The Pentose Phosphate Cycle

The pentose phosphate cycle is only a subsidiary pathway for the breakdown of glucose. In higher plants its importance lies primarily in its supplying NADPH + H . This is because the formation of pentoses from glucose-6-phosphate seems to be less critical in photosynthetically active 

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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... 

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. 

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... 

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.

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.

Just as with the pentose phosphate cycle, an exact reversal of the glycolysis sequence (Eq. 17-53) is precluded on thermodynamic grounds. Even at very high values of the phosphorylation state ratio Rp, the reaction ... 

Photosynthesis. The formation of carbohydrates in green plants by the process of photosynthesis is described in ihc entry on Photosynthesis. The synthetic mechanism involves the addition of carbon dioxide to ribulose-1,5-diphosphate and the subsequent formation of two molecules of 3-phosphoglyccric acid which are reduced to glyceraldehyde-3-phosphate. The triose phosphates are utilized to again from ribulose-5-phosphates by enzymes of the pentose phosphate cycle Phosphorylation or ribulose-5-phosphate with ATP regenerates ribulose-1.5-diphosphate to accept another molecule of carbon dioxide. See also Phosphorylation (Photosynthetlc). 

So far. Sect. 2 of this article has generally dealt with biosynthetic studies on deoxysugars and deoxysugar-containing natural products. This last subsection provides a brief excursion into two aspects of carbohydrate metabolism that are important for biosynthetic studies of deoxysugars (1) the pentose phosphate cycle in the metabolism of Streptomycetes and other microorganisms and (2) the short activation pathway of deoxygenated carbohydrates. 

Unexpected Results from Incorporation Experiments ("Scrambling" Through the Pentose Phosphate Cycle)... 

E. Melendez-Hevia and A. Isidore. 1985. The game of the pentose phosphate cycle J. Theor. Biol. 117 251-263. (PubMed)... 

Hydrogenase purified from the bacterium Pyrococcus furiosus has been combined with the enzymes of the pentose phosphate cycle to produce hydrogen from glucose-6-phosphate and NADP (Woodward et al., 2000). Similar experiments have been made using hydrogenase from Thiocapsa ro-... 

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