Hydrolysis (; from Ancient Greek hydro- 'water' and lysis 'to unbind') is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution and elimination reactions in which water is the nucleophile. Biological hydrolysis is the cleavage of biomolecules where a water molecule is consumed to effect the separation of a larger molecule into component parts. When a carbohydrate is broken into its component sugar molecules by hydrolysis (e.g., sucrose being broken down into glucose and fructose), this is recognized as saccharification. Hydrolysis reactions can be the reverse of dehydration synthesis (a specific type of condensation reaction) in which two molecules join into a larger one and eject a water molecule. Thus hydrolysis adds water to break down molecules, whereas condensation joins molecules through the removal of water.
Types Hydrolysis is a chemical process in which a molecule of water is added to a substance, causing both the substance and water molecule to split into two parts. In such reactions, a chemical bond is broken, with one fragment of the target molecule (or parent molecule) gaining a hydrogen ion, and the other gaining a hydroxide. In living systems, most biochemical reactions (including ATP hydrolysis) take place during the catalysis of enzymes. The catalytic action of enzymes allows for the hydrolysis of proteins, fats, oils, and carbohydrates.
Esters and amides
Ester and amide hydrolysis occurs through nucleophilic acyl substitution where water acts as a nucleophile (a nucleus-seeking agent, e.g., water or hydroxyl ion), attacking the carbon of the carbonyl group of the ester or amide. Under acidic conditions, the carbonyl group is activated via protonation, allowing for direct nucleophilic attack by water. In an aqueous base, hydroxyl ions are better nucleophiles than polar molecules such as water due to the negative charge localized on the oxygen and therefore directly attack the carbonyl group. Upon hydrolysis, an ester is converted into a carboxylic acid plus an alcohol, while an amide converts into a carboxylic acid and an amine or ammonia (which in the presence of acid are immediately converted to ammonium salts). One of the two oxygen groups on the carboxylic acid are derived from a water molecule and the amine/ammonia or alcohol gains the hydrogen ion. Perhaps the oldest commercially practiced example of ester hydrolysis is saponification (formation of soap). It is the hydrolysis of a triglyceride (fat) with an aqueous base such as sodium hydroxide (NaOH). During the process, glycerol is formed, and the fatty acids react with the base, converting them to salts. These salts are called soaps, commonly used in households. Under biological conditions, this reaction is catalyzed by lipases for the digestion of fats, acting when adsorbed to an oil-water interface. Other esterases function in water, serving a variety of biological functions. A key biological application of amide hydrolysis is the digestion of proteins into amino acids. Proteases, enzymes that aid digestion by causing hydrolysis of peptide bonds in proteins, catalyze the hydrolysis of peptide bonds in peptide chains, releasing polypeptide fragments two to six amino acids long. Those fragments are then broken down into single amino acids via carboxypeptidases secreted by the pancreas.
However, proteases do not catalyze the hydrolysis of all kinds of proteins. Their action is stereo-selective: Only proteins with a certain tertiary structure are targeted as 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 undergo hydrolysis. This specificity preserves the integrity of other proteins such as hormones, and therefore the biological system continues to function normally. Many polyamide polymers such as nylon 6,6 hydrolyze in the presence of strong acids. The process leads to depolymerization. For this reason, nylon products fail by fracturing when exposed to small amounts of acidic water. Polyesters are also susceptible to similar polymer degradation reactions. The problem is known as environmental stress cracking.
ATP
Hydrolysis is related to energy metabolism and storage. All living cells require a continual supply of energy for two main purposes: the biosynthesis of micro and macromolecules, and 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) that release energy when needed. ATP can undergo hydrolysis in two ways: Firstly, the removal of terminal phosphate to form adenosine diphosphate (ADP) and inorganic phosphate, with the reaction:
ATP + H2O → ADP + Pi Secondly, the removal of a terminal diphosphate to yield adenosine monophosphate (AMP) and pyrophosphate. The latter usually undergoes further cleavage into its two constituent phosphates. This results in biosynthesis reactions, which usually occur in chains, that can be driven in the direction of synthesis when the phosphate bonds have undergone hydrolysis.
Polysaccharides
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