Carbapenem-hydrolyzing β-lactamase from
Serratia marcescens FHSM4055 was purified 926-fold by means of carboxylmethyl Sephadex C-50, Sephacryl S-200, and Mono S column chromatography. The molecular weight was 30, 000 by SDS-PAGE and the isoelectric point was 8.7. The enzyme activity was inhibited by EDTA, and restored by adding zinc (II) or manganese (II). It was inhibited by
p-chloromercuribenzoate and iodine as well as the heavy metals, Hg (II), Fe (II), Fe (III), and Cu (II). These results indicate that the enzyme is a metallo-β-lactamase and that the SH-group of only one cysteine residue probably binds to the metal ion, thus contributing to the stability of the enzyme active center. The specific constant (
kcat/
Km) showed that the enzyme hydrolyzed various β-lactam antibiotics such as carbapenems, cephalosporins, moxalactam, cephamycins, and penicillins other than monobactams. Ampicillin and piperacillin with respective amino- and imino-groups, ceftazidime with a carboxypropyloxyimino-group, and cefclidin with a carbamoylquinuclidine-group were poor substrates among the β-lactam antibiotics other than the monobactams tested. The plots of the turnover number (
kcat) against pH for the hydrolysis of cephaloridine gave an asymmetrical curve with the ‘tail’ on the acid side (p
K1, 5.9; p
K2, 9.0; p
K3, 10.8), whereas those of
kcat/
Km gave a bell-shaped curve (p
K1 5.8; p
K2, 9.8). Both results suggest that two ionic forms of an intermediate yield the same product at different rates and that the enzyme is stable under alkaline conditions. Since the N-terminal amino acid sequence of 27 residues determined was consistent with that of the metalloenzyme (Antimicrob. Agents Chemother., 1994, 38: 71-78), the above enzymatic characteristics seem to coincide.
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