Ketoses configurations

For 2-ketoses, configurational prefixes are given in the same way as for aldoses (see 2-Carb-8.2 and 2-Carb-8.3). 

Aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers (Chapter 4). The nomenclature for such molecules must specify the configuration about each asymmetric center, and drawings of these molecules must be based on a system that clearly specifies these configurations. 

FIGURE 7.3 The Structure and stereochemical relationships of D-ketoses having three to six carbons. The configuration in each case is determined by the highest numbered asymmetric carbon (shown in gray). In each row, the new asymmetric carbon is shown in red. 

The natural acceptor aldehyde can be considerably varied among phosphorylated as well as unphosphorylated hydroxyaldehydes, which are both converted at comparable rates (Table 5)13-44 47. Although the catalytic reaction creates only a single stereocenter, the enzymes from yeast or spinach efficiently distinguish between adjacent configurations with preference for (3SAR)-i>yn isomeric ketose products44 47, which nicely parallel those derived from FruA reactions (Section 1.3.4.6.1). 

Restrictions for the substrates of the transketolase-catalyzed reaction only arise from the stereochemical requirements of the enzyme. The acceptor aldehyde must be formaldehyde9,20, glycolaldehydel6,17 or a (R)-2-hydroxyaldehyde10,17. The donor ketose must exhibit a (3(7,4 R) configuration10. The enzyme selectively adds the hydroxyacetyl moiety to the Re-face of the acceptor aldehyde leading to a 3(7 configuration of the products. 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

Note 2. Since all aldoses up to the hexoses have trivial names that are preferred, the systematic names apply only to the higher aldoses. However, the configurational prefixes are also used to name ketoses (see below) and other monosaccharides. 

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

Studies202 on the substrate specificity of transketolase suggest that this enzyme will only attack ketoses with the threo configuration at C3 and C4. 

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.

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

Names of cyclised (hemi-acetalised) aldoses and ketoses contain the infixes pyran or furan to indicate the six- or five-membered heterocyclic structure and a stereodescriptor, a or p, to indicate the configuration of the anomeric or hemi-acetal carbon atom. 

The substrate spectrum of SuSyl from yeast is well documented for a variety of acceptors [24, 29, 30]. In the series of ketoses we concluded that SuSyl favors the 3S,4R configuration because L-sorbose 4 and D-xylulose 5 are accepted and D-tagatose 6, D-psicose 7, and D-sorbose 8 are not substrates (Fig. 2.2.6.1 and... 

Problem 22.7 (a) Explain how in basic solution an equilibrium is established between an aldose, its C epimer (a diastereomer with a different configuration at one chiral C) and a 2-ketose. (b) Will fructose give a positive Fehling s test which is done in a basic solution <4... 

---