Purification and Properties of Potato 1,4‐α‐d‐Glucan: 1,4‐α‐d‐Glucan 6‐α‐(1,4‐α‐Glucano)‐Transferase: Evidence against a Dual Catalytic Function in Amylose‐Branching Enzyme

Q‐Enzyme, the enzyme that synthesizes the 1,6‐α‐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 °C of 15. The energy of activation is 25 kJ/mol below 15 °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, degree of β‐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.