Hexoses form an equilibrium mixture of six isomers in aqueous solution: the α- and β-pyranoses, α- and β-furanoses, and two open-chain forms (the carbonyl and the hydrated carbonyl) (1). Enzymes which act on hexoses and their derivatives usually display certain specificities (diastereo-specificities) toward these forms. Since mutarotation is quite slow compared to the rate of enzymatic reactions (2, 3), this anomeric bias cannot be overlooked, although there exists a number of mutarotases that greatly accelerate mutarotations thereby aiding subsequent transformations (4–6). Differential recognition of α- and β-d-glucopyranose by enzymes was first reported more than sixty years ago (7–9) and reviews of this anomeric specificity have been in the literature for some time (4–6). It is therefore surprising to see errors in current textbooks as shown in Table 1.
aAlthough hexokinase acts on both anomers equally, the texts are uniform in specifying α-D-glucopyranose as the substrate. It is notable that a textbook published in 1971 described the anomeric specificity of hexokinase correctly (18). bThe structure of the sugar is not specified.
We show here a small sample of current texts and an even smaller sample of the data available on the anomeric specificity of enzymes. We hope by means of this note to encourage future textbook writers and instructors in Biochemistry courses to examine the existing literature and avoid errors related to diastereospecificities of enzymes.
Acknowledgments
We are grateful to Ronald Bentley of the University of Pittsburgh for his valuable comments.
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Citation
Behrman, E. J.; Gopalan, Venkat. J. Chem. Educ.2007, 84, 1608.
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