Sugar phosphates interconversions

The nonoxidative phase consists of a series of sugar-phosphate interconversions that result in the conversion of ribulose 5-phosphate to ribose 5-phosphate (Figure 6-3). 

Pentose phosphate pathway Summary of the path way Reduced coenzymes produced by the pathway PENTOSE PHOSPHATE PATHWAY (p. 143) Also called the hexose monophosphate shunt, or6-phosphogluconate pathway, the pentose phosphate pathway is found in all cells. It consists of two irreversible oxidative reac tions followed by a series of reversible sugar-phosphate interconversions. No ATP is directly consumed or produced in the cycle, and two NADPH are produced for each glu cose 6-phosphate entering the oxidative part of the pathway. 

The PPP occurs in the cytosol of the cell. It comprises two irreversible oxidative reactions followed by a series of reversible sugar-phosphate interconversions. Unlike glycolysis or the citric acid cycle, in which the direction... 

Figure 7.9 Interconversion of sugar phosphates and sugar nucleotide phosphates. Adapted from "Biotechnology of microbial exopolysaccharides". IW Sutherland, Cambridge University Press, 1990.

The glycolytic enzyme triosephosphate isomerase (TIM) catalyzes the interconversion of the sugar phosphates dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP). Although the reaction is extremely fast, the enzyme is an attractive candidate for mapping... 

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. 

Ultrasonic relaxation techniques have been used to estimate the energy barrier to syn-anti conformational interconversion in cytidine 2, 3 -mono-phosphate, in the presence and absence of ethidium bromide. Raman spectroscopic measurements on a number of dinucleoside monophosphate crystals of precisely known sugar-phosphate structure have been compared with those of B- and A-form DNA, to give information on the comparative rigidity of the backbone in the nucleic acids. The same technique, applied to poly(dA-dT) and poly[d(A-T)] at different temperatures, suggests that some of the furanose... 

Sugar skeletons are interconverted by way of three classes of compound, sugar phosphates, sugar nucleotides and cyclitols and the major known pathways which interconnect them are summarised in the schemes later in this chapter. The sugar nucleotides are the main intermediates in these interconversions and it is their metabolism which forms the bulk of this chapter. The cyclitols provide an important and, sometimes, dominant route from hexose to uronic acid and, thence, pentoses in plants, but their role in animals is unclear. Sugar phosphates are of importance as the entry to the sugar nucleotide pathways, but of themselves contribute little to metabolic interconversion directly for anabolic purposes. 

The role of soluble, cytoplasmic enzymes in saccharide assembly seems chiefly to be in the phosphorylation of sugars and the interconversion of sugar phosphates. The enzymes of sugar nucleotide interconversion are generally membrane-associated and must, in most cases, be firmly membrane-bound. They are, therefore, likely to be integral membrane proteins and must, for the most part, be constituents of the endoplasmic reticulum. 

Similar reactions have recently been found in plants to be involved in pentose metabolism.D-Xylose-l-phosphate was found to react with UTP to form UDPXy. A second enzyme, which may be different from the epimerase of glucose-galactose interconversions, epimerizes carbon 4 of the pentose to produce the UDP derivative of L-arabinose. The number of specific pyrophosphorylases for sugar phosphates in plant tissue is not known at this time. 

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