Synthesis of Amylose by Potato D-Enzyme

Abstract

THE synthesis of the chain-forming α-1: 4-bonds of potato starch (amylose + amylopectin) has for long been ascribed to phosphorylase acting on glucose-1-phosphate and a preformed starch-like primer¹. We now report the synthesis of amylose-like material by another enzyme system from potato. This synthesis makes use of D-enzyme, discovered in 1953 by Peat and co-workers². D-enzyme catalyses the reversible disproportionation of maltodextrins, the glucose oligosaccharides containing the α-1: 4-linkage. For example, maltotriose is acted on to give, as the first products of reaction, glucose and malto-pentaose, and at equilibrium glucose and a whole series of maltodextrins are present. However, with maltotriose as the initial substrate, none of the synthetic oligosaccharides is of sufficient length to form a coloured complex with iodine. The minimum length for colour formation is at least 12 glucose units³. Incubation of D-enzyme with a maltodextrin and glucose lowers the average length of the polymer because of transfer of some of its glucose residues to the added glucose. It was realized that a reversal of this procedure, the removal of glucose, should increase the length of the polymers and might ultimately result in the synthesis from a small maltodextrin molecule of an iodine-staining, amylose-like polysaccharide. If, in addition to D-enzyme, the potato is equipped with such a glucose-removing enzyme system, then it is presumably capable of synthesizing amylose by a route other than that provided by phosphorylase. A suitable glucose-removing system is hexokinase and adenosine tri-phosphate, and hexokinase is known to be in the potato⁴. This will convert glucose into glucose-6-phosphate, and D-enzyme does not transfer chain segments from maltodextrins to the sugar phosphate as it does to glucose⁵. The system (maltodextrin + D-enzyme + hexokinase + adenosine triphosphate) should therefore bring about amylose synthesis. This has been achieved.