Glycolysis

Resources

Glycolysis, a series of enzymatic steps in which the six-carbon glucose molecule is degraded to yield two three-carbon pyruvate molecules, is a central catabolic pathway in plants, animals, and many microorganisms.

In a sequence of 10 enzymatic steps, energy released from glucose is conserved by glycolysis in the form of adenosine triphosphate (ATP). So central is glycolysis to life that its sequence of reactions differs among species only in how its rate is regulated, and in the metabolic fate of pyruvate formed from glycolysis.

In aerobic organisms (some microbes and all plants and animals), glycolysis is the first phase of the complete degradation of glucose. The pyruvate formed by glycolysis is oxidized to form the acetyl group of acetyl-coenzyme A, while its carboxyl group is oxidized to CO₂. The acetyl group is then oxidized to CO₂ and H₂O by the citric acid cycle with the help of the electron transport chain, the site of the final steps of oxidative phosphorylation of adenosine diphosphate molecules to high-energy ATP molecules.

In some animal tissues, pyruvate is reduced to lactate during anaerobic periods, such as during vigorous exercise, when there is not enough oxygen available to oxidize glucose further. This process, called anaerobic glycolysis, is an important source of ATP during very intense muscle activity.

Anaerobic glycolysis also serves to oxidize glucose to lactic acid with the production of ATP in anaerobic microorganisms. Such lactic acid production by bacteria sours milk and gives sauerkraut its mildly acidic taste.

A third pathway for pyruvate produced by glycolysis produces ethanol and CO₂ during anaerobic glycolysis in certain microorganisms, such as brewer’s yeast—a process called alcoholic fermentation, an anaerobic process by which glucose or other organic nutrients are degraded into various products to obtain ATP.

Because glycolysis occurs in the absence of oxygen, and living organisms first arose in an anaerobic environment, anaerobic catabolism was the first biological pathway to evolve for obtaining energy from organic molecules.

Glycolysis occurs in two phases. In the first phase, there are two significant events. The addition of two phosphate groups to the six-carbon sugar primes it for further degradation in the second phase. Then, cleavage of the doubly phosphorylated six-carbon chain occurs, breaking fructose 1,6-diphosphate into two 3-carbon isomers. These are fragments of the original six-carbon sugar dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

In the second phase, the two 3-carbon fragments of the original 6-carbon sugar are further oxidized to lactate or pyruvate.

Entry into the second phase requires the isomer to be in its glyceraldehyde 3-phosphate form. Thus, the dihydroxyacetone phosphate isomer is transformed into glyceraldehyde 3-phosphate before being further oxidized by the glycolytic pathway.

Glycolysis produces a total of four ATP molecules in the second phase, two molecules of ATP from each glyceraldehyde 3-phosphate molecule. The ATP is formed during substrate-level phosphorylation-direct transfer of a phosphate group from each 3-carbon fragment of the sugar to adenosine diphosphate (ADP), to form ATP. But because two ATP molecules were used to phosphorylate the original six-carbon sugar, the net gain is two ATP.

The net gain of two ATP represents a modest conservation of the chemical energy stored in the glucose molecule. Further oxidation, by means of the reactions of the Kreb’s cycle and oxidative phosphorylation are required to extract the maximum amount of energy from this fuel molecule.

See also Adenosine triphosphate; Krebs cycle.

Resources

BOOKS

Atkinson, D.E. Cellular Energy Metabolism and Its Regulation. New York: Academic, 1977.

Lehninger, A.L. Principles of Biochemistry. New York: Worth Publishers, Inc., 1982.

OTHER

Indiana State University. “Digestion of Dietary Carbohydrates” <http://web.indstate.edu/thcme/mwking/glycolysis.html> (accessed November 25, 2006).

University of Texas, Institute for Cellular and Molecular Biology. “Glycolysis” <http://biotech.icmb.utexas.edu/glycolysis/glycohome.html> (accessed November 25, 2006).

Marc Kusinitz

Glycolysis

Glycolysis, a series of enzymatic steps in which the six-carbon glucose molecule is degraded to yield two three-carbon pyruvate molecules, is a central catabolic pathway in plants, animals and many microorganisms .

