Pentose monophosphates

Energy burst in the neutrophils derived significantly from the pentose monophosphate pathway generates NADP, NAD, and H2O2. 

Pentose phosphates occur naturally in the nucleic acids, each of which is made up of four mononucleotides, or glycosides of pentose monophosphate (p. 130). 5-phosphoribose occurs in the nucleic acid of animal chromatin, and in the inosinic and adenylic acid of muscle (p. 291). 3-phosphoribose occurs in the guanylic acid, xanthylic acid and nucleic acid found in yeast (p. 348). 

Cells require a constant supply of N/ X)PH for reductive reactions vital to biosynthetic purposes. Much of this requirement is met by a glucose-based metabolic sequence variously called the pentose phosphate pathway, the hexose monophosphate shunt, or the phosphogluconate pathway. In addition to providing N/VDPH for biosynthetic processes, this pathway produces ribos 5-phosphate, which is essential for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. 

R. Goldberg and J. Tewari, Thermodynamic and transport properties of carbohydrates and their monophosphates The pentoses and hexoses, J. Phys. Chem. Ref. Data, 18 (1989) 809-822. 

The hexose monophosphate pathway has several names just to confuse you. It s called the hexose monophosphate shunt or pathway (HMP shunt or pathway), or the pentose phosphate pathway, or the phospho-gluconate pathway (Fig. 15-1). The pathway in its full form is complicated and has complicated stoichiometry. Usually it s not necessary to remember all of it. The important points are that it makes NADPH for biosynthesis and riboses (C-5 sugars) for DNA and RNA synthesis. 

Under basal conditions 5% of brain glucose is metabolized via the pentose phosphate shunt (PPS), also termed the hexose monophosphate pathway [66], a pathway active in both neurons and astrocytes. The PPS has... 

Pentose phosphate pathway (= hexose monophosphate shunt) generation of NADPH G6PDH deficiency... 

The hexose monophosphate (HMP) shunt (pentose phosphate pathway) occurs in the cytoplasm of all cells, where it serves two major functions ... 

This pathway is variously known as the pentose phosphate, hexose monophosphate or phosphogluconate pathway, cycle or shunt. Although the pentose phosphate pathway achieves oxidation of glucose, this is not its function, as indicated by the distribution of the pathway in different tissues. Only one of the carbons is released as CO2, the key products are NADPH and ribose 5-phosphate, both of which are important for nucleotide phosphate formation and hence for synthesis of nucleic acids (Chapter 20). The... 

Figure 20.10 The positions in the pathway for de novo purine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. The pathway begins with glucose which provides ribose 5-phosphate, via the pentose phosphate pathway (Chapter 6). Glutamine provides its amide nitrogen in two reactions formation of 5-phosphoribosylamine and formation of guanosine monophosphate (GMP) from xantho-sine 5-phosphate (XMP).

The pentose phosphate pathway (PPP, also known as the hexose monophosphate pathway) is an oxidative metabolic pathway located in the cytoplasm, which, like glycolysis, starts from glucose 6-phosphate. It supplies two important precursors for anabolic pathways NADPH+H+, which is required for the biosynthesis of fatty acids and isopren-oids, for example (see p. 168), and ribose 5-phosphate, a precursor in nucleotide biosynthesis (see p. 188). 

A, The pentose phosphate pathway (PPP), also called the hexose monophosphate shunt, is an alternate pathway of glucose metabolism that supplies the NAD PH required by many biosynthetic pathways. 

Ribose 5-phosphate derived from the pentose phosphate pathway or from dietary sources is the starting material that eventually gives rise to inosine monophosphate (IMP) (Figure 10-1). 

The oxidative pentose phosphate pathway (phosphogluconate pathway, or hexose monophosphate pathway) brings about oxidation and decarboxylation at C-l of glucose 6-phosphate, reducing NADP+ to NADPH and producing pentose phosphates. 

Pentose phosphate pathway Summary of the path way Reduced coenzymes produced by the pathway PENTOSE PHOSPHATE PATHWAY (p. 143) Also called the hexose monophosphate shunt, or6-phosphogluconate pathway, the pentose phosphate pathway is found in all cells. It consists of two irreversible oxidative reac tions followed by a series of reversible sugar-phosphate interconversions. No ATP is directly consumed or produced in the cycle, and two NADPH are produced for each glu cose 6-phosphate entering the oxidative part of the pathway. 

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... 

Transketolase reactions leading via the pentose or hexose monophosphate shunt pathway of glucose oxidation to the eventual production of pentoses for RNA/DNA synthesis and NADPH for the biosynthesis of fatty acids... 

The hexose monophosphate (Helly, 1976) and pentose phosphate (Hochachka and Hayes, 1962 Yamaguchi et al., 1976 Walsh, 1985 Malinovskaya, 1988 Kudryavtseva, 1990) shunts have also been found to increase in importance. The activity of transketolase, the enzyme which inhibits the peptide-phosphate pathway, is greater in fish from cold water, e.g. trout and smelt, than in those from warm water (Kudryavtseva, 1990). In the Black Sea horse-mackerel, a sharp decline in adenine nucleotide content (AMP, ADP and ATP) in white and red muscle tissues and in liver occurs at low temperature (Trusevich, 1978). In this case, the ATP is mosdy resynthesized by glycolysis. The increase in the glucose content of the blood of fish at low ambient temperatures may be of the same nature (Prosser, 1967 ... 

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