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.
Effect of side-chain amide thionation on turnover of β-lactam substrates by β-lactamases. Further evidence on the question of side-chain hydrogen-bonding in catalysis
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R F Pratt, R Krishnaraj, H Xu; Effect of side-chain amide thionation on turnover of β-lactam substrates by β-lactamases. Further evidence on the question of side-chain hydrogen-bonding in catalysis. Biochem J 15 September 1992; 286 (3): 857–862. doi: https://doi.org/10.1042/bj2860857
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