The pH-dependence of class B and class C β-lactamases

The classification by structure allots beta-lactamases to (at present) three classes, A, B and C. The pH-dependence of the kinetic parameters for class B and class C have been determined. They differ from each other and from class A beta-lactamases. The class B enzyme was beta-lactamase II from Bacillus cereus 569/H/9. The plots of kcat against pH for the hydrolysis of benzylpenicillin by Zn(II)-requiring beta-lactamase II and Co(II)-requiring beta-lactamase II were not symmetrical, but those of kcat/Km were. A similar feature was observed for the hydrolysis of both benzylpenicillin and cephalosporin C by a class C beta-lactamase from Pseudomonas aeruginosa. The results have been interpreted by a scheme in which two ionic forms of an intermediate can give product, but do so at differing rates.