The bacterial target of beta-lactam antibiotics consists of a set of multiple, membrane-bound receptors. Some of them have been characterized as DD-carboxypeptidases. The DD-carboxypeptidases catalyse the opening of amide bonds and transfer the carbonyl carbon to an exogenous nucleophile, and are specifically designed to operate on the D-Ala-D-Ala linkageof L-R-D-Ala-D-Ala terminated peptides (where R is most often a diamino acid residue). The R61, R39 and several Bacilli DD-carboxypeptidases are known to be serine-enzymes and the G DD-carboxypeptidase has been characterized as a metallo (Zn ions) enzyme. Both the R61 and the G enzymes have been crystallized. In turn, the S. faecalis 43,000-Mr DD-carboxypeptidase, which is inhibited by low dose levels of pCMB, might be a thiol-enzyme. The goal pursued is the understanding of the mechanistic properties and functioning of the active centers of the DD-carboxypeptidases at the molecular and atomic levels. The research program involves 1) further characterization of the S. faecalis enzyme (which can be obtained in a water-soluble form); 2) isolation of various Streptomyces membrane-bound enzymes in a truly water-soluble form; 3) sequencing of the G and R61 enzymes; 4) the 2.8 A structure analysis of the G enzyme. (A similar study is conducted by Dr. J.R. Knox at the University of Connecticut, on the R61 enzyme, which enzyme is prepared and purified in this laboratory and then sent to Storrs); 5) conformational studies and quantitative structure activity relationships (QSAR).