Enzymatic synthesis of amylopectin.

The problem of branching in glycogen and starch has been the subject of interest ever since Cori and Cori (I, 2) isolated the phosphorolytic enzyme from animal tissues and Hanes (3) found a similar enzyme in plants. Examination of polysaccharides produced by these enzymes when glucose-l-phosphate was used as a substrate showed that, similar to amylose, they had a linear configuration in which the glucose units are combined through 1,4-glucosidic linkages only (4, 5). However, naturally occurring glycogen and most starches also contain 1,6 linkages. It was therefore assumed that synthesis in vivo of glycogen and of the amylopectin starch component requires two enzymes and that in the process of preparation of the muscle or potato phosphorylase the enzyme producing the 1,6 linkages is eliminated. Proceeding on this assumption, Cori and Cori (6) produced evidence for the existence of a branching factor in liver and heart, capable of synthesizing 1,6 linkages. The combined action of the branching enzyme and crystalline muscle phosphorylase resulted in the formation of a polysaccharide which closely resembled glycogen. Later, Haworth, Peat, and Bourne (7) reported the isolation from potatoes of an enzyme, the so called Q enzyme, which, in association with potato phosphorylase, catalyzed the synthesis of amylopectin from glucose-l-phosphate. In a series of papers, Peat and his collaborators (8-14) presented evidence that the Q enzyme converts linear amylose into branched amylopectin. Since this reaction proceeds without the presence of inorganic phosphate, they hold the view that Q enzyme functions as a non-phosphorolytic enzyme. Their evidence is based chiefly on the following experimental facts. When Q enzyme is allowed to act upon amylose, a product is produced that gives a purple-red color with iodine; the synthetic material is soluble in water, and does no,t retrograde from solution; it is attacked by P-amylase with the liberation of maltose, but hydrolysis is arrested before conversion is completed; end-group analysis by the methylation method indicates the existence of chains of about 20 glucose units in average length (8).