The LD‐Carboxypeptidase Activity in Gaffkya homari

The effects in vitro of D‐amino acids or glycine on the formation of wall‐bound peptidoglycan were studied with wall membrane enzyme preparations from Gaffkya homari . These amino acids inhibited the incorporation of nascent peptidoglycan into the preformed polymer (e.g. ID 50 values for D‐alanine, D‐leucine, and glycine = 5.6 mmol/1, 1.3 mmol/l, and 11 mmol/l, respectively). The inhibiton was accompanied by an incorporation of the inhibitor into position 4 of the peptide subunit Ala 1 ‐DGlu 2 (Lys 3 ‐DAla 4 ), where the indices refer to the position of an amino acid residue within the peptide subunit. It is suggested that the reaction is catalyzed by an LD‐carboxypeptidase. Therefore, this enzyme has also D‐amino acid exchange activity. At inhibitory concentration fewer tripeptide subunits were formed in the nascent peptidoglycan in favour of the formation of tetrapeptide subunits bearing the inhibitor at the C termini. The tripeptide subunits are assumed to be necessary in order that nascent peptidoglycan is utilized as substrate in the transpeptidation reaction. Thus an essential role of the LD‐carboxypeptidase is indicated.