Hydrolysis

Hydrolysis literally means reaction with water. It is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. One fragment of the parent molecule gains a hydrogen ion (H⁺) from the additional water molecule. The other group collects the remaining hydroxyl group (OH⁻). To illustrate this process, some examples from real life and actual living systems are discussed here.

The most common hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water autoionizes into negative hydroxyl ions and hydrogen ions. The salt breaks down into positive and negative ions. For example, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with hydroxyl ions whereas acetate ions combine with hydrogen ions to produce neutral acetic acid, and the net result is a relative excess of hydroxyl ions, causing a basic solution.

However, under normal conditions, only a few reactions between water and organic compounds occur. Generally, strong acids or bases must be added in order to achieve hydrolysis where water has no effect. The acid or base is considered a catalyst . They are meant to speed up the reaction, but are recovered at the end of it.

Acid–base-catalyzed hydrolyses are very common; one example is the hydrolysis of amides or esters . Their hydrolysis occurs when the nucleophile (a nucleus-seeking agent, e.g., water or hydroxyl ion) attacks the carbon of the carbonyl group of the ester or amide. In an aqueous base, hydroxyl ions are better nucleophiles than dipoles such as water. In acid, the carbonyl group becomes protonated, and this leads to a much easier nucleophilic attack. The products for both hydrolyses are compounds with carboxylic acid groups.

Perhaps the oldest example of ester hydrolysis is the process called saponification. It is the hydrolysis of a triglyceride (fat) with an aqueous base such as sodium hydroxide (NaOH). During the process, glycerol, also commercially named glycerin, is formed, and the fatty acids react with the base, converting them to salts. These salts are called soaps, commonly used in households.

Moreover, hydrolysis is an important process in plants and animals, the most significant example being energy metabolism and storage. All living cells require a continual supply of energy for two main purposes: for the biosynthesis of small and macromolecules, and for the active transport of ions and molecules across cell membranes. The energy derived from the oxidation of nutrients is not used directly but, by means of a complex and long sequence of reactions, it is channeled into a special energy-storage molecule, adenosine triphosphate (ATP) .

The ATP molecule contains pyrophosphate linkages (bonds formed when two phosphate units are combined together) that release energy when needed. ATP can be hydrolyzed in two ways: the removal of terminal phosphate to form adenosine diphosphate (ADP) and inorganic phosphate, or the removal of a terminal diphosphate to yield adenosine monophosphate (AMP) and pyrophosphate. The latter is usually cleaved further to yield two phosphates. This results in biosynthesis reactions, which do not occur alone, that can be driven in the direction of synthesis when the phosphate bonds are hydrolyzed.

In addition, in living systems, most biochemical reactions, including ATP hydrolysis, take place during the catalysis of enzymes. The catalytic action of enzymes allows the hydrolysis of proteins, fats, oils, and carbohydrates. As an example, one may consider proteases, enzymes that aid digestion by hydrolyzing peptide bonds in proteins. They catalyze the hydrolysis of interior peptide bonds in peptide chains, as opposed to exopeptidases, another class of enzymes, that catalyze the hydrolysis of terminal peptide bonds, liberating one free amino acid at a time.

However, proteases do not catalyze the hydrolysis of all kinds of proteins. Their action is stereo-selective: Only proteins with a certain tertiary structure will be targeted. The reason is that some kind of orienting force is needed to place the amide group in the proper position for catalysis. The necessary contacts between an enzyme and its substrates (proteins) are created because the enzyme folds in such a way as to form a crevice into which the substrate fits; the crevice also contains the catalytic groups. Therefore, proteins that do not fit into the crevice will not be hydrolyzed. This specificity preserves the integrity of other proteins such as hormones, and therefore the biological system continues to function normally.

see also Enzymes; Tertiary Structure.

Joseph Bariyanga

Bibliography

Freifelder, David (1987). Molecular Biology, 2nd edition. Boston: Jones and Bartlett.

Hydrolysis

Hydrolysis is a chemical reaction in which water (H₂O) reacts with another substance to split it into two or more new substances. Examples of the process of hydrolysis include the conversion of starch to glucose in water under the action of a suitable catalyst; conversion of sucrose to fructose and glucose in water in the presence of an acidic or enzyme catalyst; conversion of fats into fatty acids and glycerol in water in soap manufacture; and reactions of dissolved salts in water to form various products. Hydrolytic reactions are important in many biochemical processes, particularly in enzyme-catalyzed reactions such as those used in digestion. The processes of hydrolysis and condensation are often considered opposite processes since the later process brings together (rather than separates) two substances while involved with water.

