FIG 5.

The biphasic kinetics observed for the C69F variant can be explained by two different branched kinetic schemes. (A) As previously described by Faraci and Pratt (36), loss of the R2 of cephalosporins is a common mechanism during cephalosporin hydrolysis that can lead to a branched mechanism, depicted by the rate constant k₃. (B) Alternatively, earlier, Citri et al. (34) found that β-lactamases can undergo a potentially reversible conformation change to a less active form, E*, here depicted by the rate constant k₅, resulting in an alternative branched mechanism. (C) Chemical structures of hydrolysis of ceftazidime (S), acyl-enzyme complexes, loss of the R2 side chain (S^−R2), and potential final products (P and P^−R2).