The active centers of Streptomyces griseus protease 3, alpha-chymotrypsin, and elastase: enzyme-substrate interactions close to the scissile bond.

Kinetic constants are reported for alpha-chymotrypsin-, Streptomyces griseus protease 3 (SGP3)-, and elastase-catalyzed hydrolysis of a number of peptides. SGP3, like alpha-chymotrypsine, hydrolyzes most readily amide bonds whose immediate acyl group (P1) is a large, hydrophobic, amino acid residue. SGP3, however, has a broader specificity for P1 residues than does alpha-chymotrypsin, primarily because the most important interactions between SGP3 and residue P1 of the substrate involve the Cbeta and Cgamma groups of the P1 side chain. For substrates of all three proteases, the amino acid residue contributing the amino group of the scissile bond (P1') is less important than P1 in determining kcat/Km for the hydrolysis reaction. Each enzyme interacts favorably with a hydrophobic P1' side chain. A substate with a large P1' side chain is bound more strongly, but is hydrolyzed less rapidly, than that with P1' Ala. The observation that strong binding of P1' is not necessarily conducive to rapid hydrolysis is consistent with the idea that parts of P1' must undergo a considerable displacement during substrate hydrolysis.