IMPERATIVES FOR ENZYMATIC CATALYSIS OF ISOMERIZATION OF SUGARS AND SUGAR PHOSPHATES

The relative yields of the products of isomerization of D,L-glyceralde hyde in D2O with intramolecular transfer of a hydride ([1-H-1]DHA) and with proton transfer ([1-H-2]DHA) were determined by high-resolution H-1 NMR analyses. A study of the catalysis of this isomerization by de uteroxide ion, buffer anions and Zn2+ established the following: (1) i somerization with proton and hydride transfer occurs at approximately equal rates in dilute solutions of sodium deuteroxide; (2) Bronsted ba ses catalyze isomerization with proton transfer in a bimolecular react ion; (3) Zn2+ catalyzes isomerization with hydride transfer in a bimol ecular reaction; and (4) Zn2+ and acetate ion react in concert to cata lyze isomerization with proton transfer in a termolecular reaction. Th ese results show that the various pathways for isomerization with prot on and hydride transfer proceed via transition states of similar energ ies, so that there is no strong imperative for enzymatic catalysis by any particular reaction mechanism. The relevance of these results to t he mechanism of action of xylose isomerase is discussed briefly. (C) 1 998 John Wiley Br Sons, Ltd.