Kinetics of action of pepsin on fluorescent peptide substrates

Oligopeptide substrates of porcine pepsin (E) of the type A-Phe-Phe-B (S) that are cleaved solely at the Phe-Phe bond under the conditions of these studies, and bearing an amino-terminal fluorescent probe group (mansyl or dansyl), have been used for stopped-flow measurements of the rate of formation of the A-Phe product. These experiments were conducted under conditions of [E] greater than [S], and the kinetic data were compared with those obtained under conditions of [S] greater than [E] for the formation of the Phe-B product (the same in all cases). The results for substrates with A = mansyl-Gly, mansyl-Gly-Gly, and dansyl-Gly-Gly support the conclusion that the rate-limiting step in the over-all catalytic process is associated with the scission of the Phe-Phe bond in the first detectables ES complex. Although the rate of this step varies widely with the nature of the A portion of A-Phe-Phe-B, the magnitude of the dissociation constant of ES is relatively invariant. This supports the view that, in the cleavage of oligopeptide substrates by pepsin, secondary enzyme--substrate interactions may cause conformational changes at the catalytic site, and that a portion of the total binding energy may be used for the attainment of the transition state in the bond-breaking step. With substrates that are hydrolyzed extremely rapidly (A = dansyl-Gly-Ala, dansyl-Ala-Ala), the rate of formation of the A-Phe product appears to be faster than the steady-state rate, suggesting that an additional step has become kinetically significant in the over-all process. This step may be associated with the return of the conformation of the active site to its original state.