Structural Mechanisms of Starch Dephosphorylation (769.2)

Starch is comprised of glucose chains long enough to interact and self‐assemble into tightly packed helices that make starch water‐insoluble. Plants utilize reversible phosphorylation of glucose residues within starch to disrupt the helices and render the granule surface accessible to hydrolyzing enzymes.Glucan dikinases solubilize starch outer glucan chains by phosphorylating different positions on glucose units and allow starch degradation to initiate. However, β‐amylase activity is inhibited when a phosphate group is reached and glucan phosphatases must then act. We initially discovered glucan phosphatase activity and now define the structural basis of dephosphorylation by glucan phosphatases.We determined the structure of two Arabidopsis starch glucan phosphatases Starch Excess 4 (SEX4) and Like Sex Four2 (LSF2) both with phospho‐glucan bound and without glucan. SEX4 utilizes a carbohydrate binding module to position the oxygen of the C6 carbon into its catalytic cleft. LSF2 binds maltohexaose‐phosphate using an aromatic channel within an extended phosphatase active site and positions the C3 carbon into its active site.These structures are the first of glucan‐bound glucan phosphatases, and they provide insights into the molecular basis of this agriculturally‐ and industry‐relevant enzyme family, and their unique mechanisms of catalysis, substrate specificity, and interaction with starch.Grant Funding Source: NSF CAREER MCB1252345, NIH R01NS070899, KSEF 2268‐RDE‐014