Sugar phosphates sedoheptulose-7-phosphate

Ribulose-5-phosphate (3.13) can be converted to ribose-5-phosphate (3.14) and xylulose 5-phosphate (3.15), by the enzymes ribose-5-phosphate isomerase and ribulose 5-phosphate 3-epimerase, respectively. The two pentose-phosphate molecules, 3.14 and 3.15, are converted to a C3 and a C7 sugar-phosphate, glyceraldehyde 3-phosphate (3.4) and sedoheptulose-7-phosphate (3.16), respectively, via the action of atransketolase. 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

As shown in Figure 12-2, the 7-carbon sugar, sedoheptulose-7-phosphate, and the 3-carbon sugar, glyceraldehyde-3-phosphate, react again, in a reaction catalyzed by transaldolase (3-carbon transfer) ... 

The remainder of the pathway consists of a series of isomerase and group tranx/er reactions (Fig. 11-26). These produce sugar phosphates ranging in size from the 3-carbon glyceraldehyde 3-phosphate to the 7-carbon sedoheptulose 7-phosphate. Also derived from ribulose 5-phosphate is ribose 5-phosphate, an essential component of ribonucleosides and ribonucleotides. The group exchange reactions are catalyzed by transaldolase... 

After a time, several other sugar phosphates were identified. Most important among these were the seven-carbon compounds, sedoheptulose-7-phosphate (IX) and sedoheptulose-l,7-diphosphate (SDP) (X), and the five-carbon compounds, ribulose-l,5-diphos-phate (RuDP) (II) and ribose-5-phosphate (XI), xylulose-5-phosphate (XII), and ribulose-5-phosphate (I) (Benson, et al., 1952). The roles of these compounds in the path of carbon in photosynthesis became more clear after they had been degraded to locate the position of radiocarbon atoms within the individual molecules (Bassham et al., 1954). 

Transketolase requires an acceptor it cannot split a ketol to form aldehydes. Presumably an intermediate active glycolaldehyde" is formed by combination of the two-carbon unit with the enzyme. The enzyme will react with a number of aldehydes, and the reactions in many cases have been shown to be reversible. The donor requirements are not completely understood. Among the sugars only those with the hydroxyl on carbon 3 on the l side are substrates. Before the discovery of epimerase, ribulose-5-phosphate was thought to be a substrate, but it has been shown that a mixture of ribose and ribulose phosphates does not undergo the transketolase reaction until epimerase is added. Besides xylulose phosphate, fructose-6-phosphate, sedoheptulose-7-phosphate, and hydroxypyruvate are glycolaldehyde donors. 

Transaldolase. When ribose-5-phosphate is the acceptor in the transketolase reaction, the product is the 7-carbon sugar, sedoheptulose-7-phosphate. The further metabolism of this compound was found to involve another type of transfer reaction, in which a dihydroxyacetone group is shifted to a phosphoglyceraldehyde molecule. The products of this transfer are fructose-6-phosphate and a 4-carbon sugar, erythrose-4-phosphate (VI). The transfer of a dihydroxyacetone group resembles the... 

A rapid, high yield synthesis of C-enriched intermediates of the pentose-phosphate pathway has been developed based on a combination of chemical and enzymic reactions. [l- C]Ribose and [l- C]arbinose 5-phospha,tes, available by the classical Kiliani method, were converted to a variety of specifically labelled 5-, 6-, 7-, and 8-carbon sugar phosphates (e.g.. D-erythro-pentulosc 5-phosphate, sedoheptulose mono- and di-phosphates) with the help of aldolase, transaldolase, and transketolase. ... 

This enzyme [EC 2.7.1.11], also known as phosphohexo-kinase and phosphofructokinase 1, catalyzes the reaction of ATP with D-fructose 6-phosphate to produce ADP and D-fructose 1,6-bisphosphate. Both D-tagatose 6-phosphate and sedoheptulose 7-phosphate can act as the sugar substrate. UTP, CTP, GTP, and ITP all can act as the nucleotide substrate. This enzyme is distinct from that of 6-phosphofructo-2-kinase. See also ATP GTP Depletion... 

The requirement for NADPH far exceeds an equal requirement for ribose 5-phosphate (necessary for the production of nucleic acids and nucleotides), and so the second phase of the pentose phosphate pathway converts the C5 sugar, by a series of reversible reactions, into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. This interconversion is shown in Fig. 11-27. Not only does the second phase of the pathway conserve all the carbon atoms of the C5 sugar, but it produces erythrose 4-phosphate (C4), xylulose 5-phosphate (C5), and sedoheptulose 7-phosphate (C7), which are available to other metabolic processes. 

The sugar metabolism is a source of many enzymes, the transketolase (TK) being one of them. TK transfers an a-hydroxy carbonyl fragment from D-xylu-lose-5-phosphate onto D-ribose-5-phosphate, forming D-sedoheptulose-7-phos-phate and D-glyceraldehyde-3-phosphate (Scheme 5.14). Since this reaction is an equilibrium reaction and starting materials and products are of similar stability, it is not very versatile for organic synthesis. Fortunately TK also accepts pyruvate instead of xylulose. Under these modified circumstances carbon dioxide... 

Until recent years, the only reducing sugars of the 7-carbon series to have been found in Nature were two ketoheptoses these were o-manno-heptulose and D-aifro-heptulose (sedoheptulose), both of which were isolated from plants, Heptulose phosphates have now been recorded from several sources, for example, from yeast, from early products of photo-... 

Transaldolase transfers a 3-carbon keto fragment from sedoheptulose 7-phos-phate to glyceraldehyde 3-phosphate to form erythrose 4-phosphate and fructose 6-phosphate (Fig. 29.9). The aldol cleavage occurs between the two hydroxyl carbons adjacent to the keto group (on carbons 3 and 4 of the sugar). This reaction is similar to the aldolase reaction in glycolysis, and the enzyme uses an active amino group, from the side chain of lysine, to catalyze the reaction. 

Two enzymes, transhetolase and transaldolase, are responsible for the reshuffling of the carbon atoms of sugars such as ribose-5-phosphate and xylulose-5-phosphate in the remainder of the pathway, which consists of three reactions. Transketolase transfers a two-carbon unit. Transaldolase transfers a three-carbon unit. Transketolase catalyzes the first and third reactions in the rearrangement process, and transaldolase catalyzes the second reaction. The results of these transfers are summarized in Table 18.2. In the first of these reactions, a two-carbon unit from xylulose-5-phosphate (five carbons) is transferred to ribose-5-phosphate (five carbons) to give sedoheptulose-7-phosphate (seven carbons) and glyceraldehyde-3-phosphate (three carbons), as shown in Figure 18.15, bottom, red numeral 1. 

In the event, as there is now one more carbon in erythrose 4-phosphate than in glyceraldehyde 3-phosphate, a seven-carbon sugar, sedoheptulose 1,7-bisphosphate,... 

The presence of a ketone group in a sugar is usually indicated by the ending -ulose, e.g. sedoheptulose, a sugar containing seven carbon atoms and a ketone group. This is formed as an intermediate in the pentose phosphate pathway of carbohydrate metabolism (Figure 17.2). 

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