Staphylococcus aureus and of the DD-peptidase of Streptomyces R61 after their inactivation by cephalosporins

It has been shown previously [Faraci & Pratt (1985) Biochemistry 24, 903-9 10; (1986) Biochemistry 25, 2934-2941; (1986) Biochem. J. 238, 309-312] that certain ,6-lactam-processing enzymes form inert acylenzymes with cephems that possess good leaving groups at the C-3' position. These inert species arise by elimination of the leaving group from the initially formed and more rapidly hydrolysing acyl-enzyme, which has the 'normal' cephalosporoate structure. The present paper shows that a strong nucleophile, thiophenoxide, can catalyse the re-activation of three examples of these inert acyl-enzymes, generated on reaction of cephalothin and cefoxitin with the PClI,-lactamase of Staphylococcus aureus and of cephalothin with D-alanyl-D-alanine transpeptidase/carboxypeptidase of Streptomyces R61. In view of the reversibility of the elimination reaction, demonstrated in model systems [Pratt & Faraci (1986) J. Am. Chem. Soc. 108, 5328-5333], this catalysis is proposed to arise through nucleophilic addition to the exo-methylene carbon atom of the inert acyl-enzyme to regenerate a more rapidly hydrolysing normal cephalosporoate. Strong support for this scenario was obtained through comparison of the kinetics of the catalysed re-activation reaction with those of turnover of the relevant 3'-thiophenoxycephems, thiophenoxycephalothin and thiophenoxycefoxitin. The enzymes appear to stabilize the products of the elimination reaction with respect to the normal cephalosporoate, but more strongly to destabilize the transition states. The effects of other nucleophiles, including cysteine, glycine amide and imidazole, on the above enzymes and on other ,lactamases can be understood in terms of the model reaction kinetics and thermodynamics.