Hydrolysis of 7-substituted cephalosporins catalysed by ?-lactamases I and II from Bacillus cereus and by hydroxide ion

Kinetic parameters are reported for the Bacillus cereus-lactamase I- and -lactamase II-catalysed hydrolysis of a series of thirty-seven cephalosporins substituted in the 7-position. These are compared with the second-order rate constants for the hydroxide ion-catalysed hydrolysis of these derivatives. There is no significant dependence of the rate of the base-catalysed hydrolysis upon the nature of the side-chain substituent. For -lactamase I, kcat/Km varies over 2 × 105 but for -lactamase II the variation with substituents is only 10. For alkyl substituents, kcat/Km increases with chain length and passes through a maximum, for -lactamase I this is with the undecyl derivative and for -lactamase II the octylcephalosporin. For -lactamase I, but not for -lactamase II, the t-butylcephalosporin is a very poor substrate. There is no evidence for a significant cavity in either enzyme to host aromatic residues. An ionised carboxylate residue on the side-chain significantly reduces reactivity with -lactamase I but not -lactamase II. It is suggested that a carboxy group on -lactamase I acts as a general catalyst facilitating -lactam C–N bond fission.