Mechanism of acyl transfer by the class A serine f - lactamase of

Optimization by energy minimization of stable complexes occurring along the pathway of hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G ,-lactamase has highlighted a proton shuttle that may explain the catalytic mechanism of the /J-lactamases of class A. Five residues, S70, S130, N 132, T235 and A237, are involved in ligand binding. The y-OH group of T235 and, in the case of benzylpenicillin, the y-OH group of S130 interact with the carboxylate group, on one side of the ligand molecule. The side-chain NH2 group of N132 and the carbonyl backbone of A237 interact with the exocyclic CONH amide bond, on the other side of the ligand. The backbone NH groups of S70 and A237 polarize the carbonyl group of the scissile /3-lactam amide bond. Four residues, S70, K73, S130 and E166, and two water molecules, WI and W2, perform hydrolysis of the bound ,-lactam compound. E166, via WI, abstracts the proton from the y-OH group of S70. While losing its proton, the O-y atom of S70 attacks the carbonyl carbon atom of the ,-lactam ring and, concomitantly, the proton is delivered back to the adjacent nitrogen atom via W2, K73 and SI 30, thus achieving formation of the acyl-enzyme. Subsequently, E166 abstracts a proton from WI. While losing its proton, W1 attacks the carbonyl carbon atom of the S70 ester-linked acyl-enzyme and, concomitantly, re-entry of a water molecule W'1 replacing WI allows E166 to deliver the proton back to the same carbonyl carbon atom, thus achieving hydrolysis of the ,-lactam compound and enzyme recovery. The model well explains the differences found in the kcat. values for hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G ,-lactamase. It also explains the effects caused by site-directed mutagenesis of the Bacillus cereus /,-lactamase I [Gibson, Christensen & Waley (1990) Biochem J. 272, 613-619].