D-Ribulose-5-phosphate-3-epimerase

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. 

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

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. 

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. 

The fungal oxidoreductive catabolism of these two pentoses is unique for fungi and produces ultimately D-xylulose 5-phosphate, which is an intermediate of the canonical pentose phosphate pathway (Fig. 18.3). In contrast, prokaryotes use an isomerase step to convert D-xylose to D-xylulose, and L-arabinose to L-ribulose. o-xylulose 5-phosphate is then either formed by the action of xylulokinase (in the case of D-xylose) or a sequence of L-ribulokinase and L-ribulose-5-phosphate 4-epimerase (in the case of L-arabinose) (Mishra and Singh 1993). 

Other reactions which are used for the preparation of some esters are those catal) ed by isomerases. Thus fructose 6-phosphate may be converted to glucose 6-phosphate, ribose 5-phosphate to ribulose 5-phosphate. Specific epimerases which lead to inversion at positions 3 or 4 have been employed. For instance D-xylulose 5-phosphate can be converted to L-ribulose 5-phosphate by a 4-epimerase, or to D-ribulose 5-phosphate by a 3-epimerase. 

Ribulose-5-phosphate may undergo two different types of isomerization. The first involves carbon 1 and carbon 2 of the pentose to yield D-ribose-5-phosphate, and the second concerns carbon 3 and yields D-xylu-lose-5-phosphate. The first reaction is catalyzed by an isomerase, the second by an epimerase. Both enzymes are widely distributed in nature and have been found in most tissues purified isomerase was first prepared from alfalfa, and spleen epimerase has also been extensively purified. 

D-Xylulose 5-phosphate is a substrate for transketolase. It is formed from ATP and D-xylulose with a liver enzyme or a bacterial enzyme . It has been prepared from D-ribose 5-phosphate by the combined action of an isomerase and a 3-epimerase with ribulose 5-phosphate as intermediate . 

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