Finding granule-bound starch synthase enzymes with high activity using free-energy calculations.

Starch is a branched glucose polymer comprising amylopectin, of high molecular weight with many short branches, and amylose, of lower molecular weight and only a few long-chain branches. Granule-bound starch synthase I (GBSSI) is one of the main enzymes controlling amylose synthesis and structure. Production of different GBSSI mutants is laborious, this study retrospectively examined the relationship between mutation-induced changes in glucan binding and experimentally reported GBSSI activity. Atomistic molecular dynamics simulations and free-energy calculations were performed for GBSSI complexed with glucan fragment G5 (a pentasaccharide fragment) to predict the effects of single-point mutations on enzyme activity and their relationship to starch characteristics. Comparing structural changes and binding free energy of G5 to wild type GBSSI and GBSSI mutants, it was found that mutants with a more negative binding energy are more likely to have higher enzyme activity and amylose content compared to the wild type. This methodology might help develop grains with improved functional properties.