1. The optically pure p-nitrophenyl esters of the d and l enantiomers of N-acetyl-tryptophan, N-acetylphenylalanine and N-acetyl-leucine, and the p-nitrophenyl ester of N-acetylglycine, have been prepared. 2. These materials are all substrates of alpha-chymotrypsin, and the rates of deacylation of the corresponding acyl-alpha-chymotrypsins have been determined. 3. As the size of the amino acid side chain increases, the l series deacylate progressively faster than the N-acetylglycyl-enzyme, and the d series progressively more slowly. 4. The results are interpreted in terms of a three-locus model of the enzyme's active site, which accounts for the interrelationship between substrate specificity and stereospecificity observed. 5. The concepts of negative specificity and of specificity saturation are introduced.

1. The pH-dependence of the binding to pepsin of four dipeptide competitive inhibitors is reported. Values of K(i) obtained from equilibrium-dialysis experiments agree closely with those from kinetic measurements. 2. The binding of uncharged N-acyl-dipeptide amides to pepsin is essentially independent of pH from 0.2 to 5.8. Values of K(i) for the corresponding N-acyl-dipeptide acids rise rapidly above pH3.5, and depend on the ionization of a group of apparent pK(a) 3.6. 3. The data indicate that pepsin does not undergo any gross conformation change (at least none that affects binding) over the whole pH range of its catalytic activity. The pH-dependence of the dipeptide acid inhibitors indicates that the acid anions do not bind to pepsin, presumably because of electrostatic repulsion between the inhibitor anion and a negative centre at or near the active site of the enzyme. 4. The binding of all four stereoisomers of N-acetylphenylalanylphenylalanine, of the depside analogues of the l-l- and d-l-compounds and of N-acetylglycyl-l-phenylalanine and N-acetyl-l-phenylalanylglycine was studied at pH2.2. 5. These results throw further light on the binding specificity of pepsin and on the charge nature of the active site of this enzyme.

Pepsin reacts stoicheiometrically with the active-site-directed irreversible inhibitor N-diazoacetyl-l-phenylalanine methyl ester, with concomitant loss of all proteolytic and peptidolytic activity. The reagent esterifies a unique aspartic acid residue in pepsin, which is in the sequence:Ile-Val-Asp-Thr-Gly-Thr-Ser