2 . 8 - A Structure of Penicillin - sensitive D - Alanyl Carboxypeptidase - transpeptidase from Streptomyces R 61 and Complexes

The crystallographic structure of the penicillin-sensitive D-alanyl carboxypeptidase-transpe tidase from Streptomyces R61 has been solved to 2.81 resolution. The 38,000-dalton serine peptidase has two regions of secondary structure, an (Y/B cluster, and a region which contains five helical segments. The j3 sheet is composed of five /3 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 B-lactam (a penicillin, a cephalosporin, a monocyclic B-lactam) and a desazacyclobutanone has been observed in Fourier difference maps. The binding site sequence is Val-Gly-Ser-Val-Thr;Lys. The &lactam ring lies near the enzyme’s catalytic serine at position 37, and the C3 substituent of a cephalosporin falls near lysine 40.