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Anomeric specificity of L-fucose dehydrogenase: a stereochemical imperative in aldopyranose dehydrogenases?

A set of hypotheses is proposed that explains the anomeric specificity of aldopyranose dehydrogenases in terms of an evolutionarily selected function. The first hypothesis, based on stereoelectronic theory, argues that, in the "allowed" transition state for oxidation at the anomeric carbon, the two oxygens attached to the anomeric carbon each bear a lone pair of electrons antiperiplanar to the departing "hydride". The second hypothesis is that the dehydrogenase is functionally constrained to bind the anomer that has this arrangement of lone pairs in its lowest energy chair conformer. The anomeric specificity of L-fucose dehydrogenase is experimentally examined. The enzyme oxidizes preferentially the beta-anomer, consistent with the prediction made by these hypotheses. Available experimental data for other enzymes (D-glucose-6-phosphate dehydrogenase, D-glucose dehydrogenase, D-galactose dehydrogenase, D-abequose dehydrogenase, and D-arabinose dehydrogenase) are found to be also consistent with the proposed hypotheses.