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Structure of Enzymes

All enzymes are proteins. Proteins are high molecular weight macromolecules. An enzyme may consist of a single polypeptide chain, e.g. beef ribonuclease, or an aggregate of polypeptide chains. The polypeptide chain is made up of a number of amino acid units linked by peptide bonds. The sequence and number of the 20 amino acid which make up enzymes varies in different enzymes. This sequence is specific for a particular enzyme and determines the properties of the enzymes.

The amino acid sequence and the three dimensional structure have been determined for the enzymes egg-white lysozyme, ribonuclease, car. Boxypeptidase, chymotrypsin and papain. Lysozyme hydrolyses links between amino sugars, and is found in saliva and egg white. Ribonuclease degrades RNA into small fragments. Cymotrypsin and trypsin are proteolytic enzymes found in the pancreas. Chymotrypsinogen, the inactivge precursor of chymotrypsin, consists of a single polypeptide chain. Removal of two dipeptides results in the active enzyme chymotrypsin, which consists of three polypeptide chains. Lysozyme, ribonuclease, carboxypeptidase, chymotrypsinogen and papain each consist of a single polypeptide chain which is highly folded into a spherical shape. Trypsinogen consists of two polypeptide chains, and chymotrypsin of three chains. The number of amino acid residues is 33 in carbonic anyhydrase, 124 in ribonuclease, 129 in egg-white lysozyme and 249 in chymotrypsinogen and trypsinogen.

beef ribonuclease

The polypeptide chain has an amino (-NH2) terminal and a carboxy (-COOH) terminal. Biosynthesis of the enzyme begins at the amino terminal.

The different parts of the polypeptide chain are linked by disulphide (-S-S-) bridges, which are most commonly found between two cysteine amino acids. Disulphide brideges may be within a polypeptide chain (intrachain) or may connect two polypeptide chains (interchain). Egg-white lysozyme and ribonuclease have four disulphide bridges, chymotrypsinogen has five and trypsinogen six.

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