Glycogen and its metabolism: some new developments and old themes

Abstract

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

Figure 1. Glycogen structure

(A) Polymerizing α-1,4-glycosidic linkages and a branching α-1,6-glycosidic linkage are shown. (B) The tiered model for glycogen organization in which inner B-chains on average carry two branches and the outer A-chains are unbranched. The black circle denotes glycogenin.

Figure 2. Glycogen particles

Shown are the well-established glycogen-associated proteins: the metabolic enzymes (mauve) glycogenin (GN), glycogen synthase (GS), phosphorylase (PH) and debranching enzyme (DBE); the protein kinases (red) phosphorylase kinase (PH kinase) and AMPK; the phosphatases (green) type 1 catalytic subunit (PP1c) and laforin (LF); the PP1 glycogen-targeting subunits (blue) R_GL, G_L and PTG; and the putative membrane-anchoring protein Stbd1. Malin has been suggested to bind glycogen via interaction with laforin. Phosphorylase kinase, Stbd1 and R_GL bind membranes. Numerous protein–protein interactions are either known or proposed to exist among these glycogen-binding proteins.

Figure 3. Overview of glycogen metabolism

Glc_out, extracellular glucose; Glc_in, intracellular glucose; HK, hexokinase; G6Pase, glucose-6-phosphatase; PGM, phosphoglucomutase; UP, UDP-glucose pyrophosphorylase; UGPPase, UDP-glucose pyrophosphatase; GN, glycogenin; GS, glycogen synthase; BE, branching enzyme; PH, glycogen phosphorylase; DBE, debranching enzyme; GAA, lysosomal α-glucosidase; GNG, gluconeogenesis.

Figure 4. Structure of mammalian glycogenin

(A) Ribbons representation of the glycogenin dimer. The active sites are denoted by the bound substrate UDP-glucose (magenta). The location of Tyr¹⁹⁵ near the dimer interface is indicated using magenta colouring of the residue. (B) The active site of glycogenin. Residues discussed in the text are labelled, and the position of the catalytically essential Mn²⁺ ion is shown using a purple sphere.

Figure 5. Regulatory features of glycogen synthase

Shown is a comparison of the general architecture of yeast and mammalian glycogen synthases in terms of phosphorylation sites (light blue, not to scale) and the arginine-rich cluster implicated in conferring sensitivity to activation by glucose 6-phosphate (green). The conserved arginine residues and the phosphorylated residues are in black and marked by dots. Some of the protein kinases involved in phosphorylating the mammalian enzyme are linked to sites they modify. See the legend to Table 2.

Figure 6. Ribbons diagram of eukaryotic glycogen synthase

The diagram highlights the sequence insertions and deletions of glycogen synthase that confer its allosteric regulation and preference for UDP-glucose. The secondary-structural elements conferring its tetrameric arrangement are coloured magenta. The location of the ten-residue deletion relative to the bacterial enzymes that conveys glucose 6-phosphate regulation is coloured blue. The inserted loop of residues that confer preference for UDP-glucose is coloured green.

Figure 7. Glucose 6-phosphate activation

The active sites of glycogen synthase are occluded in the absence of glucose 6-phosphate (A), but are opened and freed for glycogen access in the activated state (B). Glucose 6-phosphate is bound at the interface between subunits with multiple charged residues interacting with the phosphate moiety and relatively few contacts with the glucose moiety (C).

Figure 8. Interactions at the regulatory interface in different conformational states of yeast glycogen synthase

(A) The binding of glucose 6-phosphate reorganizes the interface and positions the regulatory helices approximately 12Å apart. (B) One of the basal state conformations of yeast glycogen synthase where two sulfate molecules are bound next to Arg⁵⁸⁹ on the opposite face of the regulatory helix from where glucose 6-phosphate is bound. The regulatory helices are positioned approximately 8 Å apart in this conformation. (C) Another conformational state observed for glycogen synthase when a single sulfate molecule is bound between the regulatory helices and pulls the helices to within approximately 5 Å of one another. This state may resemble the inhibited phosphorylated state.

Figure 9. Maltodextran binding in glycogen phosphophorylase and glycogen synthases

(A) A ribbons representation of phosphorylase with a maltodextran bound in the ‘glycogen storage site’. (B) A ribbons representation of the glycogen synthase monomer from E. coli displaying maltodextran-binding sites ‘c’ and ‘d’ that are located on its N-terminal domain. (C) A ribbons representation of a single subunit in yeast glycogen synthase displaying the maltodextran bound to site-1 located in the N-terminal domain. (D) A ribbons representation of a single subunit of yeast glycogen synthase displaying the locations of all four maltodextran-binding sites.

Figure 10. Active site of yeast glycogen synthase

Those residues in Gys2p responsible for recognizing and binding the donor nucleotide sugar substrate are labelled. The glutamate residues present in the EX₇E motif probably participate in glucosyl transfer from the donor to acceptor substrate (Glu⁵⁰⁹) and in positioning the uridine ribose moiety (Glu⁵¹⁷).

Figure 11. Possible mechanism for glycogen phosphorylation

The usual glycogen synthase reaction is shown on the left where glucose from UDP-glucose is added to the non-reducing end to form a new α-1,4-glycosidic linkage. The proposed mechanism for the introduction of phosphate would involve the formation of either glucose-1,2-cyclic phosphate (a) or glucose-1,3-cyclic phosphate (b) in the enzyme active site. Reaction of C-1 of the cyclic phosphate would lead to addition of either a glucose 2-phosphate or a glucose 3-phosphate to the non-reducing end.

Figure 12. Control of glycogen synthesis in muscle

Both insulin and exercise increase glucose uptake via GLUT4. Increased glucose 6-phosphate (Glc-6-P) levels provide feedforward activation of GS (glycogen synthase). Insulin also causes dephosphorylation and activation of glycogen synthase by promoting the inactivation of GSK3 by Akt. The effect of exercise on glycogen synthase phosphorylation is more complex, potentially dephosphorylating via a PP1G containing R_GL and well as increasing phosphorylation via activation of protein kinases such as AMPK. PhK, phosphorylase kinase.

Figure 13. Degradation of glycogen

Glycogen is converted into glucose (Glc) by two pathways: (a) the classic cytosolic pathway controlled by cAMP and PKA, and mediated by glycogen phosphorylase (PH) and debranching enzyme (DBE); and (b) the lysosomal pathway in which degradation is ultimately catalysed by the lysosomal GAA. The latter pathway is poorly understood mechanistically, but probably resembles autophagy. It may be an example of selective autophagy with cargo specificity conferred by Stbd1 which would anchor glycogen to membranes and interact with the ATG8 family member GABARAPL1. The model also depicts the possibility that abnormally phosphorylated and/or branched glycogen is preferentially disposed of by this pathway. GS, glycogen synthase; LF, laforin.

Figure 14. Reported interactions among laforin, malin and glycogen-metabolizing proteins

The Figure summarizes some of the interactions reported for laforin and malin, as discussed in the text, based on studies in vitro, in cell systems and genetically modified mice. Blue single-headed arrows depict an enzyme–substrate relationship. Red double-headed arrows indicate a protein–protein interaction. Dashed green arrows indicate signalling pathways. Asterisks indicate some instances where analyses of Epm2a ^−/− and/or Epm2b ^−/− mice do not seem consistent with the proposed interaction. LF, laforin; LB; Lafora bodies; GS, glycogen synthase; DBE, debranching enzyme.