Hydrolysis of 3-substituted cephalosporins catalysed by β-lactamases I and II from Bacillus cereus and by hydroxide ion

Second-order rate constants for the alkaline hydrolysis of 3-thiol substituted cephalosporins are independent of the pK_a of the thiol over a pK_a range of 9. If there is a leaving group at C-3′ it is expelled after the β-lactam ring is opened and the expulsion of the leaving group does not enhance the rate of β-lactam C-N bond fission. The zinc enzyme β-lactamase II is about a 100-fold better catalyst than the serine enzyme β-lactamase I for the hydrolysis of the same cephalosporin. The second-order rate constant k_cat/K_m for both β-lactamase enzymes shows no dependence on the nature of the substituent at C-3′ which is not explicable by the different chemical reactivity of the cephalosporins. There is no evidence for a significant recognition site in either enzyme for the C-3′ substituent. The kinetic parameters k _cat and K_m for the β-lactamase I-catalysed hydrolysis may be complicated by the formation of intermediates.