Understanding the Binding of Starch Fragments to Granule-Bound Starch Synthase

Granule-bound starch synthase (GBSS) plays a major role, that of chain elongation, in the biosynthesis of amylose, a starch component with mostly (1 → 4)-α connected long chains of glucose with a few (1 → 6)-α branch points. Chain-length distributions (CLDs) of amylose affect functional properties, which can be controlled by changing appropriate residues on granule-bound starch synthase (GBSS). Knowing the binding of GBSS and amylose at a molecular level can help better determine the key amino acids on GBSS that affect CLDs of amylose for subsequent use in molecular engineering. Atomistic molecular dynamics simulations with explicit solvent and docking approaches were used in this study to build a model of the binding between rice GBSS and amylose. Amylose fragments containing 3–12 linearly linked glucose units were built to represent the starch fragments. The stability of the complexes, interactions between GBSS and sugars, and difference in structure/conformation of bound and free starch fragments were analyzed. The study found that starch/amylose fragments with 5 or 6 glucose units were suitable for modeling starch binding to GBSS. The removal of an interdomain disulfide on GBSS was found to affect both GBSS and starch stability. Key residues that could affect the binding ability were also indicated. This model can help rationalize the design of mutants and suggest ways to make single-point mutations, which could be used to develop plants producing starches with improved functional properties.