Pentose-phosphate-cycle

To provide for all steps of the pentose phosphate cycle, at least three glucose 6-phosphate molecules are required. Let us consider separate reactions of this cycle. 

Glucose-6-phosphate dehydrogenase is a dimer with a molecular mass of about 135 000. Up to eight electrophoretically separable isoenzymes for this enzyme are known. A specific feature of the above reaction is the formation of NADP Hr The reaction equilibrium is strongly shifted to the right, since the lactone formed is liable to hydrolysis, which is spontaneous or lactonase-assisted. 

Dehydrogenation of 6-phosphogluconate to ribulose 5-phosphate. This reaction is catalyzed by 6-phosphogluconate dehydrogenase according to the scheme  

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below  

Two other pentose phosphates (ribose 5-phosphate and xylulose 5-phosphate), which are derived from ribulose 5-phosphate, are important for the subsequent reaction of the cycle. Two molecules of 

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There is known one more catabolic route for carbohydrates commonly referred to as the pentose phosphate cycle (also called hexose mono phosphate shunt, or phosphogluconate pathway). 

As a tribute to the biochemists who have played a decisive role in its investigation, the pentose phosphate cycle is also referred to as the Warburg-Dickens-Horecker pathway. 

The pentose phosphate cycle represents a multienzyme system in... 

Thus, in the course of reactions catalyzed by the intrinsic enzymes of the pentose phosphate cycle, two fructose 6-phosphate molecules, one glyceraldehyde 3-phosphate molecule, and three carbon dioxide molecules are produced from three glucose 6-phosphate molecules. In addition, six NADP -H2 molecules are formed. The overall scheme for the pentose phosphate cycle is ... 

The biological function of the pentose phosphate cycle involves the production of two compounds NADP H2, which is a reductive force in the synthesis of various materials, and the metabolite ribose... 

Fig. 7.—The Pentose Phosphate Cycle for the Incorporation of Carbon Dioxide into Carbohydrates During Photosynthesis.86-, 38...

The literature concerning the pentose phosphate cycle has been reviewed (D5, DIO, M5) and, especially the literature for erythrocytes (A7). 

Since only less than 10% of G-6-P is channeled into the pentose phosphate cycle (under physiological conditions this percentage varies depending on the different tissues), the question must be discussed, what is the importance of this shunt. With regard to the resulting compounds Eqs. [(3), (5), (6), (7)] one mole NADPH2 appears twice. Furthermore, pentose phosphates are furnished for biosynthesis of nucleotides, nucleic acids, and fatty acids (D5, D6, DIO, H13, M5). 

With respect to the problem of hormonal influences on enzymes of the pentose phosphate cycle, investigations of the enzyme levels in... 

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%. 

The way in which aromatic amino compounds lead to the formation of MHb is of some interest in regard to the role played by the first reaction of the pentose phosphate cycle in this reaction system. It has been stated (L5) that nitrosobenzene effects within one hour the conversion of Hb to MHb to the extent of 80% of total pigment according to the following reactions ... 

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

Figure 5.4. First site of NADPH generation in the pentose phosphate cycle.

Pentose phosphate cycle Pentose and glucuronate interconversions... 

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. 

Figure 21. Secondary metabolism blocks and amino acid derivation. Note that shikimic acid can be derived directly from photosynthesis and glycolysis through the pentose phosphate cycle, or alternatively as a pyruvic acid postcursor.

Wolosiuk, R., Ballicora, M., Hagelin, K. (1993) The reductive pentose phosphate cycle for photosynthetic C02 assimilation enzyme modulation. FAS EB J. 7, 622-637. 

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... 

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