Starch UDPglucosyl-, transferase

Starch-UDPglucosyl transferase uridine 5-(D-glucosyl pyrophosphate) maltosaccharide of DP 3, amylose, amylopectin — 7.5-8.2 28... 

The activity of starch-UDPglucosyl transferase in potato tubers is high, and fractionation of the subcellular particles in sucrose-citrate media has given a starch-granule fraction showing 400-2000 times the activity of the above bean preparation. The presence of an enzyme which is active only when bound to starch granules (where the structural relationship between enzyme and substrate is similar to that in plant cells) is suggestive of an in vivo function. 

Recondo and Leloir " have found that adenosine 5-(i>-glucosyl pyrophosphate) will also act as a glucosyl donor for starch-UDPglucosyl transferase and that the rate of transfer to starch is about 10 times that with uridine 5-(n-glucosyl pyrophosphate). The transferred glucose is present... 

At the present time, it would be premature to assign a purely degradative function to P-enzyme, and, in the later discussion, it will be assumed that both P-enzyme and starch-UDPglucosyl transferase are concerned with the synthesis of (1—>4)-a-D-glucosidic linkages in plant starches. Alternative metabolic pathways are not uncommon in Nature. 

Many starches contain small proportions of phosphorus which is not solvent-extractable but is present as phosphate ester of the C-6-hydroxyl group of a few n-glucose residues. Potato starch contains about 0.1% of phosphorus, most of which (0.08%) is present in the amylopectin component. Cereal starches may contain only 0.02% of phosphorus. The origin of the phosphorus in potato starch is not known. One possibility, wliich has not been examined experimentally, is that traces of n-glucose 1,6-diphosphate are incorporated into the polysaccharides by P-enzyme or by starch-UDPglucosyl transferase. 

At the outset, it must be emphasized that no single theory can, as yet, accommodate all the available experimental observations and, until it is established whether P-enzyme or starch-UDPglucosyl transferase (or both) catalyzes the formation of (1—>4)-linkages, this discussion is bound to be tentative and incomplete. 

The specificity of rat-muscle glycogen-UDPglucosyl transferase has been examined by Goldemberg. A marked difference from starch-UDPglucosyl transferase is that adenosine 5- D-glucosyl pyrophosphate) is only 50% as effective as the uridine analog. In contrast to the rabbit-muscle enzyme, maltoheptaose (and maltosaccharides of DP 4, 5, 6, and 8) will slowly act as acceptors of low efficiency. Maltose and malto-triose are very inefficient acceptors, and at concentrations of 0.64 and... 

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