In a sequence of 10 enzymatic steps, energy released from glucose is conserved by glycolysis in the form of adenosine triphosphate (ATP). So central is glycolysis to life that its sequence of reactions differs among species only in how its rate is regulated, and in the metabolic fate of pyruvate formed from glycolysis.

In aerobic organisms (some microbes and all plants and animals), glycolysis is the first phase of the complete degradation of glucose. The pyruvate formed by glycolysis is oxidized to form the acetyl group of acetylcoenzyme A, while its carboxyl group is oxidized to CO2. The acetyl group is then oxidized to CO2 and H2O by the citric acid cycle with the help of the electron transport chain, the site of the final steps of oxidative phosphorylation of adenosine diphosphate molecules to high-energy ATP molecules.

In some animal tissues, pyruvate is reduced to lactate during anaerobic periods, such as during vigorous exercise , when there is not enough oxygen available to oxidize glucose further. This process, called anaerobic glycolysis, is an important source of ATP during very intense muscle activity.

Anaerobic glycolysis also serves to oxidize glucose to lactic acid with the production of ATP in anaerobic microorganisms. Such lactic acid production by bacteria sours milk and gives sauerkraut its mildly acidic taste.

A third pathway for pyruvate produced by glycolysis produces ethanol and CO2 during anaerobic glycolysis in certain microorganisms, such as brewer's yeast—a process called alcoholic fermentation . Fermentation is an anaerobic process by which glucose or other organic nutrients are degraded into various products to obtain ATP.

Because glycolysis occurs in the absence of oxygen, and living organisms first arose in an anaerobic environment, anaerobic catabolism was the first biological pathway to evolve for obtaining energy from organic molecules.

Glycolysis occurs in two phases. In the first phase, there are two significant events. The addition of two phosphate groups to the six-carbon sugar primes it for further degradation in the second phase. Then, cleavage of the doubly phosphorylated six-carbon chain occurs, breaking fructose 1,6-diphosphate into two 3-carbon isomers. These are fragments of the original six-carbon sugar dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

In the second phase, the two 3-carbon fragments of the original 6-carbon sugar are further oxidized to lactate or pyruvate.

Entry into the second phase requires the isomer to be in its glyceraldehyde 3-phosphate form. Thus, the dihydroxyacetone phosphate isomer is transformed into glyceraldehyde 3-phosphate before being further oxidized by the glycolytic pathway.

Glycolysis produces a total of four ATP molecules in the second phase, two molecules of ATP from each glyceraldehyde 3-phosphate molecule. The ATP is formed during substrate-level phosphorylation-direct transfer of a phosphate group from each 3-carbon fragment of the sugar to adenosine diphosphate (ADP), to form ATP. But because two ATP molecules were used to phosphorylate the original six-carbon sugar, the net gain is two ATP.

The net gain of two ATP represents a modest conservation of the chemical energy stored in the glucose molecule. Further oxidation, by means of the reactions of the Kreb's cycle and oxidative phosphorylation are required to extract the maximum amount of energy from this fuel molecule.

See also Adenosine triphosphate; Krebs cycle.

Resources

books

Atkinson, D.E. Cellular Energy Metabolism and Its Regulation. New York: Academic, 1977.

Lehninger, A.L. Principles of Biochemistry. New York: Worth Publishers, Inc., 1982.

Marc Kusinitz

Glycolysis

Glycolysis is the sequence of enzymatic reactions that oxidize the six-carbon sugar glucose into two three-carbon compounds with the production of a small amount of adenosine triphosphate (ATP) . Glycolysis has two basic functions in the cell. First, it metabolizes simple six-carbon sugars to smaller three-carbon compounds that are then either fully metabolized by the mitochondria to produce carbon dioxide and a large amount of ATP or used for the synthesis of fat for storage. Second, glycolysis functions to produce

a small amount of ATP, which is essential for some cells solely dependent on that pathway for the generation of energy.

