Other Starch Degrading and Synthesizing Enzymes

Many other starch-degrading and -synthesizing enzymes have been described in the literature, but often these have never been obtained in a purified form, and very little is known regarding their properties and action pattern. A few of these enzymes are considered in this Section. 

Amylo-(1 6)-glucosidase is the a-D-(l— 6)-debranching enzyme found in animal skeletal muscle, where it debranches glycogen. Methods of preparation have been described, but there is no knowledge regarding its structure or physical properties. 

Q-Enzyme is the enzyme thought to be responsible for effecting a D-glucosyl exchange reaction in which an a-D-(l- 4)-linkage in amy-lose is converted into an a-D-(l— 6)-linkage. Probably present in all plants, the enzyme was first isolated, and later crystallized, from the potato. Purification was achieved by fractional precipitation with ethanol at low temperatures. Few properties of the crystalline enzyme were detailed, but it was found to be very heat-labile, to be stabilized to some extent by the presence of substrate, and to be activated by inorganic ions. Later experiments have shown this preparation of enzyme to be impure.  

In view of its possible importance in starch synthesis, surprisingly little is known about the specificity and mechanism of action of Q-enzyme. It has been suggested both that G40 is not a substrate for the enzyme, and that G40 is the minimum length for rapid action. Also, it has been reported that Q-enzyme acts more rapidly on maize amylopectin than on the amylose thereof.  

R-Enzyme was one of the first enzymes reported to hydrolyze the a-D-(l- 6)-linkages in starch. The enzyme has been isolated from 

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