Galactose-glucose interconversion

Galactose is converted into glucose 6-phosphate in four steps. The first reaction in the galactose—glucose interconversion pathway is the phosphorylation of galactose to galactose 1 -phosphate by galactokinase. 

The problem of the D-galactose-D-glucose interconversion was discussed on p. 176 in connection with the biosynthesis of lactose, although, in fact, the mechanism was largely elucidated by studying the metabolism of D-galactose in j/eas[Pg.179]

The network as depicted in the figure is a very simplified one. It does not incorporate epi-lactose (C-2 epimer of lactose, 4-0-p-D-galacto-pyranosyl-D-mannose) and further reactions of D-galactose. However, it does describe the major events the interconversion of lactose and lactulose, the formation of D-galactose by an elimination reaction and the simultaneous formation of acidic by-products from the glucose moiety. 

The interconversion of glucose and galactose occurs at the UDP-hexose level (fig. 16.2) with a 4-epimer-ase. The epimerase is unusual in that it has a covalently bound NAD +. What function do you propose for the NAD+, and what intermediate do you expect in the reaction ... 

An analogous approach was used to show that galactose-1-P uridylyltransferase catalyzes the interconversion of UDP-glucose and galactose- -P with UDP-galactose and glucose-1-P with retention of configuration at PQ [21]. The stereochemical transformations used in the analysis are shown in Equation 14. 

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. 

Mannose is found in the diet in small amounts. Tike galactose, it is an epimer of glucose, and mannose and glucose are interconverted by epimerization reactions. The interconversion can take place either at the level of fructose 6-phosphate to mannose 6-phosphate, or at the level of the derivatized sugars (see Fig. 30.8). 

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