Carbohydrates pentose phosphate pathway

M5. Marks, P. A., A newer pathway of carbohydrate metabolism the pentose phosphate pathway. Diabetes 5, 276-283 (1956). 

The ozone treatment apparently initiated changes in the pathways of carbohydrate metabolism, with glycolysis being reduced while the activity of the pentose phosphate pathway was increased. Rat lung tissue exposed to ozone also exhibited the ozone-induced depression of GPD and enhancement of G6PD activity (, 10), The activation of the pentose phosphate pathway is a characteristic featiire of diseased plants (11, 12),... 

Glucose 6-phosphate is the key intermediate in carbohydrate metabolism. It may be polymerized into glycogen, dephosphorylated to blood glucose, or converted to fatty acids via acetyl-CoA. It may undergo oxidation by glycolysis, the citric acid cycle, and respiratory chain to yield ATP, or enter the pentose phosphate pathway to yield pentoses and NADPH. 

Cells differ considerably in their use of the pentose f phosphate pathway. In muscle, a tissue in which carbohydrates are utilized almost exclusively for generation of mechanical energy, the enzymes of the pentose phosphate pathway are lacking. By contrast, red blood cells are totally dependent on the pentose phosphate pathway as a source of NADPH for which they need to keep the iron of hemoglobin in its normal +2 valence state. A deficiency in glucose-6-phosphate dehydrogenase, the first enzyme in the pentose phosphate pathway, can lead to the wholesale destruction of red blood cells and a condition known as hemolytic anemia. 

The transaldolase (EC 2.2.1.2) is an ubiquitous enzyme that is involved in the pentose phosphate pathway of carbohydrate metabolism. The class I lyase, which has been cloned from human [382] and microbial sources [383], transfers a dihydroxyacetone unit between several phosphorylated metabolites. Although yeast transaldolase is commercially available and several unphosphorylated aldehydes have been shown to be able to replace the acceptor component, preparative utilization has mostly been limited to microscale studies [384,385] because of the high enzyme costs and because of the fact that the equilibria usually are close to unity. Also, the stereochemistry of transaldolase products (e.g. 38, 40) [386] matches that of the products from the FruA-type DHAP aldolase which are more effortlessly obtained. 

An alternative pathway to the Embden-Meyerhof pathway of glycolysis for conversion of carbohydrates to pyruvate is the pentose-phosphate pathway (Fig. 5.13). Its main role is not ATP production but to provide NADPH for fat synthesis, and pentoses (in particular, D-ribose-5-phosphate) for nucleic acid synthesis. The pathway can also convert pentoses to hexoses, which can then be further metabolised by glycolysis. With regard to cestodes, a... 

Transketolase is involved in the pentose phosphate pathway, which is the major pathway of carbohydrate metaholism in some tissues and a significant alternative to glycolysis in all tissues. The main importance of the pentose phosphate pathway is in the production of NADPH for use in hiosynthetic reactions (and especially lipogenesis) and the de novo synthesis of rihose for nucleotide synthesis. 

Conceptual Insights, Overview of Carbohydrate and Fatty Acid Metabolism. View this media module to gain a "bigger picture" understanding of the roles of the pentose phosphate pathway in the context of other metabolic pathways (glycolysis, citric acid cycle, glycogen and fatty acid metabolism). 

The assembly of biological molecules, including proteins and nucleic acids, requires the generation of appropriate starting materials. We have already considered the assembly of carbohydrates in regard to the Calvin cycle and the pentose phosphate pathway (Chapter 20). The present chapter and the next two examine the assembly of the other important building blocks—namely, amino acids, nucleotides, and lipids. 

The KQ is influenced by the net rate of fatty acid biosynthesis. Fatty acid synthesis involves the conversion of carbohydrate, via the acetyJ-CoA intermediate, to long-chain fatty acids. The synthesis of fatty acids requires reduced NADP as a co/ac-tor. It involves the consumption of two molecules of NADPH + H+ for each 2-carbon unit incorporated into the fatty acid. The NADPH + H is supplied by two separate pathways the pentose phosphate pathway (PPP) and the malic enzyme/citrate Lyase pathway. 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

---