Carbohydrate metabolism pentose phosphate pathway

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

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. 

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. 

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). 

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. 

In the general metabolism of carbohydrates, sedoheptulose functions in the hexose monophosphate shunt (or pentose phosphate pathway), which operates through the following reactions. 

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... 

Transaldolase is a critical enzyme in the metabolism of carbohydrates directed through the pentose phosphate pathway [63]. It is responsible for transferring a... 

---