Glucopyranose enzymic preparation

Enzymic hydrolysis can often provide the means for solving a problem involving the partial hydrolysis of esters. A synthesis145 of 1-O-ab-scisoyl-/3-D-glucopyranose proved difficult at the last step, but the use of a crude enzyme-preparation obtained from ripe seeds of Helianthus annus allowed deacetylation to be achieved at 0-2, 3, 4, and 6 of the D-glucosyl group without rupture of the ester bond to the anomeric center. 

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. 

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. 

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