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. 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, 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, 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: :{{chem2 | ATP + H2O -> ADP + P_{i} }} 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 . The glycoside bond is represented by the central oxygen atom, which holds the two monosaccharide units together.
Monosaccharides can be linked together by
glycosidic bonds, which can be cleaved by hydrolysis. Two, three, several or many monosaccharides thus linked form
disaccharides,
trisaccharides,
oligosaccharides, or
polysaccharides, respectively. Enzymes that hydrolyze glycosidic bonds are called "
glycoside hydrolases" or "glycosidases". The best-known disaccharide is
sucrose (table sugar). Hydrolysis of sucrose yields
glucose and
fructose.
Invertase is a
sucrase used industrially for the hydrolysis of sucrose to so-called
invert sugar.
Lactase is essential for digestive hydrolysis of
lactose in milk; many adult humans do not produce lactase and
cannot digest the lactose in milk. The hydrolysis of polysaccharides to soluble sugars can be recognized as
saccharification.
DNA Hydrolysis of
DNA occurs at a significant rate in vivo. For example, it is estimated that in each human cell 2,000 to 10,000 DNA
purine bases turn over every day due to hydrolytic depurination, and that this is largely counteracted by specific rapid
DNA repair processes. The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as :{{chem2 | M(H2O)_{n}^{m+} + H2O M(H2O)_{n-1}(OH)^{(m-1)+} + H3O+ }} Thus the aqua
cations behave as acids in terms of
Brønsted–Lowry acid–base theory. This effect is easily explained by considering the
inductive effect of the positively charged metal ion, which weakens the bond of an attached water molecule, making the liberation of a proton relatively easy. The
dissociation constant, pKa, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion. Ions with low charges, such as are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as , , and have a pKa of 6 or more and would not normally be classed as acids, but small divalent ions such as undergo extensive hydrolysis. Trivalent ions like and are weak acids whose pKa is comparable to that of
acetic acid. Solutions of salts such as or in water are noticeably
acidic; the hydrolysis can be
suppressed by adding an acid such as
nitric acid, making the solution more acidic. Hydrolysis may proceed beyond the first step, often with the formation of polynuclear species via the process of
olation. are well characterized. Hydrolysis tends to proceed as
pH rises leading, in many cases, to the precipitation of a hydroxide such as or . These substances, major constituents of
bauxite, are known as
laterites and are formed by leaching from rocks of most of the ions other than aluminium and iron and subsequent hydrolysis of the remaining aluminium and iron.
Mechanism strategies Acetals,
imines, and
enamines can be converted back into
ketones by treatment with excess water under acid-catalyzed conditions: ; ; . ==Catalysis==