Comprehensive Notes on Glycogen Metabolism

Glycogen Metabolism

Overview

  • Glycogen breakdown (degradation) and synthesis occur through different pathways.
  • Phosphorylase, a key enzyme in glycogen breakdown, is regulated by allosteric interactions and reversible phosphorylation.
  • Epinephrine and glucagon signal the need for glycogen breakdown.
  • Glycogen breakdown and synthesis are reciprocally regulated.

Glycogen: Structure and Function

  • Glycogen is a large polymer of glucose molecules that can be rapidly broken down into glucose monomers when needed.
  • While not as energy-rich as fatty acids, glycogen serves to:
    • Buffer blood glucose levels between meals.
    • Provide a glucose storage mechanism for strenuous muscular activity.
  • Glycogen is a readily mobilized form of stored glucose.
  • It can be converted to glucose directly, fueling anaerobic activities and playing a role in fight-or-flight responses.

Glycogen Storage

  • Glycogen is a highly branched homopolymer of glucose found in all tissues.
  • The largest glycogen stores are in the liver and muscle.
  • The liver breaks down glycogen and releases glucose into the blood to provide energy for the brain and red blood cells.
  • Muscle glycogen is mobilized to provide energy for muscle contraction.
  • Glucose units are joined by α-1,4 linkages (straight lines).
  • G represents glycogenin.

Glycogen Structure

  • A glycogen molecule consists of a branched series of glucose monomers joined through α-1,6 and α-1,4-glycosidic bonds.
  • Most glucose-glucose bonds in glycogen are of the α-1,4 variety.

Fate of Glycogen Breakdown

  • The metabolic fate of glucose-6-phosphate (glucose-6-P) produced from glycogenolysis varies depending on tissue type.
  • The activity of glucose-6-phosphatase is crucial in determining this fate.

Enzymes in Glycogen Breakdown

Four distinct enzymatic activities are required to generate glucose-6-P for further energy metabolism:

  1. Glycogen degradation: glycogen phosphorylase
  2. Remodeling glycogen to ensure the chain remains a substrate for degradation:
    • Transferase
    • α-1,6-glucosidase (debranching enzyme)
  3. Converting the product of glycogen breakdown into a useful form (glucose-6-P):
    • Phosphoglucomutase

Glycogen Phosphorylase

  • Glycogen phosphorylase cleaves the glycogen chain through the addition of phosphate (Pi), yielding glucose-1-P and a shortened glycogen chain:
    Glycogenn+Pi→Glucose−1−P+Glycogenn−1Glycogenn + Pi \rightarrow Glucose-1-P + Glycogen_{n-1}

Glycogen Phosphorylase: Important Facts

  • The glycogen phosphorylase reaction is readily reversible in vitro (very small ΔG°\Delta G°).
  • Inside the cell, the equilibrium favors glycogen breakdown because the concentration of glucose-1-P is typically less than 1% of free phosphate (Pi).
  • The phosphorolytic cleavage of glycogen is energetically advantageous because the released sugar is already phosphorylated, conserving ATP.
  • It also traps the “activated glucose” molecule within the cell.

Glycogen Transferase and Debranching Enzyme

  • Transferase shifts a block of three residues from the outer branch to the acceptor branch.
  • Debranching enzyme produces a free glucose and a linear chain.
  • Glycogen phosphorylase stops cleaving the glycogen chain when its terminal residue is four residues from the branch point.

Remodeling Enzymes

  • Glycogen phosphorylase degrades the glycogen chain to a limited extent, stopping when it reaches an α-1,6-glycosidic bond.
  • Inside the cell, glycogen phosphorylase stops cleaving the glycogen chain when its terminal residue is four residues from the branch point.
  • Since a branch point is encountered approximately every 10 residues, glycogen breakdown would cease after only six glucose equivalents are released.
  • Further breakdown requires two glycogen remodeling enzymes:
    • Transferase
    • Debranching enzyme (α-1,6 glucosidase)

Phosphoglucomutase

  • The glucose-1-P produced by glycogen phosphorylase is already phosphorylated but not a substrate for downstream metabolism.
  • To enter the major pathways for energy production or free glucose release, glucose-1-P must first be converted to glucose-6-P.
  • This isomerization reaction is catalyzed by phosphoglucomutase.

