Spontaneous and Enzymatically Catalyzed Anomerization of Glucose 6-phosphate and Anomeric Specificity of Related Enzymes.

Enzymes acting on the first carbon of n-glucose are specific for a particular anomer. Two well known cases are the glucose oxidase from Penicillium not&urn and glucose dehydrogenase from liver, both of them specific for @-glucose1 (I, 2). It is likely that enzymes acting at the same level of n-glucose 6-phosphate will also have an anomeric specificity, but there was no definite information on this point, with the excepticn of phosphoglucomutases, which act on either a-n-glucose l-phosphate (3) or its /3 anomer (4). The rate of spontaneous anomerization (mutarotation) of glucose and many other free sugars is rather slow at physiological pH (5) and there is an enzyme, mutarotase, that catalyzes the interconversion oc-n-glucose = P-n-glucose (6-8). However, there was no information on the rate of spontaneous anomerization of glucose B-phosphate or on the possibility of its enzymatic catalysis. Hexokinase was known to act on both aand P-glucose giving rise, presumably, to o(and P-glucose 6-phosphate, respectively (9). At the level of glucose g-phosphate there is a metabolic cross-road in which at least three enzymes are involved, glucose phosphate isomerase, glucose 6-phosphate dehydrogenase, and phosphoglucomutase, whose anomeric specificities for glucose B-phosphate were unknown. The glucose phosphate isomerase reaction seems to involve the formation of an enediol as an intermediate, which presumably would require the prior opening of cyclic hexose 6-phosphate, spontaneously or catalyzed by glucose phosphate isomerase (10). On the basis of some observations of competitive inhibition of the enzyme by open and cyclic structurally related compounds, it has also been suggested that glucose phosphate isomerase would probably act on the open forms of its substrates (11, 12). These suggestions raised the problem of the conversion mechanism of glucose 6-phosphate, which is a product of hexokinase and phosphoglucomutase reactions, into the acyclic form. Taking into account the high glycolytic activity of yeast and its low intracellular pH (13), it appeared important to determine if the rate of glucose 6-phosphate anomerization, spontaneously or enzymatically catalyzed, was high enough not to be a limiting step in glycolysis. On the other hand, if glucose phosphate isomerase acted on cyclic