- A dramatic change in the rate-limiting step of beta-lactam hydrolysis results from the substitution of the active-site serine residue by a cysteine in the class-C beta-lactamase of Enterobacter cloacae 908R. Dubus, A., Monnaie, D., Jacobs, C., Normark, S., Frère, J.M. Biochem. J. (1993)