Pentose intermediate

The carbamyl phosphate is condensed with a molecule of aspartate giving ureidosuccinic acid, from which orotic acid is formed by cyclization and oxidation. In the presence of PRPP and a pyrophosphorylase this acid forms a ribotide and decarboxylation yields uridine monophosphate. The decarboxylation of the product of amination of orotidine phosphate gives cytidine monophosphate (Fig. 75). It can be seen that the pentose intermediate in pyrimidine nucleotide biosynthesis is PRPP, the same as for purine nucleotide biosynthesis. 

The base transfer reaction probably involves an enzyme-deoxyribose intermediate which is not susceptible to attack by phosphate or water. The reaction is not phosphate-dependent and deoxyribose 1-phosphate and deoxyribose are inactive in the system. A net synthesis of deoxyriboside does not take place. The activity of the enzyme is summarized in Fig. 3. Nucleoside phosphorylase does not catalyze this type of transfer reaction (6S). In this case an enzyme-pentose intermediate can be postulated which is only susceptible to attack by phosphate. 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

MORE NADPH THAN RmOSE-5-P IS NEEDED BY THE CELL Large amounts of N/VDPH can be supplied for biosynthesis without concomitant production of ribose-5-P, if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates. As shown in Figure 23.39, this alternative involves a complex interplay between the transketolase and transaldolase reac-... 

In the Webb and Levy test (60) for 2-deoxy pentoses, the same hydroxyaldehyde intermediate (4) (30), formed by treating the sugars with trichloracetic acid, reacts with p-nitrophenylhydrazine to yield the pyridazinium salt (5) from which a quinonoid dye (6) absorbing at 560 m/i, is formed in alkaline medium. 

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

A number of lyases are known which, unlike the aldolases, require thiamine pyrophosphate as a cofactor in the transfer of acyl anion equivalents, but mechanistically act via enolate-type additions. The commercially available transketolase (EC 2.2.1.1) stems from the pentose phosphate pathway where it catalyzes the transfer of a hydroxyacetyl fragment from a ketose phosphate to an aldehyde phosphate. For synthetic purposes, the donor component can be replaced by hydroxypyruvate, which forms the reactive intermediate by an irreversible, spontaneous decarboxylation. 

Pyruvate-dependent lyases serve catabolic functions in vivo in the degradation of sialic acids and KDO (2-keto-3-deoxy-manno-octosonate), and in that of 2-keto-3-deoxy aldonic acid intermediates from hexose or pentose catabolism. 

Derivatives of trioses, tetroses, and pentoses and of a seven-carbon sugar (sedohepmlose) are formed as meta-bohc intermediates in glycolysis and the pentose phosphate pathway. Pentoses are important in nucleotides. 

D-Ribose Nucleic acids. Structural elements of nucleic acids and coenzymes, eg, ATP, NAD, NADP, flavo-proteins. Ribose phosphates are intermediates in pentose phosphate pathway. ... 

Figure 25-7. Metabolism of adipose tissue. Hormone-sensitive lipase is activated by ACTH, TSH, glucagon, epinephrine, norepinephrine, and vasopressin and inhibited by insulin, prostaglandin E, and nicotinic acid. Details of the formation of glycerol 3-phosphate from intermediates of glycolysis are shown in Figure 24-2. (PPP, pentose phosphate pathway TG, triacylglycerol FFA, free fatty acids VLDL, very low density lipoprotein.)...

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. 

Very little work on the synthesis of pentitols had been carried out before 1928. Those pentitols which had been synthesized were made by the reduction of pentoses. Several pentitol intermediates had been synthesized" and it seemed desirable to attempt to complete the synthesis of pentitols from the unsaturated intermediates, using the methods which had been successfully employed in the synthesis of hexitols. 

Neher and Lewis177 obtained 2-furaldehyde from 2,3,4-tri-O-methyl-L-arabinose by heating with dilute acid after preliminary enolization with alkali. Isbell83 proposed a mechanism for this conversion similar to that for the conversion of tetra-0-methyl-(2-hydroxy-D-glucal) into 5-(hydroxymethyl)-2-furaldehyde XLIV was suggested as an intermediate. In Hurd and Isenhour s178 scheme for the formation of 2-furaldehyde from free pentose, the enol (XLV) of a 3-deoxypentosone was regarded as an inter-... 

It has been suggested15 that apiose [3-(hydroxymethyl)-D-g cero-tetrose] (LVII), a branched-chain pentose,47 originates from the aldol reaction of dihydroxyacetone with glycolaldehyde. The origin of this and all other branched-chain sugars so far encountered in natural products is uncertain, but they may arise from intermediate branched-chain carboxylic acids which are believed to be formed in the fixation of carbon dioxide (see above). 

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]. 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

The C-glycosylation of pentose glycals with silylacetylenes or allylsilanes through oxocarbenium ion intermediates proceeds with high regio- and stereo-selectivity, giving the 1,4-anti compounds as the main products. ... 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

A positive AG for this reaction conld also be achieved by a marked decrease in the concentration of glucose 6-phosphate. Why is this not feasible Glucose 6-phosphate is an important metabolic intermediate and is involved in several metabolic pathways (e.g. glycogen synthesis, glycolysis, pentose phosphate pathway). Lowering its concentration by the two orders of magnitude, which would be necessary, would markedly decrease the rates at which these important pathways could proceed. 

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