Glucose-6-Phosphatase

  • Glucose-6-P is effectively trapped within the cell.
  • Glucose-6-phosphatase releases free glucose into the blood following glycogen breakdown:
    Glucose−6−P+H2O→Glucose+PiGlucose-6-P + H_2O \rightarrow Glucose + Pi
  • The liver releases glucose into the bloodstream during exercise and between meals; this glucose is primarily absorbed by skeletal muscle and brain tissue.
  • Glucose-6-phosphatase is expressed almost exclusively in the liver and is part of both the gluconeogenic and glycogen breakdown pathways, highlighting the liver’s role in buffering blood glucose levels.
  • Because glucose-6-phosphatase opposes the function of hexokinase, these two activities must be coordinately regulated in the liver. Glycolysis is not a major energy-producing pathway for the liver; instead, it is used to fuel the TCA cycle for biosynthetic and storage purposes.

Regulatory Circuits in Glycogen Degradation

  • Glycogen breakdown is controlled through multiple biochemical mechanisms that affect each other.
  • The enzyme glycogen phosphorylase is central to this regulatory circuitry.
  • Phosphorylase is regulated by several allosteric effectors that signal the energy state of the cell.
  • The enzyme is also subject to reversible covalent modification (phosphorylation) responsive to hormones such as insulin, glucagon, and epinephrine.
  • The regulation differs between the liver and skeletal muscle. Muscle uses glycogen for its own energy production, while the liver maintains glucose homeostasis for the whole body.

Conformational Equilibrium of Phosphorylase

  • Phosphorylation modulates the equilibrium between the R (active) and T (inactive) states.
  • The phosphorylated a-form favors the R state; the unphosphorylated b-form favors the T state.
  • a and b are ensembles of R and T states with different distributions.
  • Phosphorylase is a dimer in equilibrium between an active, relaxed (R) state and an inactive, tense (T) state.
  • The equilibrium for the a-form favors the R (active) state, while the equilibrium for the b-form favors the T (inactive) state.

Muscle Phosphorylase Regulation

  • The default state of muscle phosphorylase is the b form (inactive).
  • AMP binding to muscle phosphorylase b stabilizes the R state, activating it.
  • Under resting conditions, muscle phosphorylase b is inhibited by ATP and glucose-6-P through stabilization of the T state.
  • When exercise begins, increasing levels of AMP cause allosteric activation of phosphorylase b.

Muscle Phosphorylase and Phosphorylation

  • The b-form can be converted into the a-form by phosphorylation, catalyzed by phosphorylase kinase.
  • The a form in muscle is not subject to allosteric regulation.
  • Epinephrine and/or electrical stimulation of the muscle generate the active a-form.

Phosphorylase Kinase Regulation

  • Calcium (Ca2+Ca^{2+}) and phosphorylation are required for maximal activity of phosphorylase kinase.
  • Phosphorylase kinase activates phosphorylase b by phosphorylation, generating phosphorylase a.
  • The kinase's activity is under dual metabolic control, activated by phosphorylation and increases in calcium levels.
  • Phosphorylation of phosphorylase kinase converts it from a low-activity form (b) to a high-activity form (a), accomplished by the cellular response to hormones.
  • Calcium level is a crucial control mechanism in muscle. Its release from the sarcoplasmic reticulum (SR) triggers both contraction and glycogen breakdown.

Liver Phosphorylase

  • The purpose of glycogen breakdown in the liver is to produce glucose for export to other tissues when blood glucose levels are low; therefore, the default form of liver phosphorylase is the a form.
  • Liver phosphorylase a exhibits the most responsive R - T transitions in response to glucose. Glucose shifts the enzyme from the R to the T state.
  • Insulin release leads to liver phosphorylase a to b conversion.
  • The difference between muscle and liver phosphorylase regulation illustrates the use of different isoforms of an enzyme to establish tissue-specific biochemical properties.
  • Liver phosphorylase activity is insensitive to AMP level.

Hormonal Control of Glycogen Breakdown

  • Glycogen metabolism is exquisitely responsive to blood concentrations of several hormones. Two of these, epinephrine and glucagon, stimulate glycogen degradation.
  • Epinephrine, released by the adrenal medulla during muscular exertion or its anticipation, stimulates glycogen breakdown primarily in muscle.
  • Glucagon, a peptide released by pancreatic α-cells as blood glucose levels drop, stimulates glycogen breakdown in the liver.

cAMP Signaling

  • Epinephrine (muscle) or glucagon (liver) activates a 7TM receptor.
  • This activates a G-protein (α, β, γ subunits) that converts GDP to GTP.
  • The GTP-bound α subunit activates adenylate cyclase, which produces cyclic AMP (cAMP) from ATP.
  • cAMP activates protein kinase A.
  • Protein kinase A phosphorylates phosphorylase kinase, which then phosphorylates phosphorylase b to activate it.

