Ketopentose-ribulose

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). 

Lower monosaccharides, i.e., aldo- and keto-bioses, -trioses, and -tetroses, do not exist naturally in a free state. Glyceroaldehyde and hydroxyacetone in phospho-rylated forms are the products of alcoholic fermentation and glycolytic sequence. Erythrose and erythrulose also appear in phosphorylated forms in the pentose cycle of glucose, while ketopentose-ribulose can be found as its phosphate ester (Table 5.1). 

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... 

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). 

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). 

In this phase, ribulose 3-phosphate is converted to glucose-6-phosphate. Stoichiometrically, this requires the rearrangement of six molecules of ketopentose phosphate to five molecules of aldohexose phosphate (Figure 15-20). It has been claimed that the pathway shown in Figure 15-20 occurs primarily in fat tissue and that a modified pathway involving arabinose 5-phosphate and octulose 8-phosphate occurs in liver cells. The overall scheme of the nonoxidative phase should be considered tentative. 

Write the reactions of D-ribulose (a ketopentose) with a. Tollens reagent b. NaBH4... 

The D-2-pentuloses, o-erythro-l-ptn lulose (D-ribulose, 12) and o-threo-l-pentulose (D-xylulose, 13) are able to cyclize as 2-pentulofuranoses. The 2-ketopentoses are important metabolic intermediates in glycolysis.11... 

D-Ribulose. D. erythro.2-Pentulosc D-adonose D-erythro-2-ketopentose. C5Hl0O5 mol wt 150.13. C... 

Pentoses aldoses containing five carbon atoms. P. are an important group of monosaccharides (see Carbohydrates). Naturally occurring P. include d- and l-arabinose, L-lyxose, D-xylose, D-ribose and 2-deoxy-D-ribose, and the ketopentoses (pentuloses) o-xylu-lose and D-ribulose. P. occur chiefly in the furanose form. They are not fermented by the usual yeasts. By distillation with dilute acids, P. are converted into furfural, a reaction which serves for the detection of P. and their differentation from hexoses. 

There are obviously many aldose monosaccharides and many ketose monosaccharides, but the discussion will focus only on the d diastereomers, as with the aldoses. The triose is l,3-dihydroxy-2-propanone (41 also called glycerone) and the tetrose is d-glycero-tetrulose (42). Two pentoses are named d-ribulose (43) and d-xylulose (44) and four hexoses are named d-psicose (45), d-fructose (46), d-sor-bose (47), and d-tagatose (48). All of these compounds are ketoses and are further classified according to the number of carbon atoms, as noted in Section 28.1. For example, 42 is a ketotetrose, 43 is a ketopentose, and 45 is a ketohexose. For 43-48, cyclization is possible to form a hemiketal. Just as aldehydes and alcohols react to form a hemiacetal, ketones and alcohols react to form a hemiketal (see Chapter 18, Section, 18.6). For 43, a five-membered ring (a furanose) is formed if the terminal CH2OH unit (in violet) reacts with the ketone carbonyl. The two anomers formed are 49 (a-d-ribulofuranose) and 50 (P-d-ribulofuranose). 

Most recently the enzyme of yeast catalyzing the cleavage of ribulose-5-phosphate has been crystallized and designated transketolase. " The pentose phosphate isomerase has been removed from the crystalline transketolase, which maintains its activity on ribulose-5-phosphate. The purified enzyme also catalyzes the formation of the ketopentose phosphate from glyceraldehyde-3-phosphate and a 2-carbon donor such as hy-droxypyruvate. Transketolase contains thiamine pyrophosphate as a coenzyme. 

The mono- and diphosphates of ribulose have been reported to be present among early products of photosynthetic CO2 fixation in algae, e.g., Scenedeamus. A pathway for the origin of this diphosphate has not yet been suggested, although Marmur and Schlenk have described the aldolase-catalyzed combination of dihydroxyacetone phosphate and glycolaldehyde phosphate to form what must have been a ketopentose diphosphate. ... 

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