Substrate Requirements of the Streptomyces albus G DD Carboxypeptidase

Streptomyces albus G secretes a carboxypeptidase which hydrolyzes C-terminal D-alanylglycine, C-terminal N°-(D-alanyl)-D, and with a much lower efficiency, C-terminal glycyl-D-alanine linkages. The side chain of the C-terminal d residue may be a long peptide sequence. The K_m values for the binding of peptides ending in L → D-alanyl → d sequences, to the carboxypeptidase are essentially controlled by the structure of the L-amino acid that precedes the C-terminal D-alanyl-D linkages. The substrate requirements of the soluble DD carboxypeptidase and of the membrane-bound transpeptidase involved in wall synthesis present striking similarities. It appears that the peptides which act as carboxyl donors in the transpeptidation reaction are those which are recognized by the DD carboxypeptidase. It is proposed that the soluble DD carboxypeptidase (or a very closely related enzyme) acts as a transpeptidase when it is integrated in the cell membrane.