The glycolytic pathway is nearly ubiquitous, being found in every cell of virtually all living creatures. It is catalyzed by soluble enzymes located in the cytosol of cells. Although the glycolytic pathway is most commonly thought of as metabolizing glucose, other common monosaccharides such as fructose, galactose , and mannose are also metabolized by it. The glycolytic pathway operates in both the presence (aerobic) or absence of oxygen (anaerobic).

The metabolism of fuel molecules in the cell can be thought of as an oxidation process. In glycolysis, glucose is the fuel molecule being oxidized. As the glucose is oxidized by the glycolytic enzymes, the coenzyme nicotinamide adenine dinucleotide (NAD⁺) is converted from its oxidized to reduced form (NAD⁺ to NADH). When oxygen is available (aerobic conditions), mitochondria in the cell can reoxidize to NADH to NAD⁺. However, if either oxygen levels are insufficient (anaerobic conditions) or mitochondrial activity is absent, NADH must be reoxidized by the cell using some other mechanism. In animal cells, the reoxidation of NADH is accomplished by reducing pyruvate , the end-product of glycolysis, to form lactic acid. This process is known as anaerobic glycolysis. During vigorous exercise, skeletal muscle relies heavily on it. In yeast, anaerobic conditions result in the production of carbon dioxide and ethanol from pyruvate rather than lactic acid. This process, known as alcoholic fermentation, is the basis of wine production and the reason why bread dough rises.

Although some cells are highly dependent on glycolysis for the generation of ATP, the amount of ATP generated per glucose molecule is actually quite small. Under anaerobic conditions, the metabolism of each glucose molecule yields only two ATPs. In contrast, the complete aerobic metabolism of glucose to carbon dioxide by glycolysis and the Krebs cycle yields up to thirty-eight ATPs. Therefore, in the majority of cells the most important function of glycolysis is to metabolize glucose to generate three-carbon compounds that can be utilized by other pathways. The final product of aerobic glycolysis is pyruvate. Pyruvate can be metabolized by pyruvate dehydrogenase to form acetyl coenzyme A (acetyl CoA). Under conditions where energy is needed, acetyl CoA is metabolized by the Krebs cycle to generate carbon dioxide and a large amount of ATP. When the cell does not need energy, acetyl CoA can be used to synthesize fats or amino acids.

see also Insulin; Krebs Cycle.

Robert Noiva

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W.H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.

glycolysis The first stage in the production of energy by breakdown of glucose in body cells; a chain of chemical events requiring a specific set of enzymes, and resulting in formation of ATP (adenosine triphosphate). In aerobic metabolism subsequent sequences produce several times more ATP, thereby providing a greater quantity of energy per molecule of glucose, utilizing oxygen, and producing carbon dioxide and water — comparable to burning organic fuels in air. In anaerobic metabolism (metabolism which does not use oxygen) glycolysis is the only means of energy production from glucose, and lactate is the end-product. This occurs in cells which cannot utilize oxygen (red blood cells), predominately in some components of skeletal muscle (fast, ‘white’ fibres), and probably to some extent in all cells when there is a shortage of oxygen. However, it also occurs in the first 1–3 minutes after a sudden increase of demand in cells which will subsequently make all the necessary ATP aerobically, because the glycolytic system can respond within seconds whereas both the biochemical pathways of aerobic metabolism and the systems for supplying them with oxygen take time to adjust; the start of vigorous exercise, even in a trained athlete with large numbers of ‘red’ muscle fibres, is the most obvious example.

Sheila Jennett

glycolysis (Embden-Meyerhof pathway) The stepwise anaerobic degradation of glucose to produce as end-products ethanol and carbon dioxide in the cells of fungi and plants (or lactic acid in animal cells). One mole of glucose yields 1 mole each of ethanol and carbon dioxide in fungi and plants (or 2 moles of lactic acid in animals). In both cases the reaction sequence has a net yield of 2 moles of ATP. However, in most cells, under aerobic conditions, the pathway serves primarily to provide pyruvate, which is oxidized via the citric acid cycle, and intermediate compounds for biosynthetic processes.