Pentose phosphate isomerase and

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

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 ... 

Ribulose-5-phosphate (3.13) can be converted to ribose-5-phosphate (3.14) and xylulose 5-phosphate (3.15), by the enzymes ribose-5-phosphate isomerase and ribulose 5-phosphate 3-epimerase, respectively. The two pentose-phosphate molecules, 3.14 and 3.15, are converted to a C3 and a C7 sugar-phosphate, glyceraldehyde 3-phosphate (3.4) and sedoheptulose-7-phosphate (3.16), respectively, via the action of atransketolase. 

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... 

Zhou, H., Cheng, J.-s., Wang, B.L, Fink, G.R. et al. (2012) Xylose isomerase overexpression along with engineering of the pentose phosphate pathway and evolutionary engineering enable rapid xylose utilization and ethanol production by Saccharomyces cerevisiae. Metab. Eng., 14, 611-622. 

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. 

Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenosis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexose-isomerase, which involves an aldose-ketose isomerization. 

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. 

This intractable problem may now be close to being solved. A Saccharomyces species that expressed the xylose isomerase gene from an anaerobic fungus was found to grow slowly on pentoses [29]. Improvement resulted from a combination of rational engineering - overexpression of the pentose phosphate-converting enzymes (see Fig. 8.5) - and classical strain improvement [30]. The authors conclude The kinetics of xylose fermentation are no longer a bottleneck in the industrial production of ethanol with yeast ... 

Figure 6.4. Role of transketolase in the pentose phosphate pathway. Glucose 6-phosphate dehydrogenase, EC 1.1.1.49 phosphogluconate dehydrogenase, EC 1.1.1.44 rihulose-phosphate epimerase, EC 5.1.3.1 phosphorihose isomerase, EC 5.3.1,6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.I.2.

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