Uridine diphosphate, UDP galactose

UDP uridine diphosphate UDP-galactose uridine diphosphate galactose UDP-glucose uridine diphosphate glucose UMP uridine monophosphate UTP uridine triphosphate Val valine... 

The galactose 1-phosphate is then converted to its epimer at C-4, glucose 1-phosphate, by a set of reactions in which uridine diphosphate (UDP) functions as a coenzyme-like carrier of hexose groups (Fig. 14-11). The epimerization involves first the oxidation of the C-4 —OH group to a ketone, then reduction of the ketone to an —OH, with inversion of the configuration at C-4. NAD is the cofactor for both the oxidation and the reduction. 

Fig. 20.5 The Leloir pathway of galactose metabolism and the uridine diphosphate (UDP) glucose pyrophosphorylase pathway. Gaik, galactokinase gait, galactose 1-phosphate uridyltrans-ferase gale, UDP-galactose 4-epimerase hex, hexokinase pgm, phosphoglucomutase Ugp, UDP-glucose pyrophosphorylase...

So, the process shown in Figure 11.9 that leads to sucrose need not actually occur by a nucleophilic substitution reaction at the anomeric carbon. Indeed, the derivative of galactose (shown as guanosine diphosphate [GDP]) may be the uridine diphosphate (UDP) or ADP instead, and, of course, the displacement is enzyme mediated. 

Most nucleotide sugars are formed in the cytosol, generally from reactions involving the corresponding nucleoside triphosphate. GMP-sialic acids are formed in the nucleus. Formation of uridine diphosphate galactose (UDP-Gal) requires the following two reactions in mammahan tissues ... 

Fig. 2. Reactions catalyzed by galactosyltransferase (GT). (a) The incorporation of galactose (Gal) into a (1—>4) linkage with JV-acelylglucosamine (NAG) to form N-acetyl-lactosamine (NAL). UDP, Uridine diphosphate, (b) Modification of the activity of GT by a-lactalbumin (a-LA) to convert it to a lactose synthase catalyzing the formation of lactose from UDP-Gal and glucose.

Galactose 1-phosphate then acquires a uridyl group from uridine diphosphate glucose (UDP-glucose), an intermediate in the synthesis of glycosidic linkages (Section 21.4.2). 

Uridine diphosphate galactose (109) was prepared using seven enzymes involved in three biosynthetic pathways, immobilised on super-bead columns. This method, which converted 50% of uracyl monophosphate into UDP-galactose (109), was superior to the solution approach as enzyme stability was improved. To study the biosynthesis of the pseudosaccaharide acarbose, thymidine 5 -diphospho-4-amino-4,6-dideoxy-a-D-glucopyranose(110) was synthesised from galactose in sixteen steps. 

Scheme 22 The whole biosynthetic pathway of sugar nucleotides. ATP, adenosine triphosphate Gal-1 -P, galactose-1-phosphate UTP, uridine triphosphate UDP, uridine diphosphate NAD, nicotinamide adenine dinucleotide Fru, fructose AcCoA, acetyl coenzyme-A PEP, phosphoenolpyruvate CTP, cytidine triphosphate NADP, nicotinamide adenine dinucieotide phosphate GTP, guanosine triphosphate.

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