Pentose Metabolic sequence

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

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. 

There is increasing evidence that CAM is controlled to a great extent at the enzyme level. As outlined earlier (see Chap. 3), the main metabolic sequence linking malate to the reductive pentose cycle is ... 

The first step of xylose catabolism is its conversion to xylulose. In bacteria, it takes place by the direct isomerization catalysed by xylose isomerase. In PeniciUium chrysogenum, a sequence of enzymes in the initial steps of pentose metabolism was observed that differs from xylose isomerization in bacteria [91, 92]. These enzymes were common in yeast and filamentous fungi. In this oxido-reductive pathway, xylose is first reduced to the xyhtol in the presence of NAD(P)-linked xylose reductase, which is then reoxidized by NAD(P)-hnked dehydrogenase to give xylulose (Fig. 1). It has been assumed that this oxido-reductive pathway is common among fungi [93]. Both the enzymes involved, xylose reductase and xylitol dehydrogenase, were found to be inducible and relatively specific for the D-xylose and xyhtol in F. oxysporum, whereas D-xylose isomerase was not detected. 

Because the generation of C02 from [l- Cjglucose is a measure of pentose phosphate activity, the initiating step in the P5C-mediated metabolic sequence, it was used as the metabolic endpoint of parametabolic regulation. When hepatocytes or erythrocytes were incubated separately, proline had no effect on pentose phosphate activity in either cell and in the absence of proline the total activity in coincubations was the... 

Fig. 1. Pathways of glucose metabolism in eubacteria and eukaryotes. The three major catabolic pathways are the Embden-Meyerhof glycolytic sequence (solid lines), the Entner-Doudoroff pathway (heavy solid lines) and the pentose phosphate pathway (dashed lines). The sequence from glyceraldehyde 3-phosphate to pyruvate is common to all three pathways.

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.. ... 

Such anaerobic metabolism can sustain life in animal cells only for short periods. However, if there is free access of oxygen, pyruvic acid passes from the above Meyerhof sequence to the tricarboxylic acid cycle (see Section 4.5), where it is completely oxidized to carbon dioxide and water. Alternative pathways exists for degrading glucose, through pentose phosphate in vertebrates, the choice of reactions varying with different tissues (see under bacteria below). All of the cell s ribose, so important in the synthesis of nucleic acids, comes from the pentose type of glycolysis. 

Insulin a polypeptide hormone, M, 5,780 (bovine), synthesized in, and secreted by, the B cells of the islets of Langerhans. The first protein primary sequence ever to be elucidated was that of I. (Fig.l) [F. Sanger etal. Biochem. J. 59 (1955) 509-518], I. is the only hormone that decreases the blood glucose concentration. It affects the entire intermediary metabolism, especially of the liver, adipose tissue and muscle. I. increases the permeability of cells to monosaccharides, amino aci and fatty acids, and it accelerates glycolysis, the pentose phosphate cycle, and, in the liver, glycogen synthesis. It promotes the biosynthesis of fatty adds and proteins. These indirect effects on various enzymes and metabolic processes are listed in the tables. 

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