Configuration of Ketoses

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. 

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... 

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.

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... 

Figure 20-2 Structure and configuration of the o-ketoses from C3 to C6. As with the aldoses (Figure 20-1), the cyclic form is predominantly an oxacyclohexane (pyranose) ring in the free sugar, but the oxacyclopen-tane (furanose) form is shown for fructose because it occurs widely in this form as the disaccharide, sucrose. Only the a anomers are shown (see Section 20-2B).

Cl-to-C2 bond, the Cl-acetoxy group of ketose acetates can be expected to participate in replacement reactions at the anomeric center. The retention of configuration obtaining in the mercaptolysis of /3-D-fructopyranose pentaacetate (CXIV) suggests that the 1,2-cyclic ion (CXV) is an intermediate in the reaction. 

The most common ketose, d-fructose, is shown below. Compare the configuration of the chiral carbon atom most remote from the keto group (C-5) with D-glyceraldehyde. 

Some ketoses are not related structurally to dihydroxyacetone. They are named by considering the configurations of all the chiral carbon atoms as a unit, ignoring the carbonyl group. 

The donor of the two-carbon unit in this reaction is xylulose 5-phosphate, an epimer of ribulose 5-phosphate. A ketose is a substrate of transketolase only if its hydroxyl group at C-3 has the configuration of xylulose rather than ribulose. Ribulose 5-phosphate is converted into the appropriate epimer for the transketolase reaction by phosphopentose epimerase (see Figure 20.11) in the reverse reaction of that which occurs in the Calvin cycle. 

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