Ribose phosphate isomerase

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...

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. 

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... 

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

Figure 6.13 BLAST search results. Part of the results from a BLAST search of the nonredundant (nr) protein sequence database with the use of the sequence of ribose-5-phosphate isomerase (also called phosphopentose isomerase, pp. 572 and S81) from coli as a query. Among the 268 sequences found is the orthologous sequence from human beings, and the alignment between these sequences is shown (highlighted in yellow). The number of sequences with this level of similarity expected to be in the database by chance is 2 X 10 as shown by the E value (highlighted in red). Because this value is much less than 1, the observed sequence alignment is highly significant.

Figure 6.6 Enediolate analogues and mechanisms of enediolate stabilisation in glucose-6-phosphate (bottom left) and ribose-5-phosphate isomerase (bottom right).

Ribose-5-phosphate isomerase, which catalyses the interconversion of ribose-5-phosphate and ribulose-5-phosphate, activates the carbonyl differently. The aldose substrate can form a furanose (but not a pyranose) ring, but the furanose forms are so disfavoured (see Table 1.1) that there are likely to be appreciable amounts of open-chain sugar present at equilibrium. Moreover, this equilibrium, involving only furanose forms whose opening is possibly assisted by general acid catalysis from the 5-phosphate, is likely to be achieved rapidly. Although the enzyme from other sources crystallise with ring forms of the substrate bound, that from Thermus thermophilus crystallises with both ribose-... 

The isomerization step (reactions 13 and 14 in Table 22.1) involves the conversion of both ribose-5-phosphate and xylulose-5-phosphate to ribulose-5-phosphate. Ribose-5-phosphate isomerase catalyzes the conversion of ribose-5-phosphate to ribulose-5-phosphate, and xylulose-5-phosphate epimerase catalyzes the conversion of xylulose-5-phosphate to iibulose-5-phosphate (Figure 22.15). The reverse of both these reactions takes place in the pentose phosphate pathway, catalyzed by the same enzymes. 

Fig. 5. A simplified metabolic scheme of ethanol formation from glucose and xylose. Enzyme abbreviations GPDH Glucose 6-phosphate 1-dehydrogenase, PGDH Phosphogluconate dehydrogenase, PGI Glucose 6-phosphate-isomerase, RKI Ribose 5-phosphate isomerase, RPE Ribulose phosphate 3-epimerase, TAL Transaldolase, TKL Transketolase, XDH Xylitol dehydrogenase, XK-. Xylulokinase, XR Xylose reductase...

---