Chymotrypsin methionine role

The function, if any, of methionine sulfoxide residues in peptides or proteins is a matter of conjecture. There is no evidence that they are of any structural significance. Indeed, since various enzymes such as ribonuclease and chymotrypsin are either partially or completely inactivated by oxidation of the methionine residues (15, 16), one hesitates to suggest any functional role for the sulfoxide. However, a role in the maintenance of oxidation-reduction potential of a biological system, as suggested by Dent (3), is conceivable. 

The involvement of methionine sulfonium salts in these chemical cleavage reactions may stimulate experiments to show the significance of special methionine residues in the active centers and the catalytic sites of enzymes. Such an important role for a particular methionine has been postulated for the enzymes phosphoglucomutase and chymotrypsin on the basis of photooxidation studies (Ray et al., 1960). 

Other results suggest that enzymatic peptide modification by Met incorporation results in increasing the nutritive value of buffalo s milk proteins [124], too. The role of dogfish chymotrypsin has been also investigated as an available catalyst in covalent amino acid enrichment [27]. Both dogfish chymotrypsin and bovine chymotrypsin were capable of incorporating methionine ester into soy protein. However, the use of bovine enzyme resulted in higher Met incorporation than that of the fish enzyme. That means that bovine chymotrypsin was more efficient in covalent methionine enrichment into soy protein [27]. 

The sidechain of methionine-192 plays an important role in a-chymotrypsin, being implicated in both the activation of the enzyme from its zymogen and the binding and orientation of a-chymotrypsin substrates during catalysis. X-ray diffraction studies reveal that the sidechain may act as a lid over a hydrophobic sidechain binding pocket known as the tosyl hole. Modification of the sidechain of Met-192 therefore has kinetic consequences thus the addition of an aromatic moiety allows it to bind in the tosyl pocket and to compete for this site with substrate or inhibitor. The behavior of such an aromatic moiety may conveniently be followed via... 

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