D-Sedoheptulose

Transketolase (TKase) [EC 2.2.1.1] essentially catalyzes the transfer of C-2 unit from D-xylulose-5-phosphate to ribose-5-phosphate to give D-sedoheptulose-7-phosphate, via a thiazolium intermediate as shown in Fig. 16. An important discovery was that hydroxypyruvate works as the donor substrate and the reaction proceeds irreversibly via a loss of carbon dioxide (Fig. 17). In this chapter, we put emphasis on the synthesis with hydroxypyruvate, as it is the typical TPP-mediated decarboxylation reaction of a-keto acid. ... 

D-xylulose 6-phMphate D-ribo e 5-pho phate HjC- OPO,Hj D-sedoheptulose 7 >lMMphate D lycerialdahydt 3-pfioiphate... 

Yamabe N, Kang KS, Park CH, Tanaka T, Yokozawa T. (2009) 7-0-galloyl-D-sedoheptulose is a novel therapeutic agent against oxidative stress and advanced glycation endproducts in the diabetic kidney. Biol Pharm Bull 2il 657-664. 

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 hydroxyethylthiamine pyrophosphates are potent nucleophiles and may add to carbonyl compounds to form carbon-carbon bonds. A good illustration of carbon-carbon bond making and breaking occurs in the reactions of transketo-lase. The enzyme contains tightly bound thiamine pyrophosphate and shuttles a dihydroxyethyl group between D-xylulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate (equations 2.55 and 2.56). 

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... 

D-Ribose 5-phosphate D-Xylulose 5-phosphate D-Sedoheptulose 7-phosphate D-Glyceraldehyde 3-phosphate... 

G3P) and D-sedoheptulose 7-P as illustrated in Scheme 5.53. In addition D-erythrose 4-phosphate can function as the ketol acceptor thus producing D-fructose-6-P and G3P (Scheme 5.53). The enzyme relies on two cofactors for activity — thiamin pyrophosphate (TPP) and Mg2+—and utilizes the nucleophilic catalysis mechanism outlined in (Scheme 5.54).83 When TPP is used as a cofactor for nucleophilic catalysis, an activated aldehyde intermediate is formed. This intermediate functions as a nucleophile, and thus TK employs a strategy that is similar to the umpolung strategy exploited in synthetic organic chemistry. 

Scheme 5.53. Transketolase transfers the C1-C2 ketol unit of D-xylulose 5-phosphate onto D-ribose-5-phosphate or D-erythrose-4-phosphate generating D-sedoheptulose-7-phosphate or D-fructose-6-phosphate respectively. P = PO32-.

Like transketolase, transaldolase (TA, E.C. 2.2.1.2) is an enzyme in the oxidative pentose phosphate pathway. TA is a class one lyase that operates through a Schiff-base intermediate and catalyzes the transfer of the C(l)-C(3) aldol unit from D-sedoheptulose 7-phosphate to glyceraldehyde-3-phosphate (G3P) to produce D-Fru 6-P and D-erythrose 4-phosphate (Scheme 5.59). TA from human as well as microbial sources have been cloned.110 111 The crystal structure of the E. coliu and human112 transaldolases have been reported and its similarity to the aldolases is apparent, since it consists of an eight-stranded (o /(3)s or TIM barrel domain as is common to the aldolases. As well, the active site lysine residue that forms a Schiff base with the substrate was identified.14112 Thus, both structurally and mechanistically it is related to the type I class of aldolases. 

Scheme 5.59. Transaldolase catalyzed the transfer of the C1-C3 aldol unit of D-sedoheptulose-7-phosphate to D-glyceraldehyde-3-phosphate (G3P) generating D-fructose-6-phosphate and D-erythrose 4-phosphate. P = P032. ...

Despite the fact that many heptoses are by far less prominent in Nature than hexoses these monosaccharides are found both as metabolic intermediates, and as structural carbohydrates of bacterial cell walls.D-Sedoheptulose 7-phosphate is an important intermediate of the pentose cycle, and D-sedoheptulose 1,7-bisphosphate is present in plants as an intermediate of the dark phase of photosynthetic reactions. L-Glycero-D-manno-heptose was isolated from the oligosaccharides obtained by partial acid hydrolysis of the lipopolysaccharide from Escherichia coli K-12 strain W3100 [153] and Haemophilus influenzae [154]. Both L-glycero-D-wtanno-heptose and D-glycero-D-ma o-heptose were isolated from the lipopolysaccharide of Vibrio parahaemolyticus [155]. 

D-sedoheptulose 7-phosphate 4- v-glycero-trioae 3-phosphate transaldolase... 

TK is one of the enzymes involved in the oxidative pentose phosphate pathway, and requires the cofactors thiamine pyrophosphate (TPP)12191 and Mg2+[218). It reversibly transfers the C1-C2 ketol unit from D-xylulose 5-phosphate to D-ribose 5-phosphate, and generates D-sedoheptulose 7-phosphate and D-Gly 3-P. D-Erythrose 4-phosphate also functions as an acceptor of the ketol unit from D-xylulose 5-phosphate, to produce Fru 6-P and D-Gly 3-P (Fig. 14.2-1). TK from baker s yeast is commercially available, and the enzyme can also be isolated from spinach[220, 2211 TK from E. coli has been overexpressed and prepared on a large scale12221. In ketol transfer reactions,... 

TA is also an enzyme of the oxidative pentose phosphate pathway[218). It catalyzes the transfer of the C1-C3 aldol unit from D-sedoheptulose 7-phosphate to D-Gly 3-P, and produces D-Fru 6-P and D-erythrose 4-phosphate (Fig. 14.2-3). TA forms a Schiffbase intermediate and does not require any co-factors. This enzyme is commercially available, and was used in a multi-enzyme synthesis of D-Fru from starch (Fig. 14.2-4) 1233] Here, it accomplished transfer of an aldol moiety from Fru 6-P to d-glyceraldehyde, and formed D-Gly 3-P and D-Fru. 

From this inlormation, you now can work backward toward the unknown The stereocenters in D-sedoheptulose must be 35, 4R, 5R, and 6/ . 

You now know these carbons are R in sugars B, A. and D-sedoheptulose as well. 

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