Purification and Properties of Potato. alpha-1,4-Glucan 6-Glycosyltransferase (Q-Enzyme)

Potato Q-enzyme (α-1,4-glucan: α-1,4-glucan 6-glycosyltransferase) has been purified at least 430-fold. Its instability in the purified state was reversed by Cleland's reagent (dithiothreitol) and the preparation was essentially free of other starch-metabolising enzymes (β-amylase, β-glucosidase, D-enzyme and R-enzyme). Studies involving successive and simultaneous actions of Q-enzyme and pullulanase on amylose showed that the final average unit-chain length and iodine stain of the products were almost identical, but gel filtration revealed that their chain-length distributions were different. Similarly, two amylopectin-like polysaccharides formed by the direct action of Q-enzyme on amylose and by the combined action of Q-enzyme and potato phosphorylase on glucose 1-phosphate were debranched with pullulanase and their unit-chain length distributions compared by gel filtration with that of a debranched native amylopectin. In neither case was the unit-chain profile of native amylopectin reproduced. The Q-enzyme preparation has been shown to introduce additional branch points into amylopectin with the formation of a significant proportion of maltohexaosyl side chains. Evidence is presented that Q-enzyme and not a second branching enzyme is responsible for this action and a mechanism for the formation of the maltohexaosyl chains is proposed.