Purification and properties of potato 1,4-alpha-D-glucan:1,4-alpha-D-glucan 6-alpha-(1,4-alpha-glucano)-transferase. Evidence against a dual catalytic function in amylose-branching enzyme.

Q-Enzyme, the enzyme that synthesizes the 1,6-alpha-glucosidic branch linkages of amylopectin, has been purified from potato to near homogeneity. The molecular weight of the enzyme is 85000. The active enzyme is a monomer, with a molar activity at pH 7.0 and 24 degrees C of 15. The energy of activation is 25 kJ/mol below 15 degrees C, changing sharply to 63 kJ/mol above that temperature. Enzyme activity is not affected by Mg2+ or ATP. There are about 11 readily titratable sulfhydryl groups per molecule. The evidence that the enzyme is a single protein entity, without hydrolytic activity towards amylose, contrasts with an earlier report that Q-enzyme consists of two components, a hydrolase with molecular weight 70000, and a transferase with molecular weight 20000. Q-enzyme acts on native and synthetic amyloses to give products resembling amylopectin in terms of average unit chain length, degress of beta-amylolysis and iodine stain. The profiles of the unit chains of these synthetic products are, however, different from that of native amylopectin. Additional branch linkages are introduced by Q-enzyme into potato amylopectin, but the product bears no resemblance to phytoglycogen.