Effect of side-chain amide thionation on turnover of beta-lactam substrates by beta-lactamases. Further evidence on the question of side-chain hydrogen-bonding in catalysis.
Two side-chain-thionated beta-lactams, a penicillin and a cephalosporin, have been prepared and found to be not significantly poorer as substrates of typical serine (classes A and C) beta-lactamases than are their oxo analogues. This result is interpreted to mean that any hydrogen-bonding site on these enzymes for the beta-lactam side-chain amide carbonyl group must be flexible and is more likely to be a passive rather than active or essential feature of the active site. Previously, data from crystal structures and site-directed mutagenesis had suggested that the side chain of Asn-132 of class-A beta-lactamases, a component of the conserved SDN loop, forms a hydrogen bond with the side-chain carbonyl of the beta-lactam substrate and may provide significant transition-state stabilization during catalysis. The thionocephalosporin was also equally as good as its oxo analogue as a substrate of the class-B beta-lactamase II of Bacillus cereus and not significantly less effective as an inhibitor of the Streptomyces R61 DD-peptidase; a tight hydrogen-bond donor site for the beta-lactam side-chain amide is apparently not present in these enzymes either.
Pratt
R F
RF
Department of Chemistry, Wesleyan University, Middletown, CT 06459.
Krishnaraj
R
R
Xu
H
H
D000577:Amides / Q000737:chemistry*; D002384:Catalysis; D002511:Cephalosporins / Q000378:metabolism*; D006860:Hydrogen Bonding; D006868:Hydrolysis; D007700:Kinetics; D010406:Penicillins / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity; D013455:Sulfur / Q000737:chemistry / Q000378:metabolism*; D001618:beta-Lactamases / Q000378:metabolism*
D000577:Amides; D002511:Cephalosporins; D010406:Penicillins; D013455:Sulfur; D001618:beta-Lactamases