Mechanistic features of pepsin-catalyzed amino transfer reactions

Peptic cleavage of N-trifluoroacetyl-L-tryptophan (CF3CO-Trp) at pH 2.45 to 5.4 in the presence of L-beta-phenyllactic acid (Pla) yields much PlaTrp. Formation of PlaTrp represents a typical peptic amino transfer reaction. In this instance PlaTrp undoubtedly derives from a reaction between the acceptor, Pla, and the carboxylate anion, CF3CO-TrpCOOO- (or a species obtained from it). This observation suggests that in general the anionic form of a substrate, such as AcPheTrpCOOO- in the case of AcPheTrp, may be the primary source of the amino acid residue transferred in amino transpeptidations. Other evidence supports the validity of the proposal. Furthermore it has the virtue of rationally explaining why AcPheTrpNH2-like substrates fail to participate in amino transpeptidations and why these reactions tend to increase in importance at high pH.