Abstract
Starch degradation occurs via a cyclic process of phosphorylation, hydrolysis by amylases, and dephosphorylation by glucan phosphatases. Glucan phosphatases belong to the dual‐specificity phosphatase (DSP) family of proteins within the larger protein tyrosine phosphatases (PTPs) superfamily. Plants contain two glucan phosphatases: Starch EXcess4 (SEX4) and Like Sex Four2 (LSF2). However, little is known about the role of glucan phosphatases in agriculturally relevant cereal crops. In this study, we overexpressed and purified Oryza sativa (rice) and Zea mays (corn) SEX4 enzymes in E. Coli cells, and determined the kinetics using both generic and substrate‐specific glucan phosphatase assays. Rice and corn SEX4 display sigmoidal kinetics against soluble amylopectin substrate, a signature feature of kinetic cooperativity. We explored the mechanism for this positive kinetic cooperativity and how these enzymes can be utilized to enhance starch degradation. Our findings provide insights into the essential role of SEX4 in reversible starch phosphorylation and advance our understanding of the glucan phosphatase family of enzymes.
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