8-Hydroxypenillic Acid from 6-Aminopenicillanic Acid: A New Reaction Catalyzed by a Class C β-Lactamase
1996
The hydrolysis of 6-aminopenicillanic acid (APA) at neutral pH, catalyzed by the class C β-lactamase of Enterobacter cloacae P99, was observed from 1H NMR spectra to yield an unexpected product, 8-hydroxypenillic acid, in bicarbonate-containing buffers. This product probably arises from turnover of the carbamate of APA by the enzyme. 1H and 13C NMR spectra of APA in bicarbonate solutions clearly demonstrate the presence of the carbamate. Turnover of APA at saturating concentrations by the enzyme is accelerated by bicarbonate and by methanol. These results suggest that deacylation of the enzyme is rate determining and that the presence of the carbamate affects this step. A mechanism involving rate-determining intramolecular nucleophilic attack by the carbamate on the acyl enzyme is proposed to rationalize these observations and lead to formation of 8-hydroxypenillate. This reaction can be seen as an example of substrate-assisted enzymic catalysis. Quantitative analysis of the data indicated a dissociation ...
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