Sedoheptulose 7-phosphate formation

Thiamine pyrophosphate has also recently been shown to exert a coenzyme function in the metabolism of pentose phosphate. - The formation of sedoheptulose phosphate from ribulose-5-phosphate by a highly purified enzyme preparation from spinach, which loses activity upon precipitation of the protein with ammonium sulfate at a low pH, was found to be almost completely reactivated by the addition of thiamine pyrophosphate. ... 

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). 

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

D-afe-o-Heptulose (sedoheptulose) (XXXVII) has been synthesized from D-erythrose (XXXVIII) plus triose phosphate, using an aldolase preparation from peas.169 Aldolases from yeast and from rat liver also form heptu-lose phosphate from these substrates.7S(o) 170(a) Crystalline muscle aldolase causes the formation of L-jrZwco-heptulose (XXXVIIa) from a mixture of L-erythrose (XXXVTIIa) and hexose diphosphate.170(b)... 

Formation of a possible precursor of 9, namely, o-glycero-D-manno-heptose 7-phosphate, from D-sedoheptulose 7-phosphate was demonstrated in Salmonella typhimurium.238... 

The first part of the reaction is formation of a protonated Schiff base r)f sedoheptulose 7-phosphate with a lysine residue in the enzyme followed by a retro-aldol cleavage to give an enamine plus erythrose 4-j>hosphatc. Show the structure of the enamine and the mechanism by which it is formed. Hie second part of the reaction is nucleophilic addition of the enamine to glyceraldehyde 3-iihosphate followed by hydrolysis of the Sch ff base to give fructose 6-phosphate. Show the mechanism. 

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. 

The action of an enzyme, transketolase, from rat liver or from spinach, on pentose 5-phosphate results in the formation of a triose phosphate and an ester of sedoheptulose, which is presumably sedoheptulose 7-phosphate. The heptulose has been identified by the preparation of sedoheptulosan tetrabenzoate (227). 

Evidence from a number of sources indicated that pentose phosphates were metabolized in a series of reactions that resulted in the formation of hexose monophosphates and hexose diphosphates. Several enzyme steps are involved in these transformations. The reaction between D-ribulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate is catalyzed by an enzyme known as transketolase (91). This enzyme is found in plant, animal, and bacterial cells. Thiamine pyrophosphate (TPP) and Mg ions are required as cofactors. The mechanism of the reaction was suggested (92) as shown in reaction (28). 

From my laboratory and the laboratories of Ef Racker, Frank Dickens and Melvin Calvin, the years that followed witnessed a series of parallel and often highly synergistic discoveries on the nature of the pentose phosphate pathway and the path of carbon in photosynthesis. Andrew Benson and others in Calvin s laboratory, had shown that phosphate esters of ribulose and sedoheptulose were early products of CO2 fixation in photosynthesis,and the immediate precursor of phosphoglyceric acid, and therefore the primary CO2 acceptor, appeared to be ribulose diphosphate. The major problems became (1) to find the enzyme or enzymes that catalyzed the formation of phosphoglyceric acid from ribulose diphosphate and (2) to define the reactions leading to the synthesis of ribulose diphosphate from triose and hexose phosphates. 

We had already shown that sedoheptulose 7-phosphate was an intermediate in the conversion of pentose phosphate to hexosemonophosphate in crude liver extracts, since it accumulated early in the reaction and then disappeared, coincident with the formation of hexose phosphate. The enzyme that catalyzed the interconversion of sedoheptulose 7-phosphate and fructose 6-phosphate was discovered in my laboratory in 1953, and with Paul Marks and Howard Hiatt, we showed that it catalyzed a Cj-transfer from the substrate to a suitable aldehyde donor. This led to the discovery of another new phosphate ester, erythrose 4-phospWe, formed in the Cs-transfer from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate. Finally, erythrose 4-phosphate could be shown to be the acceptor for a second reaction catalyzed by transketolase, with xylulose 5-phosphate as the donor. All of the steps in the pentose phosphate pathway had now been elucidated (Fig. 2). Erythrose 4-phosphate was also shown to condense with glyceraldehyde 3-phosphate to... 

An alternative reaction to the hydrolysis of fructose 1,6-diphosphate is the condensation of glyceraldehyde 3-phosphate and erythrose 4-phosphate to form sedoheptulose 1,7-diphosphate, which is hydrolyzed by a specific phosphatase. However, this would not account for the formation of stan and other assimilation products. 

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