Glucose phosphate isomerase, reaction

Faces of molecules can also be diastereotopic an example is provided by the enediol intermediate, 44, postulated to occur in the glucose-phosphate isomerase reaction ... 

The triose phosphate isomerase reaction completes the first phase of glycolysis, each glucose that passes through being converted to two molecules of glyceraldehyde-3-phosphate. Although the last two steps of the pathway are... 

The reaction is irreversibly shifted to the right. Fructose 6-phosphate is isomerized to glucose 6-phosphate by glucose-phosphate isomerase. 

Glucose phosphate isomerase 5.3.1.9 Glucose-6-phosphate Fructose-6-phosphate Reaction with resorcinol... 

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. 

Figure 7.6. The metabolic reactions involved in the conversion of glycerol to glucose, the required precursor in the formation of sophorose. Note Reaction 1 catalyzed by triose phosphate isomerase. Reaction 2 catalyzed by aldolase. Reaction 3 catalyzed by fructose 1,6-bisphosphatase. Reaction 4 catalyzed by phosphoglucose isomerase., Reaction 6 catalyzed by glucose 6-phosphatase.

This freely reversible reaction requires Mg " " and is specific for glucose-6-phosphate and fructose-6-phosphate. It is catalyzed by glucose-phosphate isomerase. 

Glucose-phosphate isomerase is one of the best studied enzymes catalyzing the interconversion of aldo- and ketohexose phosphates. An active site carboxyl group is a possible candidate for the base catalyzing the intramolecular proton transfer reaction. The affinity label 1,2-anhydro-D-mannitol 6-phosphate (8) inactivates the enzyme by forming an ester linkage between C-l of the affinity label and an active site carboxyl of a glutamic acid residue (98). 

The substrate specificity of glucose-phosphate isomerase illuminates additional stereochemical subtleties of the isomerase reaction (JOO). In the aldose to ketose direction, the enzyme potentially operates on an equilibrium mixture of substrate forms composed of two cyclic hemiacetals (the a- and /3-anomers, of glucose 6-phosphate) and trace quantities of the acyclic aldehyde form lEq. (17)] ... 

By measuring the relative intensities of the fructose-1,6-bisphosphate, dihydroxyacetone phosphate, and glyceraldehyde, it was possible to show that the aldolase reaction is in equilibrium in the cell. Similar measurements on dihydroxyacetone phosphate and glyceraldehyde-3-phosphate showed that the triose phosphate isomerase reaction is not in equilibrium. It was also found that the adenylate kinase reaction, 2ADP ATP + AMP, is in equilibrium in the intact cell without added oxygen or glucose. 

The guilty party is the triose phosphate isomerase (TIM) reaction that interconverts DHAP and G3P. To be converted to pyruvate, the DHAP first has to be converted to G3P. TIM just moves the carbonyl group between the two carbons that don t have phosphate attached. TIM doesn t touch the phosphate. So, if the DHAP is labeled at the carbon that has the phosphate attached, the G3P that comes from DHAP will be labeled at the carbon with the phosphate attached. The carbon with the phosphate attached in the G3P that was produced directly by the aldolase reaction came from C-6 of glucose, but the carbon with the phosphate attached in the G3P that was produced from DHAP came from C-l of glucose. After TIM does it stuff, the carbon of G3P that has the phosphate will be... 

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-... 

These three biochemical reactions are catalyzed by hexokinase (EC 2.7.1.1), glucose-6-phosphate isomerase (EC 5.3.1.9), and 6-phosphofructokinase (EC 2.7.1.11), respectively. The EC numbers are from Enzyme Nomenclature (Webb, 1992). The first step is to write the conservation matrix for this reaction system at specified pH because that will show how to calculate the further transformed Gibbs energies of formation at specified [ATP] and [ADP]. 

Step 2 is the isomerization of a-D-glucose 6-phosphate, by glucose-6-phosphate isomerase, to a-D-fructose 6-phosphate. This is a freely reversible reaction. 

Phosphorylation and isomerization. Glucose, produced by the digestion of dietary carbohydrate., is first phosphorylated at the hydroxyl group on C6 by reaction with ATP in a process catalyzed by the enzyme hexokinase. The glucose 6-phosphate that results is isomerized by glucose 6-phosphate isomerase to fructose 6-phosphate. As the open-chain structures in Figure 29.4 show, this isomerization reaction takes place by keto-enol tautomerism (Section 22.1), since both glucose and fructose share a common enol ... 

---