Pentoses, oxidation

Oxidation with Bromine Water.—With 2- or 3-desoxy-hexoses or -pentoses, oxidation by bromine water follows the usual course and converts them into the corresponding 2- or 3-desoxy-hexonic or -pentonic acids. The oxidation is more rapid if the hydrobromic acid that is produced in the reaction is removed as it is formed, by barium or calcium carbonate227 or barium benzoate.228 In view of the sensitivity of 2-desoxysugars to prolonged contact with mineral acids, it is advantageous to adopt this latter procedure for their oxidation.112 Sometimes for the preparation of desoxy -pentonic and -hexonic acids the method of oxidation (or slight modifications thereof) introduced by Goebel229 is employed. 

The metabolism of pentoses has been studied in detail only recently, chiefly of ribose and to a lesser extent deoxyribose. Horecker and his coworkers (H7) have considered the steps in the hexose monophosphate shunt, and the pentose oxidative or Warburg-Dickens-Lipmann pathway. It appears that this pathway is the source of the pentose moiety of nucleotides, ribonucleic acid, and deoxyribonucleic acid. It is also undoubtedly the source of the small amounts of ribulose in normal urine. 

Fig. 2. A schematic representation of carbohydrate metabolism. The figure indicates the relationship of the point of entry of glucose, galactose, and fructose, showing that separate enzyme systems are involved. The pentose oxidative cycle is abbreviated by the symbols ...

D-Sedoheptulose is a sugar intermediate in ametabohc cycle (the pentose oxidation cycle) that ultimately converts glucose into 2,3-dihydroxypropanal (glyceraldehyde) plus three equivalents of CO2. Determine the structure of D-sedoheptulose from the following information. 

New membrane-bound dehydrogenases relating to pentose oxidation have been indicated from recent work on oxidative fermentation of acetic acid bacteria (Adachi et al. 2012). As illustrated in Fig. 13.1, enzymes catalyzing pentose oxidation to 4-keto-D-pentose and its further oxidation to 4-keto-D-pentonate, and direct oxidation of o-pentonate to 4-keto-D-pentonate, are involved. No report about the responsible enzymes is available so far. Some reports state that there is no D-ribose-oxidizing enzyme in acetic acid bacteria. Strictly speaking, D-ribose is oxidized by the quinoprotein-GDH to D-ribonate via D-ribonolactone within the... 

Fig. 13.7 Reactions concerning the new oxidative fermentation involving pentose oxidation. Red arrows D-aldopentose 4-dehydrogenase, blue arrcws 4-keto-D-aldopentose 1-dehydrogenase,...

Adachi O, Hours RA, Akakabe Y, Shinagawa E, Ano Y, Yakushi T, Matsushita K (2013) Pentose oxidation by acetic acid bacteria led to a finding of membrane-bound purine nucleosidase. Biosci Biotechnol Biochem 77(5) 1131-1133... 

Respiratory, or oxidative, metaboHsm produces more energy than fermentation. Complete oxidation of one mol of glucose to carbon dioxide and water may produce up to 36 mol ATP in the tricarboxyHc acid (TCA) cycle or related oxidative pathways. More substrates can be respired than fermented, including pentoses (eg, by Candida species), ethanol (eg, by Saccharomjces), methanol (eg, by Hansenu/a species), and alkanes (eg, by Saccharomjces lipoljticd). 

BOTH RIBOSE-5-P AND NADPH ARE NEEDED BY THE CELL In this case, the first four reactions of the pentose phosphate pathway predominate (Figure 23.37). N/VDPH is produced by the oxidative reactions of the pathway, and ribose-5-P is the principal product of carbon metabolism. As stated earlier, the net reaction for these processes is... 

MORE RIBOSE-5-P THAN NADPH IS NEEDED BY THE CELL Synthesis of ribose-5-P can be accomplished without production of N/VDPH if the oxidative steps of the pentose phosphate pathway are bypassed. The key to this route is the extrac-... 

FIGURE 23.38 The oxidative steps of the pentose phosphate pathway can be bypassed if the primary need is for ribose-5-P. 

How many of the 14 NADPH needed to form one palmitate (Eq. 25.1) can be made in this way The answer depends on the status of malate. Every citrate entering the cytosol produces one acetyl-CoA and one malate (Figure 25.1). Every malate oxidized by malic enzyme produces one NADPH, at the expense of a decarboxylation to pyruvate. Thus, when malate is oxidized, one NADPH is produced for every acetyl-CoA. Conversion of 8 acetyl-CoA units to one palmitate would then be accompanied by production of 8 NADPH. (The other 6 NADPH required [Eq. 25.1] would be provided by the pentose phosphate pathway.) On the other hand, for every malate returned to the mitochondria, one NADPH fewer is produced. 

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthesis—and also in the pentose phosphate pathway. 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

Although glucose 6-phosphate is common to both pathways, the pentose phosphate pathway is markedly different from glycolysis. Oxidation utilizes NADP rather than NAD, and CO2, which is not produced in glycolysis, is a characteristic product. No ATP is generated in the pentose phosphate pathway, whereas ATP is a major product of glycolysis. 

Figure 20-3. Role of the pentose phosphate pathway in the glutathione peroxidase reaction of erythrocytes. (G-S-S-G, oxidized glutathione G-SH, reduced glutathione Se, selenium cofactor.)...

The pentose phosphate pathway, present in the cytosol, can account for the complete oxidation of glucose, producing NADPH and COj but not ATP. 

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