Peptidoglycan Cross-Linking Activity of l,d-Transpeptidases from Clostridium difficile and Inactivation of These Enzymes by β-Lactams.

In most bacteria, the essential targets of β-lactam antibiotics are the d,d-transpeptidases that catalyze the last step of peptidoglycan polymerization by forming 4→3 cross-links. The peptidoglycan of Clostridium difficile is unusual since it mainly contains 3→3 cross-links generated by l,d-transpeptidases. To gain insight into the characteristics of C. difficile peptidoglycan cross-linking enzymes, we purified the three putative C. difficile l,d-transpeptidase paralogues LdtCd1, LdtCd2, and LdtCd3, which were previously identified by sequence analysis. The catalytic activities of the three proteins were assayed with a disaccharide-tetrapeptide purified from the C. difficile cell wall. LdtCd2 and LdtCd3 catalyzed the formation of 3→3 cross-links (l,d-transpeptidase activity), the hydrolysis of the C-terminal d-Ala residue of the disaccharide-tetrapeptide substrate (l,d-carboxypeptidase activity), and the exchange of the C-terminal d-Ala for d-Met. LdtCd1 displayed only l,d-carboxypeptidase activity. Mass spectrometry analyses indicated that LdtCd1 and LdtCd2 were acylated by β-lactams belonging to the carbapenem (imipenem, meropenem, and ertapenem), cephalosporin (ceftriaxone), and penicillin (ampicillin) classes. Acylation of LdtCd3 by these β-lactams was not detected. The acylation efficacy of LdtCd1 and LdtCd2 was higher for the carbapenems (480 to 6,600 M-1 s-1) than for ampicillin and ceftriaxone (3.9 to 82 M-1 s-1). In contrast, the efficacy of the hydrolysis of β-lactams by LdtCd1 and LdtCd2 was higher for ampicillin and ceftriaxone than for imipenem. These observations indicate that LdtCd1 and LdtCd2 are inactivated only by β-lactams of the carbapenem class due to a combination of rapid acylation and the stability of the resulting covalent adducts.