The roles of the carboxy group in ?-lactam antibiotics and lysine 234 in ?-lactamase I

The replacement of the C3 carboxylate in phenoxymethylpenicillin by a hydroxymethyl group and of the C4 carboxylate in cephalosporins by both a lactone and an aldehyde gives derivatives which are still good substrates for Bacillus cereus 569/H -lactamase I. The enzyme rate-enhancement factors for the hydrolysis of the modified -lactams vary from 104 to 106. All three modified substrates show bell-shaped (kcat/Km)–pH profiles indicative of two catalytically important ionising residues on the protein of pKa, about 5 and 9. Although lysine 234 is a highly conserved residue in class A -lactamases and has been traditionally thought to interact with the carboxylate of the -lactam antibiotic, it is not responsible for the decrease in enzyme activity at high pH corresponding to the pKaof about 9.