Ribulose-5-phosphate ketose-aldose isomerization

Now, finally, sedoheptulose 7-phosphate undergoes a transketolase-catalyzed (EC 2.2.1.2) process (as in Scheme 11.7) to remove two carbon atoms using the enzyme cofactor thiamine diphosphate to yield ribose 5-phosphate and a two-carbon fragment that has remained attached to the thiamine cofactor of transketo-lase (EC 2.2.1.1, sedoheptulose 7-phosphate) (Scheme 11.11). When the two-carbon fragment is added to glyceraldehyde 3-phosphate, the material of Scheme 11.8 again is applied and xylulose 5-phosphate results. The xylulose 5-phosphate isomerizes to ribulose 5-phosphate as in Scheme 11.9 (with intervention of ribulose phosphate 3-epimerase (EC 5.1.3.1). And, the ribose 5-phosphate, an aldose, isomerizes (an aldose-ketose isomerase, EC 5.3.1.6, ribose 5-phosphate isomerase) to ribulose 5-phosphate. 

Both transaldolase and transketolase require a ketose phosphate as the donor molecule and an aldose phosphate as the acceptor. Furthermore, both enzymes require the same steric configuration at carbons 3 and 4 as is found in glucose and fructose. Ribulose-5-phosphate, the first pentose phosphate to be formed in the pentose phosphate pathway, does not have the correct configuration to serve as a substrate for either transaldolase or transketolase. However, both a suitable donor ketose and an acceptor aldose can be made by isomerizations of ribulose-5-phosphate, and enzymes that... 

One isomerization from the ketose to aldose Turns ribulose-5-phosphate to the phosphate of ribose. 

To generate ribose 5-phosphate from the oxidative pathway, the ribulose 5-phos-phate formed from the action of the two oxidative steps is isomerized to prodnce ribose 5-phosphate (a ketose-to-aldose conversion, similar to fmctose 6-phos-phate being isomerized to glncose 6-phosphate see section III.B.l below). The ribose 5-phosphate can then enter the pathway for nucleotide synthesis, if needed, or can be converted to glycolytic intermediates, as described below for the nonox-idative phase of the pentose phosphate pathway. The pathway through which the ribose 5-phosphate travels is determined by the needs of the cell at the time of its synthesis. 

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