Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates: a model for the interconversion of D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate

Based on experimental data, a model is proposed for the interconversion of either unlabelled hexose phosphates or D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate in the reaction catalyzed by phosphoglucoisomerase. This model takes into account the known differences in maximal velocity and affinity for each substrate, the intramolecular transfer of tritium between C1 and C2, and the isotopic discrimination between unlabelled and tritiated esters. This model reveals that, in a close system characterized by the progressive detritiation of hexose phosphates, the concentration ratio of D-glucose 6-phosphate to D-fructose 6-phosphate is much higher with the tritiated than unlabelled esters, a paradoxical increase in the specific radioactivity of D-glucose 6-phosphate above its initial value being even observed during the initial period of exposure of D-[2-3H]glucose 6-phosphate to phosphoglucoisomerase. The extension of this model to an open system may be essential for the correct interpretation of radioactive data collected in intact cells exposed to D-[2-3H]glucose.     

Dehydrogenation of the phosphonate analogue of glucose 6-phosphate by glucose 6-phosphate dehydrogenase.

6,7 -Dideoxy-alpha-D-gluco-heptose 7-phosphonic acid, the isosteric phosphonate analogue of glucose 6-phosphate, was synthesized in six steps from the readily available precursor benzyl 4,6-O-benzylidene-alpha-D-glucopyranoside. The analogue is a substrate for yeast glucose 6-phosphate dehydrogenase, showing Michaelis-Menten kinetics at pH7.5 and 8.0. At both pH values the Km values of the analogue are 4-5 fold higher and the values approx. 50% lower than those of the natural substrate. The product of enzymic dehydrogenation of the phosphonate analogue at pH8.5 is itself a substrate for gluconate 6-phosphate dehydrogenase.     

Anomerization of glucose 6-phosphate. pH dependence and solvent isotope effects.

The anomerization of α-d-glucose 6-phosphate has been examined using a spectrophotometric coupled enzyme assay. The pH-rate profile for spontaneous d-glucose 6-phosphate anomerization reveals that the d-glucose 6-phosphate dianion is the species giving rise to the much higher rate of d-glucose 6-phosphate anomerization over that of d-glucose. A deuterium solvent isotope effect of is consistent with the postulated intramolecular general-base catalysis by the phosphate.     

Effect of NADP$ and glucose 6-phosphate on the sedimentation behavior of glucose 6-phosphate dehydrogenase from sweet potato

Sedimentation behavior of sweet potato glucose 6-phosphate dehydrogenasewas studied using the sucrose density gradient centrifugation.The relative s value to s_20, value of alcohol dehydrogenasewas determined to be about 6 in the absence of both NADP^$ andglucose 6-phosphate. In the presence of NADP^$, the enzyme wassedimented with a relative s value of about 9. The additionof glucose 6-phosphate did not affect the sedimentation behavior.When glucose 6-phosphate was added to the gradient medium containingNDAP^$, the enzyme was sedimented with a relative s value ofabout 6 or 7, depending on the concentration of glucose 6-phosphate. ¹ Present address: Institute of Applied Microbiology, Universityof Tokyo, Bunkyo-ku. Tokyo, Japan. (Received February 13, 1971; )