Catalytic reactions of phosphoglucose isomerase with cyclic forms of glucose 6-phosphate and fructose 6-phosphate.

Yeast phosphoglucose isomerase is shown to use and produce both the u and the fi anomeric forms of fructofuranose 6-phosphate. Rapid quench techniques show a Z&fold preference in the utilization of the a! anomer over the /3 anomeric form. The previous report that the (Y anomer of glucopyranose-6-P is the preferred substrate form is confirmed. However the /3 anomer is also shown to react at a slower rate. Studies using active site labeled phosphoglucose isomerase suggest that both anomers of each substrate react at the same enzyme site. A reaction mechanism consistent with these observations involves opening of both (Y and fl ring forms to the corresponding acyclic forms followed by isomerization via an identical enzyme-bound cis-enediol. In order to accommodate both the o( and the /3 anomers, torsion around C-C bonds is envisioned for the /3 anomers to arrive at the same cis-enediol intermediates.