Ribose-5-phosphate isomerase mechanism

To summarize these observations First, the enzymes of this segment of the photosynthetic carbon reduction cycle are not controlled co-ordinately a single inductive step does not appear to affect the activity (and presumably the production) of these three enzymes in the same way. Second, the kinds of control mechanisms which appear to occur here are (1) a direct and prompt effect of illumination, reflected in the rapid increase in ribulose-diphosphate carboxylase activity, and (2) a more indirect kind of control, possibly involving induction of the other enzymes (e.g., ribose-phosphate isomerase) by small molecules produced in photosynthesis. Unfortunately, it has not been possible to demonstrate an increase in the level of either the isomerase or kinase by administration of glucose and some other carbohydrates to etiolated leaves in darkness. 

Figure 6.6 Enediolate analogues and mechanisms of enediolate stabilisation in glucose-6-phosphate (bottom left) and ribose-5-phosphate isomerase (bottom right).

Fig. 36. The Calvin cycle (black lines) and pentose phosphate cycle (red lines). PGA = 3-Phosphoglyceric acid, PGAL = 3-phosphoglyceraldehyde, Rib. = ribose-5-phosphate, Xyl = xylulose-5-phosphate, Ru-diP = ribulose-1,5-diphosphate, C4 = erythrose-4-phosphate, FDP = fructose-1,6-diphosphate. A few of the enzymes participating are encoded, 1 = carboxydismutase, 2 = triose phosphate dehydrogenase, 3 = triose phosphate isomerase, 4 = aldolase, 5 = phosphatase, 6 = phosphoglucoisomerase. Details of the conversion of glucose-6-P into ribulose-5-P are given in Fig. 43. It should be pointed out that the pentose phosphate cycle presents only here and there a true reversal of the Calvin cycle. In many instances the mechanisms and enzymes are different.

A recurring intermediate. Phosphopentose isomerase interconverts the aldose ribose 5-phosphate and the ketose ribulose 5-phosphate. Propose a mechanism. 

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