Ketopentose phosphate

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. 

Valuable information concerning the carbohydrates formed during steady state photosynthesis has been obtained by using cellulose chro matography in conjunction with radioactive carbon (29-32,57,58,235). During the flrst few seconds of photosynthesis there is concurrent formation of fructose and sedoheptulose phosphate followed by ketopentose phosphate and later by glucose phosphates (32,57). Examination of many plants shows sedoheptulose to be of widespread occurrence (31,192). 

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. 

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). 

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). 

Preparation 52), The ketopentose has been synthesized from D-arabinose by isomerization with pyridine followed by isolation as the crystalline o-nitrophenylhydrazone. The free sugar has not been crystallized. The D-erythro-pentu ose 5-phosphate may be obtained by treatment of D-glu-conic acid 6-phosphate with yeast enzymes (Chapter XIII). 

TPP is a coenzyme for transketolase, the enzyme that catalyzes the conversion of a ketopentose (xylulose-5-phosphate) and an aldopentose (ribose-5-phosphate) into an aldotriose (glyceraldehyde-3-phosphate) and a ketoheptose (sedoheptulose-7-phosphate). Notice that the total number of carbons in the reactants and products is the same (5 + 5 = 3 + 7). Propose a mechanism for this reaction. 

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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