Unique carbohydrate binding platforms employed by the glucan phosphatases

Abstract

Glucan phosphatases are a family of enzymes that are functionally conserved at the enzymatic level in animals and plants. These enzymes bind and dephosphorylate glycogen in animals and starch in plants. While the enzymatic function is conserved, the glucan phosphatases employ distinct mechanisms to bind and dephosphorylate glycogen or starch. The founding member of the family is a bimodular human protein called laforin that is comprised of a carbohydrate binding module 20 (CBM20) followed by a dual specificity phosphatase domain. Plants contain two glucan phosphatases: Starch EXcess4 (SEX4) and Like Sex Four2 (LSF2). SEX4 contains a chloroplast targeting peptide, dual specificity phosphatase (DSP) domain, a CBM45, and a carboxy-terminal motif. LSF2 is comprised of simply a chloroplast targeting peptide, DSP domain, and carboxy-terminal motif. SEX4 employs an integrated DSP-CBM glucan-binding platform to engage and dephosphorylate starch. LSF2 lacks a CBM and instead utilizes two surface binding sites to bind and dephosphorylate starch. Laforin is a dimeric protein in solution and it utilizes a tetramodular architecture and cooperativity to bind and dephosphorylate glycogen. This chapter describes the biological role of glucan phosphatases in glycogen and starch metabolism and compares and contrasts their ability to bind and dephosphorylate glucans.

Structural biology of glucan phosphatases from humans to plants

“…At the LSF2 active site, the O3 hydroxyl of Glc3 directly interacts with the phosphate at a distance of 2.4 Å compared to 7.0 Å for the O6 position, consistent with the observed O3-specific activity of LSF2 (Figure 3D) [45]. Further, the conformation and orientation of the glucan in the active site is significantly different, with laforin displaying a highly curved glucan conformation while SEX4 and LSF2 display a more helical glucan conformation but with opposite orientations [50,51]. Thus, each glucan phosphatase engages phospho-glucans at their active site in an orientation-specific manner, providing a physical mechanism for their position-specific activity.…”

Skeletal Muscle Glycogen Chain Length Correlates with Insolubility in Mouse Models of Polyglucosan-Associated Neurodegenerative Diseases

“…LD is caused by mutations in either laforin ( EPM2A ) or malin ( EPM2B ). Laforin is a carbohydrate-binding dual-specificity phosphatase that dephosphorylates glycogen (Emanuelle et al, 2016; Figure 1B). Malin forms a complex with laforin and acts as a ubiquitin E3 ligase implicated in regulating glycogen-metabolism-related enzymes (Chan et al, 2003; Sullivan et al, 2017; Figure 1C).…”

The starch‐binding domain family CBM41—An in silico analysis of evolutionary relationships

“…4,5 This view is supported by the first crystal structure of a CBM41 module determined in complex with maltooligosaccharides in the family GH13 pullulanase from Thermotoga maritima. 6 The CBM41 is therefore considered as one of the 13 families of starch binding domains (SBDs), that is, CBM20, 21,25,26,34,45,48,53,58,68,69, and 74 in addition to CBM41. 1 In some cases a domain, classified as an SBD, may actually represent a glycogen-binding domain (GBD), for example, in laforin, genethonin-1 or the b-subunit of AMPactivated protein kinase.…”