The pathway of pepsin-catalysed transpeptidation. Evidence for the reactive species being the anion of the acceptor molecule.

1. The inhibition of pepsin-catalysed hydrolysis of N-acetyl-l-phenylalanyl-l-phenylalanylglycine by the acyl product and product analogues was studied at pH4.3. 2. The acyl product, N-acetyl-l-phenylalanine, gives rise to linear competitive inhibition at pH4.3, whereas at pH2.1 it shows linear non-competitive behaviour. 3. The extent of transpeptidation to N-acetyl-l-[(3)H]phenylalanine during the pepsin-catalysed hydrolysis of N-acetyl-l-phenylalanyl-l-phenylalanyl-glycine is significant at pH4.7, but is undetectable at pH1.3. 4. Both the inhibition and transpeptidation experiments are consistent with the anion of the acceptor molecule being the substrate in pepsin-catalysed transpeptidation. This conclusion supports the formulation of pepsin-catalysed reactions put forward by Knowles et al. (1970).