Interconversion of sugars

Figure 7.9 Interconversion of sugar phosphates and sugar nucleotide phosphates. Adapted from "Biotechnology of microbial exopolysaccharides". IW Sutherland, Cambridge University Press, 1990.

Anomeric interconversion of sugars and their derivatives is typically relatively slow, but anomerization of phos-phorylated sugars is usually faster. Values for the rate constants can be obtained by nuclear magnetic resonance techniques. For example, the conversion of /3-fructose... 

An account of the methods of synthesis of amino sugars has been given in an earlier Volume of this Series1 attention is devoted in this Section to methods recently developed. Several general methods have been employed for the synthesis of 2-amino-2-deoxy-aldoses these respectively involve direct amination of suitable sugar derivatives, interconversion of sugar series, and ascent or descent of a series with or without concomitant amination or molecular rearrangements. 

The pentose phosphate pathway meets the need of all organisms for a source of NADPH to use in reductive biosynthesis (Table 20.2). This pathway consists of two phases the oxidative generation of NADPH and the nonoxidative interconversion of sugars (Figure 20.19). In the oxidative phase, NADPH is generated when glucose 6-phosphate is oxidized to ribose 5-phosphate. This five-carbon sugar and its derivatives are components of RNA and DNA, as well as ATP, NADH, FAD, and coenzyme A. 

Part of the dark reactions of photosynthesis is interconversion of sugars with an enzyme called transketolase using thiamine pyrophosphate, TPP, as a catalyst (Section 8.12.8). Provide a reasonable mechanism for this enzymatic reaction. In addition to water, there are weak general acids and general bases present in the active site at pH 7. 

Enzymic interconversion of sugars has also been exploited as an additional fresh milk processing step to convert the normal lactose milk sugar, to which some people react unfavorably, into glucose and galactose, which are much better tolerated [82]. Products from this milk can now be enjoyed by those who could only tolerate milk substitutes previously. 

Those caramels which are prepared from plain sugars without any catalyst present the relatively simplest chemical features. In order the better to understand the complexity of caramelization specific interconversions of sugars in solution have to be kept in mind. It is well known that pyranoid and furanoid sugars are considerably more stable than the acyclic forms. Mu-tarotation of sugars is autocatalyzed by protons, " and, with D-glucose, the equilibrium slightly favors )5-D-glucopyranose over the a anomer. It has... 

Neufield, E.F., V. Ginsburg, E.W. Putman, and D. Fanshier Formation and interconversion of sugar nucleotide by plant extract Arch. Biochem. Biophys. (1957) 602-616. 

Fig. 29.10. A balanced sequence of reactions in the pentose phosphate pathway. The interconversion of sugars in the pentose phosphate pathway results in conversion of 3 glucose 6-phosphate to 6 NADPH, 3 COj, 2 fructose 6-phosphate, and one glyceraldehyde 3-phosphate.

List the two phases of the pentose phosphate pathway (oxidative generation of NADPH and nonoxidative interconversion of sugars). List the biochemical pathways that require NADPH from the pentose phosphate pathway. 

---