Glucose-6-phosphate phosphohexose isomerase reaction

FIGURE 9 Isomerization and elimination reactions (a) The conversion of glucose 6-phosphate to fructose 6-phosphate, a reaction of sugar metabolism catalyzed by phosphohexose isomerase. (b) This reaction proceeds through an enediol intermediate. The curved blue ar-... 

The reaction mechanism is similar to the reaction promoted by phosphohexose isomerase in step (2) of glycolysis (Fig. 14-4). After the triose phosphate isomerase reaction, C-1, C-2, and C-3 of the starting glucose are chemically indistinguishable from C-6, C-5, and C-4, respectively (Fig. 14-6), setting up the efficient metabolism of the entire six-carbon glucose molecule. 

In addition to serving as structural motifs, enols and enolates are involved in diverse biological processes. Several enol/enolate intermediates have been proposed to be involved in glycolysis (Section IV.A), wherein c/ -enediol 21 is proposed to be an intermediate in the catalytic mechanism of phosphohexose isomerase and an enol-containing enamine intermediate (22) has been proposed in the catalytic pathway of class I aldolase. In the case of glucose-fructose (aldose-ketose) isomerization, removal of the proton on Cl-OH produces the aldose while deprotonation of C2-OH yields the ketose, which is accompanied by protonation at the C2 and Cl positions, respectively. There are several cofactors that are involved in various biological reactions, such as NAD(H)/NADP(H) in redox reaction and coenzyme A in group transfer reactions. Pyridoxal phosphate (PLP, 23) is a widely distributed enzyme cofactor involved in the formation of a-keto acids, L/D-amino... 

Glucose-6-P is then isomerized by phosphohexose isomerase to fructose-6-P (making up 30% of the equilibrium mixture). Another kinase phosphorylates the 1-position and the resulting fructose diphosphate is cleaved in an equilibrium reaction to two trioses, namely dihydroxyacetone phosphate (C-1 to C-3) and glyceral-dehyde phosphate (C-4 to C-6). The equilibrium mixture is composed of 89% hexose and 11% triose (under the conditions of Meyerhof s measurements) condensation, therefore, is the preferred (= exergonic) reaction. The reaction is analogous to the aldol condensation described in organic chemistry (Chapt. 1-2, XV-5). Catalysis of the reverse reaction by the enzyme aldolase is explained by the fact that enzymes always catalyze up to the equilibrium. ... 

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