On The Mechanism of Amylose Branching by Potato Q‐Enzyme

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Abstract
1 When potato Q-enzyme converts amylose into an amylopectin-like molecule, the action is by a random, endo-type transglycosylation of the substrate chains. 2 Inter-chain transfer takes place during the formation of the amylopectin branch linkage. This is seen in experiments in which radioactive label was transferred between substrates of disparate molecular weight. Intra-chain transfer, leading to the formation of a branch linkage, is not excluded by these experiments. 3 The minimum length of amylose chain that can act as an acceptor in the transglycosylation reaction, under the experimental conditions described, is greater than 40 glucose units. 4 The requirement of Q-enzyme for substrate chains at least 40 glucose units in length is interpreted as meaning that a stabilized secondary and tertiary structure must be established in the substrate before it can be utilized by Q-enzyme, and that the forces that provide such conformation are sufficiently strong only when the chains are longer than the minimum. Inter-chain transfer is seen as taking place by one of two mechanisms. The first involves the reaction of the enzyme with a chain that has a stabilized (helical?) conformation. An enzyme-donor chain intermediate is formed, that then reacts with an acceptor chain to complete the transglycosylation. The second mechanism envisages the substrate for the enzyme as being a complex formed between two chains (a double helix?). The enzyme encounters the complex and carries out an inter-chain transglycosylation reaction.