Structural mechanisms of plant glucan phosphatases in starch metabolism.

Glucan phosphatases are a recently discovered class of enzymes that dephosphorylate starch and glycogen, thereby regulating energy metabolism. Plant genomes encode two glucan phosphatases, called Starch EXcess4 (SEX4) and Like Sex Four2 (LSF2), that regulate starch metabolism by selectively dephosphorylating glucose moieties within starch glucan chains. Recently, the structures of both SEX4 and LSF2 were determined, with and without phosphoglucan products bound, revealing the mechanism for their unique activities. This review explores the structural and enzymatic features of the plant glucan phosphatases, and outlines how they are uniquely adapted to perform their cellular functions. We outline the physical mechanisms used by SEX4 and LSF2 to interact with starch glucans: SEX4 binds glucan chains via a continuous glucan-binding platform comprising its dual-specificity phosphatase domain and carbohydrate-binding module, while LSF2 utilizes surface binding sites. SEX4 and LSF2 both contain a unique network of aromatic residues in their catalytic dual-specificity phosphatase domains that serve as glucan engagement platforms and are unique to the glucan phosphatases. We also discuss the phosphoglucan substrate specificities inherent to SEX4 and LSF2, and outline structural features within the active site that govern glucan orientation. This review defines the structural mechanism of the plant glucan phosphatases with respect to phosphatases, starch metabolism and protein-glucan interaction, thereby providing a framework for their application in both agricultural and industrial settings.