Pentose oxidation cycle

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. 

By a special technique the augmentation of pentose phosphate cycle activity in red cells of obese patients was shown by Sonka et al. (S16), increasing the glucose utilization by the oxidative pathway from 7.56 to 12.25%. 

Fig. 10. The connection between the pentose phosphate cycle and the C-fl oxidation pathway.

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

An oxidative pentose phosphate cycle. Putting the three enzyme systems together, we can form a cycle that oxidizes hexose phosphates. Three carbon... 

The oxidative pentose phosphate cycle is often presented as a means for complete oxidation of hexoses to C02. For this to happen the C3 unit indicated as the product in Fig. 17-8A must be converted (through the action of aldolase, a phosphatase, and hexose phosphate isomerase) back to one-half of a molecule of glucose-6-P which can enter the cycle at the beginning. On the other hand, alternative ways of degrading the C3 product glyceraldehyde-P are available. For example, using glycolytic enzymes, it can be oxidized to pyruvate and to C02 via the citric acid cycle. 

The reactions enclosed within the shaded box of Fig. 17-14 do not give the whole story about the coupling mechanism. A phospho group was transferred from ATP in step a and to complete the hydrolysis it must be removed in some future step. This is indicated in a general way in Fig. 17-14 by the reaction steps d, e, and/. Step/represents the action of specific phosphatases that remove phospho groups from the seven-carbon sedoheptulose bisphosphate and from fructose bisphosphate. In either case the resulting ketose monophosphate reacts with an aldose (via transketolase, step g) to regenerate ribulose 5-phosphate, the C02 acceptor. The overall reductive pentose phosphate cycle (Fig. 17-14B) is easy to understand as a reversal of the oxidative pentose phosphate pathway in which the oxidative decarboxylation system of Eq. 17-12 is... 

Cori D, Racker E. The oxidative pentose phosphate cycle. V. Complete oxidation of glucose 6-phosphate in a reconstructed system of the oxidative pentose phosphate cycle. Archiv. Biochem. Biophys. 1959 83 195-205. 

Compared with the investigations in the extreme halophiles, there is very little information on the operation of a pentose-phosphate pathway in other archaebacteria. The radiorespirometric analyses of glucose metabolism in Sulfolobus species [13], which established the Entner-Doudoroff type pathway (section 2.2), were also consistent with a non-cyclic pentose-phosphate pathway in S. brierleyi and a conventional oxidation cycle in Sulfolobus strain LM. Similarly, respiratory studies [15] provide evidence for a pentose phosphate cycle capable of glucose oxidation in Tp. acidophilum. No data are available for the methanogens. 

V. Klybas, M. Schramm, and E. Racker, Oxidative pentose phosphate cycle. IV. Synthesis of sedoheptulose 1,7-diphosphate, sedoheptulose 7-phosphate, glyceraldehyde 3-phosphate, and glycolaldehyde phosphate, Arch. Biochem. Biophys., 80 (1959) 229-235. 

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