Nucleotides sugar, enzymic conversion

Conversion of the a-D-glucopyranosyl derivative (94a) into the a-D-galactopyranosyl ester (95a) was demonstrated370 in 1951 as the first example of an enzymic reaction of a sugar nucleotide. The enzyme that catalyzes this reaction, namely, uridine 5 -(a-D-glu-copyranosyl pyrophosphate) 4"-epimerase,371 is common in Nature. Purified preparations have been obtained from yeast,372 373 Escherichia coli 374-376 mung-bean seedlings,377 wheat germ,378 and animal tissues.244,379 380... 

In a system where UDP-Gal regeneration is the goal, the inclusion of a one-step conversion of UTP to UDP-Gal using the enzyme UDP-Gal pyrophosphorylase would be ideal. Unfortunately, this enzyme is not commercially available, nor is it easy to prepare from biological sources [8]. Luckily, an alternative biosynthetic pathway to UDP-Gal exists. In nature, UDP-Glc is a central intermediate for the biosynthesis of UDP-Gal and UDP-GlcUA, as well as other sugar nucleotides. The enzyme UDP-Glc pyrophosphorylase (UDPGP EC 2.1.1.9) required for the conversion of UTP to UDP-Glc can be purchased. The UDP-Gal 4-epimerase... 

Hydrolysis of sugar nucleotides with unspecific pyrophosphatases has already been mentioned (Section 11,1, p. 310). A similar reaction is catalyzed by a bacterial enzyme specific for adenosine 5 -(a-D-glucopyranosyl pyrophosphate).459 The specific conversion of uridine 5 -(a-D-glucopyranosyl pyrophosphate) into a-D-glucopyranosyl phosphate, uridine, and inorganic phosphate was observed with an enzyme from Escherichia colt 459,460 a preparation from Bacillus subtilis can act in a similar manner461 on different sugar nucleotides. ... 

It has also been demonstrated that expensive substrates such as UDP-Gal can be readily prepared in situ by enzymatic conversion of the relatively inexpensive sugar nucleotide uridine 5 -diphospho-a-D-glucopyranose (UDP-Glc) using a UDP-Gal 4-epimerase enzyme. This system, coupled with an appropriate UDP-Gal transferase, provides more economic access to enzymatically galactosylated compounds. In these multienzyme systems, to increase enzyme efficiency and also avoid multiple fermentations for separate enzyme preparations, fusion proteins have been constructed that contain both the Gal-epimerase and Gal-transferase enzymes. The use of these fused enzyme systems has increased in the recent years as their catalysis of sequential reactions can have a kinetic advantage over the mixture of two separated enzymes since the product of the first enzyme travels a shorter distance before being captured by the next enzyme in the sequence. 

---