Glycogen metabolism in muscle and liver: biochemistry, pathways and regulation

Glycogen metabolism, as well as metabolism in general, has two phase: catabolism, the destructive phase, and anabolism, the constructive phase. During the constructive phase, reactions catalyzed by specific enzymes lead to glycogen synthesis, whereas during the destructive phase, reactions catalyzed by specific enzymes lead to glycogen breakdown.
In Archea, Bacteria, Mushrooms and animals, glycogen acts as a store of energy and carbon that organisms accumulate in times of nutritional plenty for utilization in times of need. In animals, glycogen is found in the cytosol of practically all cell types, as well as in lysosomes.
In mammals the major glycogen stores are found in the liver and skeletal muscle. Liver glycogen is used to regulate blood glucose levels in between meals, in the fasted state or during high intensity exercise, to provide adequate energy supply primarily to neurons and red blood cells. Muscle glycogen is used to meet the energy needs of the muscle during high intensity exercise.
Like starch, glycogen is a branched polymer of D-glucose, and allows high glucose accumulation giving rise to a much lower osmotic pressure than glucose would exert if it were in its monomeric form. Phosphate groups and the protein glycogenin are covalently bound to the polysaccharide skeleton, whereas numerous other proteins are noncovalently bound. The protein-glycogen structures are termed beta-granules and are considered a rapid energy source. Such proteins are involved in glycogen metabolism, that is regulated by allosteric effectors and covalent modifications of key enzymes of glycogen synthesis and cytosolic degradation of glycogen or glycogenolysis.

CONTENTS

Glycogen metabolism: the constructive phase

Glycogen synthesis takes place in the cytosol, appears to be associated with actin filament, and can occur from glucose derived from dietary carbohydrates, or from glucose from noncarbohydrate precursors, such as lactate and alanina.
Lactate, produced by skeletal muscle cells working under low oxygen conditions, by red blood cells, that dependent on anaerobic glycolysis for ATP production, and by other tissues, is mostly metabolized in the liver to form glucose via gluconeogenesis. Glucose diffuses from the hepatocytes into the bloodstream and is also transported to the skeletal muscle cells where it may be converted to lactate, thus closing the cycle. This cycle is known as the Cori cycle.
Alanine, a nonessential amino acid, may arise from transamination reactions in which pyruvate produced in glycolysis acts as the acceptor of the amino group of amino acids used for energy. Therefore, alanine is the means by which the carbon skeleton of pyruvate and amino groups are transported from extrahepatic tissues to the liver, where the carbon skeleton is used to form glucose via gluconeogenesis, and the amino groups is converted into urea through the urea cycle. Glucose diffuses into the bloodstream and reaches the peripheral tissues where it may be converted into pyruvate, thus closing the cycle which is known as the glucose-alanine cycle.
Glucose enters the cells through transmembrane carrier proteins known as glucose transporters (GLUT), of which GLUT4 is primarily expressed in insulin-dependent or insulin-sensitive tissues, such as skeletal and cardiac muscle, liver, and adipose tissue. GLUT4 is the insulin-responsive glucose transporter.
Once in the cell, glucose is phosphorylated to glucose 6-phosphate. This reaction is catalyzed, in hepatocytes and pancreatic beta cells, by glucokinase or hexokinase IV (EC 2.7.1.1), and by other hexokinases (EC 2.7.1.1) in other cell types.
The fate of glucose 6-phosphate depends on the metabolic status of the cell. It may enter the glycolytic pathway to provide energy and/or building blocks for many other metabolic pathways. Alternatively, the isomerization of glucose 6-phosphate to glucose 1-phosphate may occur, in the reversible reaction catalyzed by phosphoglucomutase (EC 5.4.2.2). Glucose 1-phosphate may be used for glycogen synthesis, or may enter the pentose phosphate pathway, when the reduced coenzyme NADPH is needed for reductive biosynthesis, such as the synthesis of cholesterol and fatty acids, or when ribose 5-phosphate is needed for the synthesis of nucleotides.
When glycogen synthesis  predominates, glucose 1-phosphate is converted to UDP-glucose in the reaction catalyzed by UDP-glucose pyrophosphorylase (EC 2.7.7.9), at the expense of one UTP. UDP-glucose is the donor of glucose residues. Initially, glycogenin catalyzes the addition of glucose to the hydroxyl group of Tyr194, after which the enzyme adds 6-10 more glucose residues to form an oligosaccharide chain of 7-11 units. The oligosaccharide chain acts as a substrate for glycogen synthase (EC 2.4.1.11) that, with branching enzyme, also called glycosyl-(4,6)-transferase (EC 2.4.1.18), leads to glycogen synthesis.

