Mechanism of amylopectin formation by the action of Q enzyme.

It has been previously demonstrated that linear amylose can be transformed with the Q enzyme from potatoes to a branched polysaccharide possessing the chemical and biochemical properties of amylopectin (l-3). A mechanism for its synthesis was first suggested by Bourne and Peat (4) in 1945. They postulated that glucose-l-phosphate is built up by phosphorylase to an unbranched chain, “pseudoamylose,” consisting of approximately twenty 1,4-linked glucose units. In the absence of the Q enzyme the pseudoamylose is further synthesized into long chain amylose molecules. The Q enzyme was assumed to be responsible for the establishment of the I,6 linkages between the short chains of pseudoamylose, thus forming branched amylopectin. In the conversion of amylose into amylopectin, the Q enzyme presumably performed a second function; namely, the scission of 1,4 linkages in amylose and formation of short pseudoamylose chains. However, this manner of combination, in which a terminal reducing group of one chain is united with an alcoholic hydroxyl of another chain, not involving the transfer of a phosphate of a glycosidic bond, is incompatible with the energy requirements for mechanisms of complex saccharide formation. Hobson and collaborators (5) later found that a-dextrins of an average chain length of less than 25 glucose units, after being treated with the Q enzyme, gave the same hydrolysis limit with P-amylase as untreated ones, whereas a decrease in hydrolysis limit was observed when dextrins of considerably longer chains were subjected to the action of this enzyme. These results indicated that short chain dextrins are incapable of acting as substrate for the synthesis of amylopectin. Peat and his collaborators therefore modified their original pseudoamylose hypothesis and now believe that chains of greater length than 20 glucose units are required by the Q enzyme for the conversion of amylose to amylopectin. We have obtained evidence which is in accord with the later hypothesis of Peat and his collaborators that formation of long chains is a requisite prior to synthesis of amylopectin by the action of the Q enzyme. It was