Modeling of α(1-4) chain arrangements in α(1-4)(1-6) glucans:: The action and outcome of β-amylase and Pseudomonas stutzeri amylase on an α(1-4)(1-6) glucan model

Simulated enzymic debranching of a P-limit dextrin model, prepared from a computed construct made by random extension and branching, and given the CCL value of w-maize amylopectin (and equal amounts of external chains with ECL values of 2 and 3) has been related to experimental chromatograms of the debranched P-limit dextrin of the amylopectin. The profile was similar to those from gel chromatograms and IEC-PAD chromatography. The equivalent lengths in glucosyl units of grid-links (g-links) of internal and external chains in constructs were calculated from the ICL and ECL values of amylopectin and models produced from the constructs with the appropriate lengths for internal and external chains. These derived models were subjected to simulated hydrolysis by Pseudomonas stutzeri amylase and the products compared with those of the experimental distribution from w-maize amylopectin. With the model the amounts of maltotetraose and maltodextrins released were similar to the experimental values but the distribution of branched maltodextrins was quite different. Unlike w-maize amylopectin - a polymer with the cluster structure - which has given a profile of molecular sizes of maltodextrins with low amounts of single and small numbers of internal chains and with a peak at a M-w of about 14,000 (13 chains), in the model the proportion of maltodextrin with one internal chain was high and as d.p. increased the amounts decreased exponentially. This would be expected if the distribution of internal chains in the core was random. It is suggested that in the core of a model prepared from a construct made with alternating probabilities of extension - one in which this probability is high relative to branching, and a second in which it is low - may give clusters of branched maltodextrins with short internal chains which are joined by longer chains; more closely approximating the distribution of internal chains of different lengths in amylopectin. An arrangement for amylopectin molecules in the starch granule has been proposed. In this, they have a wafer-like, discoidal shape, composed of the amorphous zone overlain with the double helical, crystalline region. The flat macromolecules are concentrically layered with the former on the inside and the latter oriented to the outside of the granule. Crown copyright (c) 2007 Published by Elsevier Ltd. All rights reserved.