Glycogen metabolism: the degradative phase

Unlike glycogen synthesis, the catabolic or degradative phase of glycogen metabolism occurs both in the cytosol and in the lysosomes, but through different metabolic pathways.
Glycogenolysis is associated with the endoplasmic and sarcoplasmic reticulum, and releases glucose for energy. Reactions catalyzed by glycogen phosphorylase (EC 2.4.1.1), α-(1,4)-glucan-6-glycosyltransferase (EC 2.4.1.25) and amylo-α-(1,6)-glucosidase or debranching enzyme (EC 3.2.1.33) lead to the release of glucose 1-phosphate, for about 90%, and free glucose, the remaining 10%. In skeletal muscle cell, hexokinase activity is so high that any free glucose molecule is immediately phosphorylated to glucose 6-phosphate, which cannot diffuse out of the cell. In hepatocytes, kidney cortex cells and enterocytes, having the gluconeogenic pathway, glucose 6-phosphatase (EC 3.1.3.9) catalyzes the dephosphorylation of glucose 6-phosphate, resulting from isomerization of glucose 1-phosphate, to glucose that can leave the cell and help regulate blood glucose levels.

Lysosomal degradation of glycogen

Although glycogen is synthesized in the cytosol, it is also found in lysosomes. Lysosomal glycogen may be the product of an autophagic mechanism, and corresponds to about 10% and 5% of total glycogen content of the liver and muscle, respectively.
According to one hypothesis, the degree of phosphorylation of glycogen would be one of the key factors in the regulation of its lysosomal metabolism. The degree of phosphorylation would be correlated with the age of the polysaccharide, acting as a quality control system: the increase in phosphorylation, leading to a decrease in solubility of granules, would be considered as a marker that directs glycogen metabolism toward lysosomal degradation, a process also known as glycophagy.
In lysosomes, glycogen breakdown is catalyzed by the enzyme acid alpha-(1,4)-glucosidase (EC 3.2.1.20). The hydrolysis leads to the release of D-glucose. As the enzyme preferably hydrolyzes the α-(1,4) glycosidic bonds, it is not yet clear how the α-(1,6) glycosidic bonds are hydrolyzed.
The importance of the lysosomal catabolism of glycogen is underscored by Pompe disease or type II glycogenosis, due to a mutation of the acid alpha-glucosidase gene. The deficiency of functional acid alpha-(1,4)-glucosidase leads to an overaccumulation of glycogen in lysosomes and vesicular structures. In its most severe form, this glycogenosis is fatal within the first year of life.

Coordinated regulation of glycogen metabolism

Glycogen synthesis and glycogenolysis are exergonic processes, so, if they operate simultaneously, they lead to a waste of energy. In the cell, the two metabolic pathways are under stringent control and  are reciprocally regulate so that when one is active, the other slows down. During evolution, this has been achieved by selecting different enzymes to catalyze the key steps of the two pathways, like what occurred for the reciprocal regulation of glycolysis and gluconeogenesis. The key enzymes are glycogen phosphorylase and glycogen synthase, whose activity is regulated through:

  • allosteric modifications, occurring on a time scale of  milliseconds, are instantly reversible, and involve as effectors calcium ions (Ca2+), glucose, and metabolites that signal the metabolic status of the cell, namely, ATP, AMP and glucose 6-phosphate;
  • covalent modifications, that is, phosphorylation and dephosphorylation of specific target proteins such as, in addition to the aforementioned enzymes, phosphorylase kinase (EC 2.7.11.19), phosphoprotein phosphatase 1 or PP1(EC 3.1.3.17), and glycogen synthase kinase 3 (EC 2.7.11.26).
    Covalent regulation occur on a time scale of seconds, and is triggered by the binding of hormones, the most important of which are insulin, glucagon and adrenaline (epinephrine), to the corresponding receptors on plasma membrane.

Allosteric and covalent mechanisms are superimposed in the coordinated regulation of glycogen metabolism.

Covalent regulation

The metabolic changes induced by the binding of insulin, glucagon and adrenaline to receptors on plasma membrane of hepatocytes and skeletal muscle cells are summarized below.

