Ribulose 5-phosphate, acid hydrolysis

The four reactions involved in this conversion are shown in figure 12.31. The first oxidation, catalyzed by glu-cose-6-phosphate dehydrogenase at C-1, converts the hemi-acetal derivative of the aldehyde group to the lactone of the corresponding acid, 6-phosphogluconic acid. After hydrolysis of the lactone, the second oxidation at C-3, converts the secondary alcohol to a ketone. The expected product, 3-keto-6-phosphogluconic acid, is decarboxylated yielding ribulose-5-phosphate. 

A regioselective dephosphorylation was used in the synthesis of 2 -carboxy-D-arabinitol 1-phosphate (Table 13-6, entry 2), a natural inhibitor of ribulose 1,5-bisphosphate carboxylase. Either acid or alkaline phosphatases can be used for the selective hydrolysis of the 1-phosphoryl group of 2 -carboxyl-D-arabinitol 1,5-bisphosphate. With acid phosphatase, the conversion was essentially quantitative yielding exclusively the 1-phosphate derivative (cleavage of the 5-phosphoryl group). On the other hand, hydrolysis with alkaline phosphatase gave a 4 1 mixture of the 1-and 5-phosphate derivatives. 

The p3nranose or furanose structure of the esters appears to influence the rate of hydrolysis. Thus glucose 6-phosphate which can form a pyranose ring is more stable than fructose 6-phosphate which can only form a furanose ring. The acid lability of some compounds such as glycolaldehyde phosphate , erythrose 4-phosphate and erythrulose phosphate may be partly due to the fact that they cannot form ring structures. Furthermore ribose 5-phosphate is more stable than ribulose 5-phosphate and in this case the difference is presumably due to the fact that the former can form a furanose ring while the latter can only exist in the linear form. 

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