Zinc-Bound Thiolate−Disulfide Exchange: A Strategy for Inhibiting Metallo-β-lactamases

The mononuclear zinc thiolate complexes [(TpPhMe)Zn(S-R)], where TpPhMe is hydrotris((3-methyl-5-phenyl)pyrazolyl)borate and (S-R) is benzyl thiolate, 4-nitrophenylthiolate, 4-trifluoromethylphenylthiolate, 4-chlorophenylthiolate,
phenylthiolate, 2-methylphenylthiolate, 4-methylphenylthiolate, 4-methoxyphenylthiolate, or 4-hydroxyphenylthiolate,
were synthesized. Representative members of the class were also characterized structurally. The benzyl thiolate
complex undergoes a thiolate−disulfide exchange reaction with a variety of diphenyl and dipyridyl disulfides. Kinetic
studies revealed that the reaction shows saturation behavior in both complex and disulfide for most of the disulfides
studied. Combined with studies of the lability of the coordinated thiolate, a mechanism is proposed where the
reactive species is the zinc-coordinated thiolate. When the free benzyl thiol was allowed to react with the same
disulfides, the reaction was slower by a factor of 20−200 than that for the zinc−thiolate complex, depending on the
particular disulfide employed. Since most metallo-β-lactamases contain one or more cysteine residues, the one in
the active site being coordinated to zinc, the present study was extended to examine whether disulfides can be
used as inhibitors of these enzymes by selective oxidation of the metal-bound cysteine. Several disulfides allowed
to react with metallo-β-lactamase CcrA from Bacteroides fragilis were moderate to potent irreversible inhibitors of
the enzyme.