Carbohydrate metabolism pentose 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),... 

The individual pathways of carbohydrate metabolism in plants overlap extensively they share pools of common intermediates, including hexose phosphates, pentose phosphates, and triose phosphates. Transporters in the membranes of chloroplasts, mitochondria, amyloplasts, and peroxisomes mediate the movement of sugar phosphates between organelles. The direction of metabolite flow through the pools changes from day to night. 

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. 

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. 

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. 

BleaUey PA, Arora KK, Williams JF. Evidence that aldolase and D-arabinose 5-phosphate are components of pentose pathway reactions in liver in vitro. Biochem. Int. 1984 8 491-500. Horecker BL, Mehler AH. Carbohydrate metabolism. Ann. Rev. Biochem. 1955 24 207-274. 

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

Various inborn errors of metabolism (Table 25-1) result from deficiencies or absence of some of the enzymes listed in Figure 25-9. Some of these are discussed later in the chapter. The relationship of carbohydrate metabolism to the production of lactate, ketone bodies, and triglycerides is also depicted in Figure 25-9. The pentose phosphate pathway, also known as the hexose monophosphate shunt, is an alternative pathway for glucose metaboUsm that generates the reduced form of nicotinamide-adenine dinucleotide phosphate (NADPH), which is used in maintaining the integrity of red blood cell membranes, in lipid and steroid biosynthesis, in hydroxylation reactions, and in other anabolic reactions. The complete picture of intermediary metabolism of carbohydrates is rather complex and interwoven with the metabolism of lipids and amino acids. For details, readers should consult a biochemistry textbook. 

In the erythrocyte, glycolysis, the pentose phosphate pathway, and the metabolism of 2,3-bisphosphoglycerate (Chapter 28) are the predominant pathways of carbohydrate metabolism. Glycolysis supplies ATP for membrane ion pumps and NADH for reoxidation of methemoglobin. The pentose phosphate pathway supplies NADPH to... 

Thiamine pyrophosphate is also an important cofactor for the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism (Fignre 15-3). These reactions are important in the reversible transformation of pentoses into the glycolytic intermediates fructose 6-phosphate and glyc-eraldehyde 3-phosphate. Again, it is the reactive carbon on the thiazole ring of TPP that reacts with a ketose phosphate (xylnlose 5-phosphate) to canse the release of an aldose phosphate with two fewer carbons (glyceraldehyde 3-phosphate). The TPP-bonnd glycoaldehyde unit is then transferred to a different aldose phosphate (ribose 5-phosphate or erythrose 4-phosphate) to produce a ketose phosphate that has two carbons more (sedoheptulose 7-phosphate or fructose 6-phosphate). 

Thiamine pyrophosphate is also an important cofactor for many dehydrogenase reactions as well as the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism. 

Carbohydrate Metabolism Glycolysis and the Pentose Phosphate Pathway. 

Occurrence and preparation. The tetrose (4) participates as D-erythrose 4-phosphate in the pentose phosphate pathway of carbohydrate metabolism.11 D-Erythrose can be prepared using a method of degradation from the pentose, D-arabinose,19 or from D-glucose.20 Synthesis of the 4-phosphate has been reported.21... 

Glycogen Metabolism, Pentose-Phosphate Pathway, and Metabolism of Other Carbohydrates... 

Gluconeogenesis has many steps in common with the production of glucose in photosynthesis, but photosynthesis also has many reactions in common with the pentose phosphate pathway. Thus, nature has evolved common strategies to deal with carbohydrate metabolism in all its aspects. 

The two ways of producing NADPH clearly indicate that all metabolic pathways are related. The exchange reactions involving malate and citryl-GoA constitute a control mechanism in hpid anabolism, while the pentose phosphate pathway is part of carbohydrate metabolism. Both carbohydrates and lipids are important energy sources in many organisms, particularly animals. 

Goenzymes are introduced in this chapter and are discussed in later chapters in the context of the reactions in which they play a role. Chapter 16 discusses carbohydrates. Chapter 17 begins the overview of the metabohc pathways by discussing glycolysis. Glycogen metabolism, gluconeogenesis, and the pentose phosphate pathway (Chapter 18) provide bases for treating control mechanisms in carbohydrate metabolism. Discussion of the citric acid cycle is followed by the electron transport chain and oxidative phosphorylation in Chapters 19 and 20. The catabolic and anabolic aspects of lipid metabohsm are dealt with in Chapter 21. In Chapter 22, photosynthesis rounds out the discussion of carbohydrate metabolism. Chapter... 

Metabolism For uptake and transport of G., see Lit. d-G. plays a central part in the carbohydrate metabolism. It is degraded to smaller molecules in complicated reaction sequences (glycolysis) with release of energy - one example is pyruvic acid, which can enter the citric acid cycle via acetyl-CoA - or (pentose phosphate pathway) can be converted to derivatives of other sugars for biosynthetic purposes under the concomitant availability of reduction equivalents. Alternatively d-G. can be stored in the liver and muscles as areserve substance glycogen (in plants starch). An antimetabolite of d-G. is 5-thio-D-glucose. For detection, see Lit.. ... 

Triose-phcfiphate isomerase (TIM) (EC 5.3.1.1) and phosphoglycerate kinase (PGK) (EC 2.7.2.3) are two ubiquitous enzymes in the major pathways of carbohydrate metabolism (glycolysis, gluconeogenesis, and pentose phosphate pathway), catalyzing the interconversion of dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate, and the phospho-group transfer between... 

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