Catalytic activity of dimeric alpha-chymotrypsin. Acylation kinetics at low pH's

Dimeric alpha-chymotrypsin (D) formed over a range of low pH (3.7-5.2) showed a different specificity from monomeric alpha-chymotrypsin (M) for the acylation step, though the catalytic system functions as well in the dimeric enzyme as in the monomeric enzyme. For aliphatic substituted phenyl esters a bulky substituent at the ortho-position of the phenyl ring reduced the acylation rate of the dimeric enzyme compared with the monomeric enzyme. Especially for aliphatic p-nitrophenyl esters, the dissociation constants of the DS complexes were always smaller than those of the MS complexes, independent of the aliphatic chain length of the substrate. The intrinsic acylation rate constant of the dimeric enzyme increased with the chain length of the substrate. These catalytic properties are discussed with reference to the structures of the catalytic and binding sites of the dimeric enzyme.