Pentose phosphate compounds

The following compound is an intermediate in the pentose phosphate pathway, an alternative route for glucose metabolism. Identify the sugar it is derived from. 

Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenosis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexose-isomerase, which involves an aldose-ketose isomerization. 

D-Xylulose 5-phosphate (ii-threo-2-pentulose 5-phosphate, XP) stands as an important metabolite of the pentose phosphate pathway, which plays a key fimction in the cell and provides intermediates for biosynthetic pathways. The starting compound of the pathway is glucose 6-phosphate, but XP can also be formed by direct phosphorylation of D-xylulose with li-xylulokinase. Tritsch et al. [114] developed a radiometric test system for the measurement of D-xylulose kinase (XK) activity in crude cell extracts. Aliquots were spotted onto silica plates and developed in n-propyl alcohol-ethyl acetate-water (6 1 3 (v/v) to separate o-xylose/o-xylulose from XP. Silica was scraped off and determined by liquid scintillation. The conversion rate of [ " C]o-xylose into [ " C]o-xylulose 5-phosphate was calculated. Some of the works devoted to the separation of components necessary while analyzing enzyme activity are presented in Table 9.8. 

The biological function of the pentose phosphate cycle involves the production of two compounds NADP H2, which is a reductive force in the synthesis of various materials, and the metabolite ribose... 

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

Since only less than 10% of G-6-P is channeled into the pentose phosphate cycle (under physiological conditions this percentage varies depending on the different tissues), the question must be discussed, what is the importance of this shunt. With regard to the resulting compounds Eqs. [(3), (5), (6), (7)] one mole NADPH2 appears twice. Furthermore, pentose phosphates are furnished for biosynthesis of nucleotides, nucleic acids, and fatty acids (D5, D6, DIO, H13, M5). 

The way in which aromatic amino compounds lead to the formation of MHb is of some interest in regard to the role played by the first reaction of the pentose phosphate cycle in this reaction system. It has been stated (L5) that nitrosobenzene effects within one hour the conversion of Hb to MHb to the extent of 80% of total pigment according to the following reactions ... 

Nevertheless, using GC-based technologies, the quantification of several important intermediates of central metabolism, especially phosphorylated intermediates, is not very reliable, presumably because these compounds and their derivatives are not thermostable. For an analysis of these groups of metabolites, an LC-MS (liquid chromatography or HPLC coupled to MS) is more suitable, because it eliminates the need for volatility and thermostability and thereby eliminates the need for derivatization. Using a triple quadrupole MS, most of the intermediates in glycolysis, in the pentose phosphate pathway, and in the tricarboxylic acid cycle were measured in E. coli [214]. 

In addition to the common pathways, glycolysis and the TCA cycle, the liver is involved with the pentose phosphate pathway regulation of blood glucose concentration via glycogen turnover and gluconeogenesis interconversion of monosaccharides lipid syntheses lipoprotein formation ketogenesis bile acid and bile salt formation phase I and phase II reactions for detoxification of waste compounds haem synthesis and degradation synthesis of non-essential amino acids and urea synthesis. 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

Carbohydrate metabolism in a typical plant cell is more complex in several ways than that in a typical animal cell. The plant cell carries out the same processes that generate energy in animal cells (glycolysis, citric acid cycle, and oxidative phosphorylation) it can generate hexoses from three- or four-carbon compounds by glu-coneogenesis it can oxidize hexose phosphates to pentose phosphates with the generation of NADPH (the ox-... 

FIGURE 20-37 Pools of pentose phosphates, triose phosphates, and hexose phosphates. The compounds in each pool are readily interconvertible by reactions that have small standard free-energy changes. 

Aromatic compounds arise in several ways. The major mute utilized by autotrophic organisms for synthesis of the aromatic amino acids, quinones, and tocopherols is the shikimate pathway. As outlined here, it starts with the glycolysis intermediate phosphoenolpyruvate (PEP) and erythrose 4-phosphate, a metabolite from the pentose phosphate pathway. Phenylalanine, tyrosine, and tryptophan are not only used for protein synthesis but are converted into a broad range of hormones, chromophores, alkaloids, and structural materials. In plants phenylalanine is deaminated to cinnamate which yields hundreds of secondary products. In another pathway ribose 5-phosphate is converted to pyrimidine and purine nucleotides and also to flavins, folates, molybdopterin, and many other pterin derivatives. 

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