Oxidative pentose cycle

The pathway of carbon dioxide fixation and assimilation to the level of sugar consists of a cyclic series of reactions sometimes referred to as the reductive pentose cycle (in contrast to the dissimilatory oxidative pentose cycle), the carbon reduction cycle or the Calvin cycle. ... 

The overall reaction by which glucose is catabolized by the oxidative pentose cycle is ... 

Chloroplasts are the organelles which carry on the total synthesis of carbohydrates from CO2 through the reductive pentose cycle driven by the assimilatory power formed in the light phase of photosynthesis and made up of NADPH and ATP, but the oxidative pentose cycle can also operate inside the chloroplasts and regenerate NADPH in the dark.< - >... 

The PRPP yield in the RBC was strikingly enhanced by addition of methylene blue or phenazine methosulfate. These artificial hydrogen acceptors stimulate the oxidative pentose cycle pathway and thereby increase the intracellular si )ply of Rib-5-P as the requisite substrate of PRPP synthetase. However, the stimulatory effect of the redox dyes failed to alter the stringent requirement for high Pi concentration. On the other hand, experiments performed with cell-free systems revealed that the activity of PRPP synthetase... 

Using the same reaction mixture as above but omitting NADP, so as to eliminate the oxidative pentose cycle pathway, we were able to reproduce essentially the characteristic response to Pi observed under anaerobic conditions in the intact RBC. As may be seen in Table 3, there was a definite shift of the Pi optimum towards physiological concentrations. The inhibitory effect of 15mM Pi seems to be attributable to interference of the Pi excess with the activity of the transaldolase-transketolase system mediating the non-oxidative pentose cycle path for Rib-5-P supply. 

Table 3. Synthesis of PRPP from glucose dependent on the non-oxidative pentose cycle effect of Pi.

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. 

A quantitatively much more important pathway of C02 fixation is the reductive pentose phosphate pathway (ribulose bisphosphate cycle or Calvin-Benson cycle Fig. 17-14). This sequence of reactions, which takes place in the chloroplasts of green plants and also in many chemiautotrophic bacteria, is essentially a way of reversing the oxidative pentose phosphate pathway (Fig. 17-8). The latter accomplishes the complete oxidation of glucose or of glucose 1-phosphate by NADP+ (Eq. 17-48) ... 

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... 

The reductive pentose cycle, or Calvin cycle. The number of arrows drawn at each step in the diagram indicates the number of molecules proceeding through that step for every three molecules of C02 that enter the cycle. The entry of three molecules of C02 results in the formation of one molecule of glyceraldehyde-3-phosphate (box on right), and requires the oxidation of six molecules of NADPH to NADP+ and the breakdown of nine molecules of ATP to ADP. 

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. 

Another central pathway by which yeasts may catabolize D-glucose is the pentose cycle (see Fig. 6), the initial stages of which are (i) the phosphorylation of D-glucose, followed by (ii) oxidation of D-glucose 6-phosphate to 6-O-phosphono-D-gluconate. The net result of the operation of this cycle is the complete oxidation of D-glucose. 

In principle, the first step by which yeasts could convert an al-dopentose into an intermediate of the pentose cycle might be (a) an epimerization, (b) a conversion into the corresponding ketose by way of the enediol, (c) conversion by oxidation and reduction, or (d) phosphorylation. For example, D-xylose might be expected to be ca-tabolized initially, either by isomerization to D-threo-pentulose, or reduction to xylitol which would then be oxidized to D-threo-pentulose. D-f/ireo-Pentulose is phosphorylated to give u-threo-... 

Again, in principle, alditols might be oxidized to the corresponding ketoses by NAD - or NADPe-linked dehydrogenases, or to the aldoses by NADP -linked dehydrogenases. Alternatively, the aldoses might be phosphorylated. Aldoses and ketoses may be phosphorylated, and alditol phosphates oxidized to aldose or ketose phosphates. These compounds are likely to be catabolized by the reactions of the Embden-Meyerhof glycolytic pathway or of the pentose cycle. 

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. 

Respiration is the reduction of O2 to H2O during the oxidation of carbohydrate to CO2. There are two types of respiration in photosynthetic organisms a dark respiration and a photorespiration [3]. Dark respiration includes O2 reduction and the oxidation of NADH and FADH2 in mitochondrial membranes, glycolysis, the Krebs cycle, and the oxidative pentose phosphate pathway. Respiration is commonly subdivided into two functional components growth respiration, supplying energy for new biomass production, and... 

Many other sugars are involved in primary metabolic pathways such as the oxidative and reductive pentose cycle, glycolysis, and photosynthesis. 

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