3-Ketopentose

L Xylulose (a 2 ketopentose excreted in excessive amounts in the urine of persons afflicted with the mild genetic disorder pentosuria)... 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

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. 

Phosphoric acid esters of the ketopentose D-ribulose (2) are intermediates in the pentose phosphate pathway (see p.l52) and in photosynthesis (see p.l28). The most widely distributed of the ketohexoses is D-fructose. In free form, it is present in fruit juices and in honey. Bound fructose is found in sucrose (B) and plant polysaccharides (e.g., inulin). 

The last enzyme in the oxidative part is phosphogluconate dehydrogenase [3], which releases the carboxylate group of 6-phosphogluconate as CO2 and at the same time oxidizes the hydroxyl group at C3 to an 0x0 group. In addition to a second NADPH+H", this also produces the ketopentose ribulose 5-phosphate. This is converted by an isomer-ase to ribose 5-phosphate, the initial compound for nucleotide synthesis (top). 

Pfitzner and Moffatt noted24 that a severe limitation existed for the synthesis of ketopentose nucleosides owing to their instability in certain media, which was attributed to loss of the nitrogenous base by elimination reactions.2... 

The reduction of ketopentose-, as well as ketohexose-, nucleosides with metal hydrides has been used to obtain biologically important nucleosides and rare sugar nucleosides, by epimerization of a chiral center. Moreover, reduction of unsaturated ketonucleosides with borohydride provides new and direct routes to unsaturated and deoxy-nucleosides. 

The simplest carbohydrates are the monosaccharides which under specified conditions are structurally characterised as polyhydroxy aldehydes or polyhydroxy ketones these are termed aldoses and ketoses respectively. Aldoses and ketoses are sub-classified, according to the number of carbons present in each molecule, into aldotetroses, aldopentoses, aldohexoses, etc., or ketotetroses, ketopentoses, etc. 

A more detailed study on the structure of D-glucose based upon physical data and chemical reactivity has revealed that the open-chain formulation of the aldopentoses, aldohexoses, ketopentoses and ketohexoses is however an oversimplification. Thus, for example, in solution D-glucose exists as an equilibrium... 

Since the compound hydroxymethyl furfural is a major product of ketohexose dehydration, its presence can be used as an indication that caramel has been added as a coloring agent in a nonheated food system (66). In contrast, ketopentose dehydration produces furfural. Another dehydration product, bis-5,5 -formylfurfurly ether, can also be used to confirm the addition of caramel color (66). 

How many stereoisomeric aldopentoses are possible How many stereoisomeric 2-ketopentoses are possible ... 

Although dihydroxyacetone does not possess a chiral carbon atom, the simple ketoses are related to it structurally by the introduction of hydroxylated chiral carbon atoms between the keto group and one of the hydroxymethyl groups. Thus there are two ketotetroses, four ketopentoses, and eight ketohexoses. 

Sugars are further classified by the number of carbons. Thus, aldoses include aldotrioses (C3 D-glyceraldehyde, HO—CH2—GHO), aldotetroses (Gy D-erythrose) aldopentoses (C-, D-ribose, D-arabinose, D-xylose) and aldohexoses (Ce D-glucose, D-mannose, D-gulose, D-galactose). Ketoses include ketotrioses (Gy dihydroxyacetonephosphate, HO-CH2-CO— CH2OH), ketotetroses (C D-erythrulose), ketopentoses (C3 D-ribulose, D-xylulose) and ketohexoses (C6 D-fructose). 

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