Epimerases ribulose-3-epimerase

FIGURE 23.31 The phosphopentose epimerase reaction interconverts ribulose-5-P and xylulose-5-phosphate. The mechanism involves an enediol intermediate and occurs with inversion at C-3. 

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 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. 

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

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

RIBULOSE-5-PHOSPHATE 4-EPIMERASE RIESKE IRON-SULFUR PROTEIN Ring reversal (or ring inversion),... 

RIBULOSE-5-PHOSPHATE 4-EPIMERASE a-XYLOSIDASE j8-XYLOSIDASE d-XYLULOKINASE l-XYLULOKINASE X-Pro dipeptidase... 

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. 

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... 

Details of these reactions, showing the structures of the molecules involved, are given in Fig. 1. These reactions require xylulose 5-phosphate as well as ribose-5-phosphate. Xylulose 5-phosphate is an epimer (see Topic Jl) of ribulose 5-phosphate and is made by phosphopentose epimerase ... 

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. 

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.

L-Ribulose-5-phosphate 4-epimerase catalyzes epimerization at the C4 position of L-ribulose-5-phosphate to form D-xylulose-5-phosphate, allowing bacteria to utilize arabi-nose as an energy source in the pentose phosphate pathway ". The enzyme from E. coli is comprised of four equal 25.5 kDa subunits and shows very close resemblance to the... 

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... 

Finally, ribose-5-phosphate is converted into ribulose 5-phosphate by phosphopentose isomerase while xylulose 5-phosphate is converted into ribulose 5-phosphate by phosphopentose epimerase. Ribulose 5-phosphate is converted into ribulose 1,5-bisphosphate through the action of phosphoribulose kinase (Figure 20,11). The sum of these reactions is... 

The donor of the two-carbon unit in this reaction is xylulose 5-phosphate, an epimer of ribulose 5-phosphate. A ketose is a substrate of transketolase only if its hydroxyl group at C-3 has the configuration of xylulose rather than ribulose. Ribulose 5-phosphate is converted into the appropriate epimer for the transketolase reaction by phosphopentose epimerase (see Figure 20.11) in the reverse reaction of that which occurs in the Calvin cycle. 

Ribulose 5-phosphate xylulose 5-phosphate Phosphopentose epimerase... 

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

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

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