pH ) work together to determine each protein's specific function.

The three-dimensional shape of proteins is derived from the sequence and properties of its amino acids and determines its function and interaction with other molecules. Each amino acid is linked to the next by a peptide bond, the name given to the link or attraction between the acid (COOH) end of one amino acid and the amino end (NH₂) of another. Proteins of various lengths are made when amino acids are linked together in this manner. A dipeptide is two amino acids joined by a peptide bond, while a tripeptide is three amino acids joined by peptide bonds.

The unique shapes of proteins enable them to perform their various tasks in the body. Heat, acid, or other conditions can disturb proteins, causing them to uncoil or lose their shape and impairing their ability to function. This is referred to as denaturation.

Functions of Proteins

Proteins act as enzymes , hormones , and antibodies . They maintain fluid balance and acid and base balance. They also transport substances such as oxygen, vitamins , and minerals to target cells throughout the body. Structural proteins, such as collagen and keratin, are responsible for the formation of bones, teeth, hair, and the outer layer of skin, and they help maintain the structure of blood vessels and other tissues. In contrast, motor proteins use energy and convert it into some form of mechanical work (e.g., dividing cells, contracting muscle).

Enzymes are proteins that facilitate chemical reactions without being changed in the process. The inactive form of an enzyme is called a proenzyme. Hormones (chemical messengers) are proteins that travel to one or more specific target tissues or organs, and many have important regulatory functions. Insulin , for example, plays a key role in regulating the amount of glucose in the blood. The body manufactures antibodies (giant protein molecules), which combat invading antigens. Antigens are usually foreign substances such as bacteria and viruses that have entered the body and could potentially be harmful. Immunoproteins, also called immunoglobulins or antibodies, defend the body from possible attack by these invaders by binding to the antigens and inactivating them.

Proteins help to maintain the body's fluid and electrolyte balance. This means that proteins ensure that the proper types and amounts of fluid and minerals are present in each of the body's three fluid compartments. These fluid compartments are intracellular (contained within cells), extracellular (existing outside the cell), and intravascular (in the blood). Without this balance, the body cannot function properly.

Proteins also help to maintain balance between acids and bases within body fluids. The lower a fluid's pH, the more acidic it is. Conversely, the higher the pH, the less acidic the fluid is. The body works hard to keep the pH of the blood near 7.4 (neutral). Proteins also act as carriers, transporting many important substances in the bloodstream for delivery throughout the body. For example, a lipoprotein transports fat and cholesterol in the blood.

Food Sources

Humans consume many foods that contain proteins or amino acids. One normally need not worry about getting enough protein or amino acids in the typical American diet. Foods from animal sources are typically rich in essential amino acids. These include chicken, fish, eggs, dairy products, beef, and pork. With the increasing emphasis on vegetarian diets, plant sources of protein are gaining in popularity. Such sources include dried beans (black, kidney, northern, red, and white beans), peas, soy, nuts, and seeds. Although plant sources generally lack one or more of the essential amino acids, when combined with whole grains such as rice, or by eating nuts or seeds with legumes , all the amino acids can be obtained.

see also Diet; Fats; Malnutrition; Nutrients; Plant-Based Diets; Protein.

Susan P. Himburg

Bibliography

Insel, Paul; Turner, R.; and Ross, Don (2001). Nutrition. Sudbury, MA: Jones and Bartlett.

Whitney, Eleanor N., and Rolfes, Sharon R. (2002). Understanding Nutrition, 9th edition. Belmont, CA: Wadsworth Group.