Effects of donor and acceptor peptides on concomitant hydrolysis and transfer reactions catalyzed by the exocellular DD-carboxypeptidase-transpeptidase from Streptomyces R39

The exocellular R39 DD-carboxypeptidase-trans-peptidase catalyzes bimolecular transfer reactions concomitantly with the hydrolysis of the peptide donor. Some of the transfer reactions studied have led to the synthesis of peptide dimers identical with or very similar to those found in the completed wall peptidoglycans of bacteria. Under conditions of nonsaturation of the donor site of the enzyme by the peptide donor, the increase of the rate of transfer caused by increasing concentrations of acceptor peptide is lower than can be accounted for by the decrease of the rate of hydrolysis, resulting in a progressive decrease of the total activity of the enzyme. Under conditions of saturation of the donor site of the enzyme by the peptide donor, the proportion of the enzyme activity which is diverted from the hydrolysis pathway because of the presence of low concentrations of peptide acceptor, is entirely channeled into the transfer pathway. The rate of the total reaction is equal to the maximal rate of hydrolysis of the peptide donor in the absence of peptide acceptor [i.e., the VHy(H2O) max value]. High concentrations of some peptide acceptors (nonamidated peptide) cause an increase of the rate of transfer higher than the decrease of the rate of hydrolysis. The rate of total reaction then exceeds the VHy(H2O) max value. The transfer reaction itself can be inhibited by an excess of some peptide acceptors (Glu-amidated peptide). The less the enzyme is saturated by the peptide donor, the lower is the concentration of peptide acceptor at which inhibition of the transfer reaction starts to occur.