Pentose phosphate pathway reaction details

The of biochemical pathways can be reversed. Examples include glycolysis and gluconeogenesis, glycogen formation and synthesis, and the pentose phosphate pathway. The details are not completely reversible. An irreversible step in one pathway tends to be replaced with another reaction, catalyzed by another enzyme. 

This is not the place to expose in detail the problems and the solutions already obtained in studying biochemical reaction networks. However, because of the importance of this problem and the great recent interest in understanding metabolic networks, we hope to throw a little light on this area. Figure 10.3-23 shows a model for the metabolic pathways involved in the central carbon metabolism of Escherichia coli through glycolysis and the pentose phosphate pathway [22]. 

The reduced coenzyme NADPH is required for the reduction reactions shown in Figure 11.5. It is also required for elongation and desaturation of fatty acids. The major source of NADPH for these reactions is the pentose phosphate pathway, which is described in detail in Chapter 6. 

Figure 1 Reactions of the oxidative segment of the pentose phosphate pathway. The conversion of [2- C] glucose to CO2 and [1- C] ribulose S-phosphate is shown. Detail of the reaction steps are discussed in the text. Reprinted from International Journal of Biochemistry, 19. Williams John. F., Arora Krishan K. and Longnecker John P. The pentose pathway A random harvest, 69 Pages, 1987, with permission from Elsevier,...

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. 

Fig. 2 The red blood cell has played a special role in the development of mathematical models of metabolism given its relative simplicity and the detailed knowledge about its molecular components. The model comprises 44 enzymatic reactions and membrane transport systems and 34 metabolites and ions. The model includes glycolysis, the Rapaport-Leubering shunt, the pentose phosphate pathway, nucleotide metabolism reactions, the sodium/potassium pump, and other membrane transport processes. Analysis of the dynamic model using phase planes, temporal decomposition, and statistical analysis shows that hRBC metabolism is characterized by the formation of pseudoequilibrium concentration states pools or aggregates of concentration variables. (From Ref...

The details of the pentose phosphate pathway will not be covered in this text. But an overview of the key reactions will allow us to understand the importance of the pathway (Figure 21.11). We can consider the pathway in three stages. The first is the oxidative stage, which can be summarized as... 

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