The reaction of cephalosporins with penicillin-binding protein 1b gamma from Escherichia coli.

The kinetics of the reaction of purified penicillin-binding protein 1b gamma from Escherichia coli with cephalosporins suggest that the enzyme exists in two kinetically distinct conformations that are in slow equilibrium. One of these forms can effect rapid hydrolysis of some beta-lactams and it is only through its deactivation by conversion to the slower reacting form that complete inhibition can be achieved. With some cephalosporins and with penicillins having simple aromatic side-chains the reaction was slower and did not exhibit the same kinetic behaviour. This could be attributed to the rate of reaction being similar to the rate of conformation change and thus sets an upper limit on the isomerization rate.