ABSTRACT
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 Ldt
Cd1
, Ldt
Cd2
, and Ldt
Cd3
, 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. Ldt
Cd2
and Ldt
Cd3
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. Ldt
Cd1
displayed only
l
,
d
-carboxypeptidase activity. Mass spectrometry analyses indicated that Ldt
Cd1
and Ldt
Cd2
were acylated by β-lactams belonging to the carbapenem (imipenem, meropenem, and ertapenem), cephalosporin (ceftriaxone), and penicillin (ampicillin) classes. Acylation of Ldt
Cd3
by these β-lactams was not detected. The acylation efficacy of Ldt
Cd1
and Ldt
Cd2
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 Ldt
Cd1
and Ldt
Cd2
was higher for ampicillin and ceftriaxone than for imipenem. These observations indicate that Ldt
Cd1
and Ldt
Cd2
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.