[en] The crystallographic structure of the penicillin-sensitive D-alanyl carboxypeptidase-transpeptidase from Streptomyces R61 has been solved to 2.8-A resolution. The 38,000-dalton serine peptidase has two regions of secondary structure, an alpha/beta cluster, and a region which contains five helical segments. The beta sheet is composed of five beta strands. The tertiary structure has no homology with the classic serine proteases or with the zinc carboxypeptidases. The binding at a common site of three types of beta-lactam (a penicillin, a cephalosporin, a monocyclic beta-lactam) and a desazacyclobutanone has been observed in Fourier difference maps. The binding site sequence is Val-Gly-Ser-Val-Thr-Lys. The beta-lactam ring lies near the enzyme's catalytic serine at position 37, and the C3 substituent of a cephalosporin falls near lysine 40.
