Seven-carbon sugars

Phenylalanine- and Tyrosine-Derived Alkaloids. Carbohydrate metaboHsm leads via a seven-carbon sugar, ie, a heptulose, derivative to shikimic acid [138-59-0] (57), C H qO, which leads in turn to prephenic acid [126-49-8] (58), (43). 

P, the seven-carbon sugar serving as the transketolase substrate. Likewise, phosphoribulose kinase carries out the unique plant function of providing RuBP from Ru-5-P (reaction 15). The net conversion accounts for the fixation of six equivalents of carbon dioxide into one hexose at the expense of 18 ATP and 12 NADPH. 

Derivatives of trioses, tetroses, and pentoses and of a seven-carbon sugar (sedohepmlose) are formed as meta-bohc intermediates in glycolysis and the pentose phosphate pathway. Pentoses are important in nucleotides. 

Recently, a new diagnostic marker was confirmed for transaldolase deficiency. The seven carbon sugars sedoheptulose and mannoheptulose are highly elevated in urine and are specific biomarkers for transaldolase deficiency [12]. 

In the second phase, transaldolase (with TPP as cofactor) and transketolase catalyze the interconversion of three-, four-, five-, six-, and seven-carbon sugars, with the reversible conversion of six pentose phosphates to five hexose phosphates. In the carbon-assimilating reactions of photosynthesis, the same enzymes catalyze the reverse process, called the reductive pentose phosphate pathway conversion of five hexose phosphates to six pentose phosphates. 

Mycaminose has been found as a component of carbomycin (Magna-mycin)83 and, structurally, it resembles desosamine84 (see Table II). On periodate oxidation, one carbon atom is split out, to yield a seven-carbon amino sugar the results of subsequent reaction with the oxidant indicated the presence of a hydroxyl group at C4. Alkaline deamination of the seven-carbon sugar proceeded at a rate similar to that of 2-amino-2-deoxy-D-glucose, whereas mycaminose itself reacted far more rapidly this indicates the dimethylamino group to be at C3. Mycaminose is believed to be 3,6-dideoxy-3-dimethylamino-/3-D-altrose.84a... 

Figure 20.10. Formation of Five-Carbon Sugars. First, transketolase converts a six-carbon sugar and a three-carbon sugar into a four-carbon sugar and a five-carbon sugar. Then, aldolase combines the four-carbon product and a three-carbon sugar to form a seven-carbon sugar. Finally, this seven-carbon fragment combines with another three-carbon fragment to form two additional five-carbon sugars.

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 answer is e. (Murray, pp 219-229. Scrivei pp 1521-1552. Sack, pp 121-138. Wilson, pp 287-317.) The pentose phosphate pathway generates reducing power in the form of NADPH in the oxidative branch of the pathway and synthesizes five-carbon sugars in the nonoxidative branch of the pathway. The pentose phosphate pathway also carries out the interconversion of three-, four-, five-, six-, and seven-carbon sugars in the nonoxidative reactions. The final sugar product of the oxidative branch of the pathway is ribulose-5-phosphate. The first step of the nonoxidative branch of the pathway is the conversion of ribulose-5-phosphate to ribose-5-phosphate or xylulose-5-phosphate in the presence of the enzymes phosphopentose isomerase and phosphopentose epimerase, respectively. Thus ribulose-5-phosphate is a key intermediate that is common to both the oxidative and nonoxidative branches of the pentose phosphate pathway. 

With the relatively minor structural modification involved in the transition from Oglycosides to C-glycosides comes the more involved question of how to name these compounds. Figure 1.1.2 shows a series of C-glycosides with their corresponding names. Compound 1 derives its name from the longest continuous chain of carbons. Since this is a seven carbon sugar, the name will be derived from heptitol. As the heptitol is cyclized between C2 and... 

Aldoses with four, five, six, and seven carbon atoms are called aldotetroses, aldopentoses, aldohexoses, and aldoheptoses, respectively. The corresponding ketoses are ketotetroses, ketopentoses, ketohexoses, and ketoheptoses. Six-carbon sugars are the most abundant in nature, but two five-carbon sugars, libose and deoxyribose, occur in the structures of RNA and DNA, respectively. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolic pathways. 

The authors next turn their attention to the pentose phosphate pathway, which is common to all organisms. The role of the pentose phosphate pathway is to produce NADPH, which is the currency of reducing power utilized for most reductive biosyntheses. In addition, this pathway generates ribose 5-phosphate needed for DNA synthesis and can produce various three-, four-, five-, six-, and seven-carbon sugars. 

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