Ribose 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 ... 

B22. Bruns, F. H., Noltmann, E., and Vahlhaus, E., Uber den Stoffwechsel von Ribose-5-phosphat in Hamolysaten. I. Aktivitatsmessung und Eigenschaften der Phosphoribose-isomerase. II. Der Pentosephosphat n Cyclus in roten Blutzellen. Biochem. Z. 330, 483-496 (1958). 

A true xylose isomerase, that did not require arsenate for its activity, was found in strains of Lactobacillus, especially L. brevis (25). In this study, it was found that the activity of isomerization of glucose and xylose were essentialfy equal, and that ribose was isomerized to ribulose at a reduced rate 24). The xylose isomerases from this strain, like those from other species, requires divalent cations for activity. In this case, Mn and Co were found to be required for activity. In other studies, Co has been found to increase xylose isomerase stability 25,26,21. ... 

Since these early discoveries, xylose isomerases have been isolated from many bacterial species, and these enzymes have been intense investigated, especially those of the genera Streptomyces, Lactobacillus, and Bacillus. The characteristics of substrate specificity (xylose glucose > ribose), divalent metal cation activation (Mg, Mn or Co ), and activity at alkaline pH are properties that most of the enzymes share to a certain extent, but significant variations exist. Some of these em es have been immobilized and patented for commercial use. There are many good reviews in the literature that describe the enzymatic characteristics of the xylose isomerases 9,28,29). 

This enzyme [EC 5.3.1.6] (also known as phosphopentos-isomerase, phosphoriboisomerase, and ribose 5-phos-... 

Ribose-5-phosphate isomerase (2BES) [72] Glu75 Gly70 Ser71 Ser71 OG 2 waters... 

Table 4.7.3 Concentrations of polyols in urines of patients with metabolic disorders, as determined by method 1. Concentrations are in mmol/mol creatinine (age-controlled reference range). Bold values are above the reference range. Dist. Disturbed peak, not quantifiable due to interference from a coeluting fragment of C6-polyols, GALT galactose-1-phosphate uridyltransferase, n.d. not detectable, RPI ribose-5-phosphate isomerase, TALDO transaldolase...

As depicted in Figure 6.8 the stability screening was based on DERA activity assay, the retro-aldol reaction of 2-deoxy-D-ribose 5-phosphate to acetaldehyde and D-glyceraldehyde 3-phosphate. D-glyceraldehyde 3-phosphate is further converted by the auxiliary enzymes triose phosphate isomerase and glycerol phosphate dehydrogenase. As the latter reaction consumes NADH it can be measured spectro-pho to metrically by the decrease in absorbance at 340 nm. 

Ribose 5-phosphate isomerase 693 D-Ribose (Rib) 162,163s, 200s Ribosome(s) 3,5, 11,233-235 structure of 233,234 Ribosylthymidine (Thd) 203 Ribosylthymidine 5 -phosphate (Thd-5 P) 203 Ribothymidine 234 of tRNA 231... 

For preparative applications, the expensive and configurationally unstable donor 128 can be simply prepared in situ by the action of ribose 5-phosphate isomerase (EC 5.3.1.6) on D-ribose 5-phosphate (39). This technique was applied to the stereoselective synthesis of d-[1-13C] fructose 6-phosphate 38 from [13C] formaldehyde [376,377] which also included a second enzymatic isomerization of the D-arafrino-3-hexulose 6-phosphate intermediate 129 into the more stable 2-hexulose derivative 38. Notable are the conflicting demands for high substrate levels (necessary to shift the fully reversible multi-component equilibrium) versus the notorious enzyme inactivation that occurs at higher formaldehyde concentrations. 

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). 

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. 

D-Glucose-6-phosphate dehydrogenase, D-gluconate-6-phosphate dehydrogenase, KDO-8-phosphate synthase, KDO-8-phosphate phosphatase and CMP-KDO synthetase have been purified to homogeneity and characterized. Munson, Rasmussen and Osborn (15) have partially purified one KDO transferase. D-Arabinose-5-phosphate isomerase is very unstable and has only been purified about 100 fold. D-Ribose-5-phosphate isomerase activity is approximately 20x that... 

Measurements of the crude specific activity (mmoles of product synthesized per minute per mg of protein in the supernatant after a 50,000 x g centrifugation) of the two isomerases in E. coli indicated that the conversion of D-ribulose-5-phosphate to D-ri-bose-5-phosphate was approximately 20- to 30-fold greater than the conversion of D-ribulose-5-phosphate to D-arabinose-5-phosphate. This rate of reaction strongly pulls the reaction substrate to D-ribose-5-phosphate, since the isomerase reaction at equilibrium strongly favors the formation of the aldo-sugar over the key intermediate D-ribulose-5-phosphate. 

The ribulose 5-phosphate is now converted to ribose 5-phosphate by isomerization, a reaction catalyzed by phosphopentose isomerase ... 

D-Ribulose 5-phosphate then undergoes an isomerization by ribose-5-phosphate isomerase to o-ribose 5-phosphate (Step 4) ... 

Decarboxylation must lead to incorporation of a proton from the solvent. The oxidative decarboxylation of 6-O-phosphono-D-gluconate gives D-erythro-pentulose-1 (S)-t 5-phosphate (with inversion of configuration).45 The R-isomer can be obtained46 from D-ribose 5-phosphate by using D-ribose 5-phosphate ketol isomerase (E.C. 5.3.1.6). 

Incorporation of tritium from D-ribose-i-t and D-mannose-J-t into rat-liver glycogen showed that the D-glucose 6-phosphate and D-mannose 6-phosphate aldo-keto isomerases have the same stereochemical characteristics in vivo as in vitro.215... 

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