Pentose phosphate isomerase

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

Alternatively, ribulose (D-eryt/iro-pentulose) 5-phosphate may be iso-merized to ribose 5-phosphate with pentose phosphate isomerase, but the same isomerase will convert D-ribose 5-phosphate into D- ryt/zro-pentulose 5-phosphate, the equilibrium being displaced by phosphorylation to the diphosphate (involving three enzyme systems). 

Dickens F, Williamson DH. Pentose phosphate isomerase and epimerase from animal tissues. Biochem. J. 1956 64 567-578. 

Pentose Phosphate Isomerase. Ribulose-5-phosphate may undergo several reactions. One is isomerization to the corresponding aldose, ribose 5-phosphate (III). The widely distributed phosphopentose... 

The presence in these preparations of a pentose phosphate isomerase results in the production of ribose-5-phosphate, the equilibrium favoring the aldose. 

In the studies described above, we had employed ribose 5-phosphate as the substrate, but the requirement for pentose phosphate isomerase in addition to transketolase suggested that the true substrate was ribulose 5-phosphate. However, this hypothesis was incompatible with the observations that sedoheptulose and fructose phosphates were substrates, since ribulose 5-phosphate possessed the opposite configuration at the critical C-3 position. The answer to this puzzle was provided by the discovery by Gilbert Ashwell and Jean Hickman at the National Institutes of Health of a new pentose ester, D-xylulose 5-phosphate, which Paul Stumpf in my laboratory also obtained as an intermediate in pentose fermentation in Lactobacillus plantarumP Jerard Hurwitz, then a post-doctoral... 

The discovery of ribulose-5-phosphate has posed the problem of whether this compound or the aldopentose phosphate is the immediate substrate for cleavage to triose phosphate. An enzyme preparation has been isolated from yeast by de la Haba and Racker, which will convert ribulose-5-phosphate to triose phosphate but which will produce this compound from ribose-5-phosphate only after a marked lag. The could be eliminated by another protein fraction from yeast presumably containing the pentose phosphate isomerase. However, it was observed that ribulose-5-phosphate alone is less readily cleaved to triose phosphate than is the reaction mixture of ribose-fi-phosphate and pentose isomerase. 

Most recently the enzyme of yeast catalyzing the cleavage of ribulose-5-phosphate has been crystallized and designated transketolase. " The pentose phosphate isomerase has been removed from the crystalline transketolase, which maintains its activity on ribulose-5-phosphate. The purified enzyme also catalyzes the formation of the ketopentose phosphate from glyceraldehyde-3-phosphate and a 2-carbon donor such as hy-droxypyruvate. Transketolase contains thiamine pyrophosphate as a coenzyme. 

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