Insulin is secreted by pancreatic beta cells in response to high blood glucose levels due to, for example, a meal rich in carbohydrates, and has an anabolic effect. When insulin binds to the receptors on the cell surface of hepatocytes and skeletal muscle cells, it triggers a cascade of reactions that results in the dephosphorylation of:

  • glycogen phosphorylase, which is inhibited;
  • glycogen synthase, which is activated.

In addition, insulin recruits GLUT4 to plasma membrane of skeletal muscle cells.

In this way, glycogen synthesis is stimulated and glycogenolysis is inhibited. This helps to lower blood glucose levels.

Glucagon is secreted by pancreatic alpha cells in response to low blood glucose levels and has an catabolic effect. When glucagon binds to the receptors on the cell surface of hepatocytes, it triggers a cascade of reactions that results in the phosphorylation of:

  • glycogen phosphorylase, which is activated;
  • glycogen synthase, which is inhibited.

Thereby glycogen synthesis is inhibited and glycogenolysis is stimulated. This helps to raise blood glucose levels.

Adrenaline is secreted by the adrenal glands in response to stimulation of the sympathetic nervous system, and plays a role in the fight-or-flight response. It may be secreted also in response to an high intensity exercise. Like glucagon, it has a catabolic effect as it causes the phosphorylation of:

  • glycogen phosphorylase, activating it;
  • glycogen synthase, inhibiting it.

However, unlike glucagon, adrenaline acts not only on hepatocytes but also on skeletal muscle cells, binding to alpha-1 and beta-1 adrenergic receptors, and  to beta-2 adrenergic receptors, respectively.

Vasopressin and adrenaline, when adrenaline binds to the alpha-1 adrenergic receptors, trigger intracellular pathways leading to the release of Ca2+ from the endoplasmic reticulum, and, as previously seen, the stimulation of glycogenolysis and inhibition of glycogen synthesis.

Allosteric regulation

The metabolic changes induced by the binding of the allosteric effectors AMP, ATP, glucose, glucose 6-phosphate, and Ca2+ to the respective binding sites on target enzymes are summarized below.

SKELETAL MUSCLE

In response to an increase in AMP concentration:

  • phosphorylase kinase is activated;
  • glycogen phosphorylase b is activated.

In response to an increase in ATP concentration:

  • phosphorylase kinase is inhibited;
  • glycogen phosphorylase b is inhibited.

In response to an increase in glucose 6-phosphate concentration:

  • PP1 is activated;
  • glycogen phosphorylase b is inhibited;
  • the phosphorylated form of glycogen synthase, glycogen synthase b, is activated via a conformational change that favors the dephosphorylation by PP1. This allosteric activation allows glycogen synthase to act as a glucose 6-phosphate sensor.

Therefore, when the cellular concentrations of ATP and glucose 6-phosphate are low and AMP concentration is high, glycogen synthase is inhibited, glycogen phosphorylase is stimulated. Consequently, glycogenolysis is stimulated and glycogen synthesis is inhibited.
Conversely, when the concentrations of ATP and glucose-6-phosphate are high, glycogen synthesis is stimulated and glycogenolysis inhibited.

In skeletal muscle cells, an increase in the intracellular concentration of Ca2+, released from the sarcoplasmic reticulum, triggers muscle contraction.  Moreover, Ca2+ binds to calmodulin, that is the δ subunit of muscle phosphorylase kinase. Ca2+-calmodulin activates the kinase that phosphorylates glycogen phosphorylase and glycogen synthase, activating the first and inhibiting the latter.

Allosteric and covalent regulation of liver and muscle glycogen phosphorylase.
Regulation of Liver and Muscle Glycogen Phosphorylase

LIVER

In the liver, glycogen phosphorylase b is not activated by AMP, whereas glycogen phosphorylase a is inhibited by an increase in blood glucose levels. How? Glucose concentration in the liver reflects the blood concentration of glucose. In response to an increase in blood glucose levels, glucose binds to an inhibitory allosteric site on glycogen phosphorylase a and triggers a conformational change that exposes phosphorylated serine residues, that are dephosphorylated by PP1, leading to glycogen phosphorylase inactivation. Therefore, glycogen phosphorylase acts as a blood glucose sensor, responding appropriately to its changes.

References

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Biochemistry, metabolism and nutrition