D-Ketohexoses

The family of D-ketoses, shown in Figure 27.5, is formed from dihydroxyacetone by adding a new carbon (bonded to H and OH) between C2 and C3. Having a carbonyl group at C2 decreases the number of stereogenic centers in these monosaccharides, so that there are only four D-ketohexoses. The most common naturally occurring ketose is D-fructose. 

Problem 27.12 Referring to Figure 27.5, which D-ketohexoses have the S configuration at C3 ... 

D. L. Bissett and R. L. Anderson, Isolation and properties of a class I D-ketohexose-1,6-diphosphate aldolase that catalyses the cleavage of D-tagatose-1,6-diphosphate, J. Biol. Chem., 255 (1980) 8750-8755. 

Ans. D-Psicose, D-fructose, D-sorbose, and o-tagatose are stereoisomers because all four have the same connectivity. Since each is neither superimposable nor the mirror image of the others, the four D-ketohexoses are diastereomers. 

Figure 24.6 summarizes the family of d ketoses that have between three and six carbon atoms. This family tree is constructed in much the same way as the family tree of aldoses, beginning with dihydroxyacetone as the parent. However, because the carbonyl group is at C2 rather than C1, ketoses have one less chirality center than aldoses of the same molecular formula. As a result, there are only four D ketohexoses, rather than eight. The most common naturally occurring ketose is D-fructose. Each compound in Figure 24.6 has a corresponding l enantiomer that is not shown. 

There are four D ketohexoses called D-psicose, D-fructose, D-sorbose, and D-tagatose. 

Ketoses also play a pivotal role in metabolism. The Fischer projections of the D-ketotetroses, D-ketopentoses, and D-ketohexoses are shown in Figure 26.3. The parent ketose is the ketotriose dihydroxyacetone. We can construct ketoses from dihydroxyacetone by inserting chiral centers (H—C—OH) one at a time between the ketone carbonyl carbon atom and the carbon atom directly below it. 

D-Ketohexose-3-epimerase, a new enzyme isolated from Pseudomonas sp., catalysed the epimerization between D-tagatose and D-sorbose, D-fructose and D-psicose, D-xylulose and D-ribulose, and between L-xylulose and L-ribulose. ... 

D Fructose (a 2 ketohexose also known as levulose it IS found in honey and IS signficantly sweeter than table sugar)... 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

The in vitro bioassays allowed to determine the inhibition constant of D-fructose transport by the CHO cells. This measure is carried out by competition with radioactive D-fructose. The study put in evidence that pentose-OZT derivatives are not recognized by the protein transporter. Only the ketohexose-OZT derivatives expressed some inhibition of GLUT5. These inhibition constants showed to be much effective with L-Sor derivatives than with D-Fru derivatives and even better than D-fructose itself (Kt = 15.5 mM) (Table 2). 

These blocked structures gave a picture of the interaction of D-fructose with the protein. It is assumed that a ketohexose C-l additional arm (absent in pentose series), is required in order to achieve some inhibition, but a certain freedom of substitution is possible (OBn, OA11 or OH). Comparison of D-Fru- and L-Sor-OZTs showed that substituted derivatives are better... 

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

The substantial amounts of this ketohexose are mainly prepared by base-catalyzed isomerization of starch-derived glucose, yet may also are generated by hydrolysis of inulin, a fructooligosaccharide. An aqueous solution of fructose—consisting of a mixture of all four cyclic tautomers (Figure 2.5), of which only the (3-D-pyranose ((3-p) form present to about 73% at room temperature is sweet — about 1.5 times sweeter than an equimolar solution of sucrose hence, it is widely used as a sweetener for beverages ( high fructose syrup ). 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

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). 

Problem 22.34 (a) Write shorthand structures for the three o-2-ketohexoses other than o-fructose. (b) Which one does not give a meso alditol on reduction (c) Which one gives the same osazone as does D-galactose (Problem 22.22)7 <... 

Problem 22.57 (a) Write shorthand open structures for a ketohexose (I) and an aldohexose (II) which form the same osazone as D-(-)-altrose (III), the C epimer of o-mannose, and for the aric acid (IV) formed by HNOj oxidation of the o-aldohexoses (III) and o-talose (V). (b) Write shorthand and conformational structures for -D-altropyranose (VI). ... 

The ketohexoses identified as components of bacterial polysaccharides include D-fructose, found in Vibrio lipopolysaccharide167 and Hemophilus influenzae capsular polysaccharide,252 and D-xylulose (d- riireo-2-pen-tulose), identified in Pseudomonas lipopolysaccharide.253 Activated forms of the monosaccharides were not determined. 

The common six-carbon sugars (hexoses) are D-glucose, D-fructose, D-galactose, and D-mannose. They all are aldohexoses, except D-fructose, which is a ketohexose. The structures of the ketoses up to Cf) are shown for reference in Figure 20-2. The occurrence and uses of the more important ketoses and aldoses are summarized in Table 20-1. 

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