Factors Determining Steric Course of Enzymic Glycosylation Reactions: Glycosyl Transfer Products Formed from 2,6-Anhydro-1-deoxy-d-gluco-hept-1-enitol by α-Glucosidases and an Inverting Exo-α-glucanase

Glycosyl transfer products were formed from 2,6-anhydro-1-deoxy-d-gluco-hept-1-enitol (heptenitol) by purified α-glucosidases from Candida tropicalis and rice and by an inverting exo-α-glucanase (glucodextranase) from Arthrobacter globiformis. The products were structurally defined through ¹H and ¹³C NMR (nuclear magnetic resonance) spectra of their crystalline per-O-acetates in comparison with those of authentic methyl 1-deoxy-α- and methyl 1-deoxy-β-d-gluco-heptuloside. 1-Deoxy-α-d-gluco-heptulosyl-(2→7)-heptenitol and 1-deoxy-α-d-gluco-heptulosyl-(2→)-D-gluco-heptulose were produced by both the Candida α-glucosidase and the glucodextranase; 1-deoxy-α-d-gluco-heptulosyl-(2→)-and 1-deoxy-α-d-gluco-heptulosyl-(2→7)-d-gluco-heptuloses by the rice α-glucosidase. These results, together with our earlier findings of sterospecific hydration of heptenitol catalyzed by the same enzymes [Hehre, E. J., Brewer, C. F., Uchiyama, T., Schlesselmann, P., & Lehmann, J. (1980) Biochemistry 19, 3557–3564], show the inadequacy of the long-accepted notion that carbohydrase-catalyzed reactions always lead to retention (or always lead to inversion) of substrate configuration. In particular, the finding that glucodextranase forms transfer products of α configuration and a hydration product of β configuration from the same substrate provides a clear example of the functioning of acceptors rather than donor substrates in selecting the steric course of reactions catalyzed by a glycosylase. The circumstances under which acceptor cosubstrates might be expected to show this significant effect are discussed. The opportunity presumably would exist whenever carbonium ion mediated reactions are catalyzed by glycosylases that provide oppositely oriented approaches of different acceptors to the catalytic center.