Pentoses formation

The rate of pentose formation from pentosan is proportional to the pentosan concentration, but is diminished by the sequence reaction of pentose to furfural, so that... 

Table II presents the quantitative results of those components volatile enough for GC analysis. At low pH the furan compounds predominate when both glucose and xylose are exposed to 300 C. This is not unexpected since all pentoses form 2-furaldehyde(2) in high yield when exposed to aqueous acid solution( ). However, the presence of 2 in the glucose reaction mixture is of interest. The major product obtained from hexoses at elevated temperatures and aqueous acid is 5-hydroxymethyl-2-furaldehyde(1) with minor amounts of 2-(hydroxyacetyl)furan(15). The 2-furaldehyde has been detected after acidic treatment of fructose(1 ), glucose(15,17), and is a major component after the thermolysis of cellulose in distilled water( ). One plausible explanation for the formation of 2 may involve loss of formaldehyde(18) from glucose with consequent pentose formation. It should be noted that the pyrolysis of 1 does produce a small amount of 2( ). However, the reaction conditions are sufficiently different to suggest a different mechanism for hydrothermolysis.

Potentially the cyclitol pathway could be more elaborate, with various epimerisation or substitutions occurring at the cyclitol level and not by way of sugar nucleotides. As Loewus has pointed out, any blockade at C-1 of myoinositol would eventually prevent decarboxylation of the uronic acid and pentose formation, while allowing UDPGalA to form. However, there is little evidence directly for or against such mechanisms. Nevertheless, the cyclitol pathway is extremely important as a major route and part of a potential regulatory system in plants, as the evidence of Rubery and Northcote (1970) shows (see Chapter 5). 

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). 

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

Glycolysis, the pentose phosphate pathway, and fatty acid synthesis are all found in the cytosol. In gluconeo-genesis, substrates such as lactate and pyruvate, which are formed in the cytosol, enter the mitochondrion to yield oxaloacetate before formation of glucose. 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver. 

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.)...

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-... 

Challenger and coworkers36 had hoped to obtain pyromeconic acid (XXVI) from pentoses (XXV) with A. oryzae, by analogy with the formation of kojic acid from hexoses. Instead, kojic acid was produced from L-arabinose and D-xylose. Corbellini and Gregorini,31 too, observed the... 

Intramolecular cycloaddition of nitrile oxides, prepared from 1,2-isopropy-lidene-protected ether-linked oligo-pentoses leads to the diastereoselective formation of chiral isoxazolines fused to 10-16-membered oxa-cycles (456). 

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 ... 

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

By careful choice of both the reaction conditions and the phenolic compound used, it may be possible to produce a colour that is characteristic of a particular carbohydrate or related group, so giving some degree of specificity to the method. Thus, Seliwanoff s test uses hydrochloric acid and either resorcinol or 3-indolylacetic acid to measure fructose with minimal interference from glucose. The colour produced by pentoses with orcinol (Bial s reagent) or p-bromoaniline is sufficiently different from that produced by hexoses to permit their quantitation in the presence of hexoses. However, none of the methods based on the formation of furfural or its derivatives can be considered to be entirely specific. 

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. 

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