Ribulose-phosphate 3-epimerase

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

RIBULOSE-5-PHOSPHATE 3-EPIMERASE Erythrose-4-phosphate epimerase,... 

RIBULOSE-5-PHOSPHATE 4-EPIMERASE RIBULOSE-5-PHOSPHATE 3-EPIMERASE... 

Ribulose 5-phosphate 3-epimerase UDP-glucose 4-epimerase D-Glyceraldehyde 3-phosphate ketol isomerase... 

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. 

A third ketopentulose ester, xlulose 5-phosphate (Xlu 5-P) Fig- 2, was isolated as a product of Rib 5-P metabolism by Ashwell and Hickman (34). It was also shown by Srere et al. (35) that Xlu 5-P, rather than the earlier assigned Ru 5-P, was a definitive substrate of TK. Ribulose 5-phosphate-3 -epimerase (Fig. 2) catalyzed the formation of Xlu 5-P and imparted the transconflguration to the hydroxyls at carbons 3 and 4, which is a necessary stereochemical condition for substrate reactivity with TK. The 3 -epimerase was purified from a bacterial source... 

There are a number of cofactor independent carbohydrate epimerases that act on activated substrates, such as keto-sugars and keto-sugar nucleotides, although there is a paucity of details about their mechanisms. D-ribulose-5-phosphate 3-epimerase catalyzes the stereoinversion of substrate about the C-3 carbon to form D-xylulose 5-phosphate (as in Fig. 7.15) [102, 103]. Solvent hydron is completely incorporated into the product at the C-3 carbon, during epimerization in the d-xylulose 5-phosphate to o-ribulose 5-phosphate direction [102], This was taken as evidence for a two-base mechanism. 

KOPRIVA, S., KOPRIVOVA, A., SUSS, K.H., Identification, cloning, and properties of cytosolic D-ribulose-5-phosphate 3-epimerase from higher plants, J. Biol Chem., 2000,275, 1294-1299. 

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 ribulose-phosphate epimerase, EC 5.1.3.1 phosphoribose isoinerase, EC 5.3.1.6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.1.2.

Ribulose phosphate epimerase, EC 5.1.3.4. Its official name is L-ribulose-5-phosphate-4-epimerase. This is a key enzyme in the pentose phosphate pathway. An epimer is a stereoisomer variant of a sugar differing in the configuration at only one carbon atom (see Chap. 11). 

Scheme 11.9. A cartoon representation of the isomerization of xylulose 5-phospate to ribulose 5-phosphate through the common enol and catalyzed by ribulose-phosphate epimerase (EC 5.1.3.1).

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 ... 

This enzyme [EC 5.1.3.1] (also known as phosphoribu-lose epimerase, erythrose-4-phosphate epimerase, and pentose-5-phosphate 3-epimerase) catalyzes the interconversion of D-ribulose 5-phosphate and D-xylulose 5-phosphate. The enzyme can also act on D-erythrose 4-phosphate. 

PFACRI N -[(5 -phosphoribosyl)formimino]-5-aminoimidazole-4-carboxamide ribonucleotide isomerase ImGPS imidazole 3-glycerol phosphate synthase OMPDC orotidine 5 -phosphate decarboxylase R5PE ribulose 5-phosphate epimerase HUMPS hex-3-ulose monophosphate... 

The 5-carbon sugar phosphates are interconverted by the action of epimerase and isomerase to yield ribulose-5-phosphate, which is phosphorylated by the enzyme ribulose phosphate kinase to make RuBP, the acceptor of C02. 

L-arabinose isomerase, which interconverts L-arabinose and L-ribulose araB, L-ribulokinase, which uses ATP to phosphorylate L-ribulose at C-5 araD, L-ribulose 5-phosphate epimerase, which interconverts L-ribulose 5-phosphate and L-xylulose 5-phosphate talB, transaldolase and tktA, transketolase. 

A. glucose-6-phosphate dehydrogenase, NADH, gluconolactone hydrolase, 6-phosphogluconate dehydrogenase, ribulose-5-phosphate epimerase... 

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...

A Ribulose phosphate 3-epimerase (xylulose phosphate 3-epimerase) (EC 5.1.3.1). 

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. 

Transketolase requires an acceptor it cannot split a ketol to form aldehydes. Presumably an intermediate active glycolaldehyde" is formed by combination of the two-carbon unit with the enzyme. The enzyme will react with a number of aldehydes, and the reactions in many cases have been shown to be reversible. The donor requirements are not completely understood. Among the sugars only those with the hydroxyl on carbon 3 on the l side are substrates. Before the discovery of epimerase, ribulose-5-phosphate was thought to be a substrate, but it has been shown that a mixture of ribose and ribulose phosphates does not undergo the transketolase reaction until epimerase is added. Besides xylulose phosphate, fructose-6-phosphate, sedoheptulose-7-phosphate, and hydroxypyruvate are glycolaldehyde donors. 

The first of the interconversions features xylulose 5-phosphate and ribose 5-phosphate. Because transketolase has the specific requirement that the hydroxyl group at C-3 must be in the xylulose configuration, xylulose 5-phosphate is produced from ribulose 5-phosphate by epimerization involving the enzyme, ribulose-phosphate 3-epimerase. Epimers are sugars which differ only in the configuration of the hydroxyl group on one specific chiral carbon atom (Section 3.2), in this case C-3, hence the name of the enzyme. Carbon atoms 1 and 2 of xylulose 5-phosphate are transferred to ribose 5-phosphate to synthesize sedoheptulose 7-phosphate which, under the influence of transaldo-... 

---