In each reaction, a water molecule separates into H⁺ and OH^-, and the compound being hydrolyzed splits into two fragments, A⁺ and B^-, where AB represents the original (generic) compound. The products of hydrolysis are AOH and HB. If, for example, ordinary sodium chloride (table salt) is placed in water, then some of the salt will form sodium and chlorine ions. These ions will then combine with the hydrogen and hydroxide ions in water to form ionized sodium hydroxide (Na⁺ + OH^-) and ionized hydrogen chloride (H⁺ + Cl^-). Sodium hydroxide and hydrogen chloride essentially exist as ions in solution because they are a strong base and strong acid, respectively. Therefore, they are neutralized by the hydroxide and hydrogen ions, and the resultant solution is neutral.

Whenever water reacts with another chemical compound, the process is called hydrolysis. Hydrolysis differs somewhat from the process called hydration, although the two can occur together. Hydration is the bonding of whole water molecules to an ion (a charged atom or molecule), usually a metal ion. Hydrolysis, on the other hand, involves an actual chemical reaction of the water molecule itself with another reactant. Aluminum ion, for example, can bond with six water molecules to form the hydrated aluminum ion. In water, however, the hydrated ion can undergo hydrolysis; some of the hydrated molecules contribute a hydrogen ion to the solution, making the solution acidic.

Solutions of non-hydrated ions often become either acidic or basic because of hydrolysis, too. In general, negative ions (anions) form basic solutions if they hydrolyze, because the negative charge on the ion attracts the positively charged hydrogen ion (H+) away from water, leaving the basic hydroxide ion (OH-) behind. Similarly, positive ions (cations) form acidic solutions if they hydrolyze, because the positive charge on the ion attracts the negatively charged hydroxide ion away from water, leaving the acidic hydrogen ion behind. Hydrolysis of these ions only occurs, however, if the ion originally came from a weak acid or base, or the salt of a weak acid or base. (A salt is an ionic chemical compound derived from an acid or base, often as the result of a neutralization reaction.) Ions do not hydrolyze if they are from strong acids or bases—such as chloride ion from hydrochloric acid or sodium ion from sodium hydroxide (a base)—or their salts.

In biochemistry, hydrolysis often involves the decomposition of a larger molecule. If a fat undergoes hydrolysis, for example, it reacts with water and decomposes to glycerol and a collection of fatty acids. Similarly, complex sugars can hydrolyze to smaller sugars, and nucleotides can hydrolyze to a five carbon sugar, a nitrogenous base, and phosphoric acid.

Hydrolysis

Whenever water reacts with another chemical compound, the process is called hydrolysis. Hydrolysis differs somewhat from hydration, although the two can occur together. Hydration is the bonding of whole water molecules to an ion (a charged atom or molecule ), usually a metal ion. Hydrolysis, on the other hand, involves an actual chemical reaction of the water molecule itself with another reactant. Aluminum ion, for example, can bond with six water molecules to form the hydrated aluminum ion. In water, however, the hydrated ion can undergo hydrolysis; some of the hydrated molecules contribute a hydrogen ion to the solution , making the solution acidic.

Solutions of non-hydrated ions often become either acidic or basic because of hydrolysis, too. In general, negative ions (anions) form basic solutions if they hydrolyze, because the negative charge on the ion attracts the positively charged hydrogen ion (H+) away from water, leaving the basic hydroxide ion (OH-) behind. Similarly, positive ions (cations) form acidic solutions if they hydrolyze, because the positive charge on the ion attracts the negatively charged hydroxide ion away from water, leaving the acidic hydrogen ion behind. Hydrolysis of these ions only occurs, however, if the ion originally came from a weak acid or base, or the salt of a weak acid or base. (A salt is an ionic chemical compound derived from an acid or base, often as the result of a neutralization reaction.) Ions do not hydrolyze if they are from strong acids or bases—such as chloride ion from hydrochloric acid or sodium ion from sodium hydroxide (a base)—or their salts.

In biochemistry , hydrolysis often involves the decomposition of a larger molecule. If a fat undergoes hydrolysis, for example, it reacts with water and decomposes to glycerol and a collection of fatty acids . Similarly, complex sugars can hydrolyze to smaller sugars, and nucleotides can hydrolyze to a five carbon sugar, a nitrogenous base, and phosphoric acid .

hydrolysis Chemical reaction in which molecules are split into smaller molecules by reaction with water; often assisted by a catalyst. For example, in digestion, enzymes catalyse the hydrolysis of carbohydrates, proteins, and fats into smaller, soluble molecules that the body can assimilate.

hydrolysis
1. Reaction between a substance and water in which the substance is split into two or more products. At the points of cleavage the products react with the hydrogen or hydroxyl ions derived from water.

2. Process of enriching the soil adsorption complex with hydrogen after exchangeable metallic ions have been replaced by hydrogen ions. Compare WEATHERING.