These enzymes bring about the hydrolysis or condensation of substrates by the addition or removal of water.
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These are more familiar as the intestinal juices of animals. They also occur in seeds and other parts of plants.
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Hydrolases are further sub-divided, according to the class of substrates on which they act, as follows:
(i) Proteases or Proteolytic enzymes:
These enzymes hydrolyse proteins into proteoses and peptones and some still further into polypeptides and amino acids.
During their course of action they simply catalyze the breaking of the peptide linkages which join amino acids, —CO.NH to —COOH and NH2.
Those proteases such as pepsin and trypsin which act on peptide linkages within the body of the protein are termed endopeptidases while o hers, the exopeptidases, hydrolyze terminal peptide jinks.
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The latter group consists of amino-peptidases which attack peptide links next to the NHS end of the molecule and carboxypeptidases which hydrolyse the links at the —COOH end.
These exopeptidases tend to act on smaller units such as the tri and dipeptides which themselves result from endopeptidase activity on proteins. Some important proteases or proteolytic enzymes are as follows:
(a) Pepsin:
It is the enzyme of gastric juice which hydrolyses the proteins into proteoses and peptones.
(b) Trypsinogen and Chymotrypsinogen:
These are inactive enzymes of the pancreatic juice which after activation are converted into trypsin and chymotrypsin respectively. These split up proteins into the stage of polypeptides.
(c) Erepsin:
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It is the enzyme of intestinal juice which splits up polypeptides into amino acids.
(d) Papain:
It is a vegetable enzyme found in Carciapapaya ordinarily hydrolyses proteins to the peptone stage but in the presence of traces of HCN, the action may go even upto polypeptide stage.
(ii) Amylolytic enzymes (amylum, starch-lysis to breakdown):
These enzymes hydrolyse the starch into dextrin and finally into maltose. Some known amylolytic enzymes are as follows:
(a) Diastase:
It converts starch into maltose.
(b) Ptyalin:
It is the enzyme of salivary juice which converts the starch into maltose.
(c) Amylopsin or Pancreatic amylase:
It hydrolyses boiled or unboiled starch into maltose.
(d) Glucogenase:
It in the liver and muscles converts glucose into glycogen and very possibly the same enzyme by reversible reaction converts glycogen into glucose.
(e) Phosphorylase:
It helps the first stage of breakdown of glycogen and also to synthesize a polysaccharide undistinguishable from glycogen, so its action is also reversible.
Glycogen or starch + inorganic phosphate glucose 1-phosphate
(f) Nucleases:
These hydrolyse nucleic acids into nucleotides which again are hydrolysed by nucleotidases into nucleosides and the latter oncb again by nucleosidases into pentose sugars, bases and phosphates out of which they are made.
(iii) Amidases:
These enzymes hydrolyze amides and include urease, arginase and purinamidases. These act on urea, arginine and purine bodies respectively to carry further dissociation of these products.
(iv) Kathepsins:
These serve like proteinases in the cells of animal tissues.
(V) Lipolytic enzymes:
They breakdown neutral fats into glycerol and fatty acids, some known lipolytic enzymes are as follows:
(a) Gastric lipase:
It splits up small amount of fat emulsified by mucin in the stomach. It also splits milk-fat.
(b) Steapsin or Pancreatic lipase:
This enzyme splits emulsified fat into glycerol and fatty acids.
(c) Intestinal lipase:
This enzyme splits phosphatides, as lecithins, into glycerol, phosphoric acid, fatty acids and choline.
(d) Esterase and phosphatases:
These enzymes act on simple esters and phosphoric esters respectively in ossification of bone and many other processes.
(vi) Invertase:
It is known as inverting enzyme which transforms disaccharides into monosaccharides.
Maltase and succus-entericus also break up maltose into molecules of glucose in the same way as invertase transforms disaccharides into monosaccharides.