Enzyme glucose 6-phosphate isomerase

Fig. 6.9. Electrophoretic patterns obtained with soluble extracts of protoscoleces removed from hydatid cysts of human(H), camel(C), sheep(S), cattle(B) and goat(G) origin from Kenya. Upper panel stained for the enzyme glucose phosphate isomerase lower panel stained for the enzyme phosphoglucomutase. (Reprinted with permission from International Journal for Parasitology, 12, Macpherson, C. N. L. McManus, D. P., A comparative study of Echinococcus granulosus from human and animal hosts in Kenya using isoelectric focusing and isoenzyme analysis, 1982, Pergamon Journals Ltd.)...

Glucose phosphate isomerase (GPI) catalyzes the reversible interconversion of glucose-6-phosphate and fructose-6-phosphate. GPI plays an essential role in carbohydrate metabolism in all cells of the body. The substrates of this enzyme, ffuc-... 

Glyceraldehyde 3-phosphate continues on in the glycolysis pathway, but dihydroxyacetone phosphate is first isomerized by the enzyme triose phosphate isomerase. As in the glucose-to-fructose conversion of step 2, the... 

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)] ... 

Marchand, M., Kooystra, U. and Wierenga, R. K. et al. (1989) Glucose-phosphate isomerase from Trypanosoma brucei. Cloning and characterization of the gene and analysis of the enzyme. Eur. J. Biochem. 184 455-464. 

Isomerases catalyze the isomerization of one compound into another. There are many important isomerization reactions in the metabolism of carbohydrates. D-Glucose-6-phosphate is converted into D-fructose-6-phosphate by phosphoglu-coisomerase. Dihydroxyacetone phosphate is converted into 3-phospho-D-glyceraldehyde by the enzyme triose phosphate isomerase. In the Calvin cycle of photosynthesis, this same enzyme converts 3-phospho-D-glyceraldehyde into dihydroxyacetone phosphate. 

The oxidative pentose phosphate cycle is often presented as a means for complete oxidation of hexoses to C02. For this to happen the C3 unit indicated as the product in Fig. 17-8A must be converted (through the action of aldolase, a phosphatase, and hexose phosphate isomerase) back to one-half of a molecule of glucose-6-P which can enter the cycle at the beginning. On the other hand, alternative ways of degrading the C3 product glyceraldehyde-P are available. For example, using glycolytic enzymes, it can be oxidized to pyruvate and to C02 via the citric acid cycle. 

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