Signal Transduction Cascades

  • Hormone binding to specific receptors on hormone-responsive cells leads to the generation of cyclic AMP (cAMP).
  • cAMP ultimately controls the activity of protein kinase A.

Steps in the signaling cascade:

  1. Hormones bind to specific seven-transmembrane receptors in the plasma membrane, activating an intracellular G-protein (a GTPase).
  2. The GTP-bound subunit activates adenylate cyclase, which produces cAMP from ATP.
  3. Elevated levels of cAMP activate protein kinase A (PKA).
  4. PKA phosphorylates its targets.
  • The signaling cascade that leads to phosphorylation-dependent activation of both phosphorylase kinase and glycogen phosphorylase is shut down immediately once the initiating hormone is no longer present. They are dephosphorylated by protein phosphatase 1 (PP1).

Glycogen Synthesis and Degradation Pathways

  • The biosynthetic and degradative pathways for glycogen do not occur through exactly the same reactions running in reverse.
  • The biosynthesis of glycogen requires UDP-glucose, an activated form of glucose.
    • Synthesis: glycogenn+UDP−glucose→glycogenn+1+UDPglycogenn + UDP-glucose \rightarrow glycogenn+1 + UDP
    • Degradation: glycogenn+1+Pi→glycogenn+glucose−1−Pglycogenn+1 + Pi \rightarrow glycogenn + glucose-1-P

UDP-glucose

  • UDP-glucose is an activated glucose carrier, similar to how ATP is a carrier of phosphate.
  • UDP-glucose is synthesized from glucose-1-P and uridine triphosphate (UTP) in a reaction catalyzed by UDP-glucose pyrophosphorylase:
  • This reaction generates PPi and is readily reversible in vitro. However, PPi is rapidly hydrolyzed into 2Pi in vivo. This irreversible reaction drives the synthesis of UDP-glucose inside the cell.
  • Many biosyntheses are driven by the hydrolysis of PPi.

Glycogen Synthase

  • New glucosyl units are added to the non-reducing termini of a growing glycogen chain.
  • The glucosyl unit of UDP-glucose is transferred to the C-4 hydroxyl group to form an α-1,4-glycosidic bond. This reaction is catalyzed by glycogen synthase, a key regulatory enzyme in the pathway.
  • The reaction requires a minimal chain of at least 4 residues and is “primed” by a protein called glycogenin.

Glycogenin

  • Glycogenin is a glycosyltransferase formed by two identical subunits. Each subunit adds a 10-20 residue glucosyl oligomer with α-1,4 linkage on its neighboring subunit.
  • Each glycogen molecule contains a glycogenin in the center.

Branching Enzyme

  • The formation of α-1,6 branches in a growing glycogen chain is catalyzed by a single enzyme called the “branching enzyme”.
  • The branching enzyme has a strict substrate requirement. It only moves groups of 7 residues from non-reducing ends, and these residues must come from a chain at least 11 subunits long.
  • The new branch point must be at least 4 positions away from its nearest neighbor.

Advantages of Branching

  • Increases its solubility.
  • Creates many terminal residues to enhance the rate of synthesis and degradation.

Glycogen Efficiency

  • Two molecules of ATP are required to incorporate dietary glucose into glycogen.
  • The complete oxidation of glucose derived from glycogen yields 31 molecules of ATP.
  • Glycogen is an efficient form of glucose storage.

Regulation of Glycogen Synthase

  • Glycogen synthase is regulated by covalent modification.
  • Phosphorylation of glycogen synthase is carried out by PKA and glycogen synthase kinase (GSK).
  • Phosphorylation converts the active (“a”) form of the synthase into an inactive (“b”) form.
  • The “b” form is activated by a high level of glucose-6-P, while the “a” form is insensitive to glucose-6-P.
  • Glycogen synthase “a” form: active, non-phosphorylated; “b” form: inactive, phosphorylated.
  • Phosphorylation has opposite effects on glycogen phosphorylase.

Insulin and Glycogen Synthesis

  • In the presence of high blood glucose levels, insulin is secreted by the pancreatic β-cells.
  • Insulin stimulates the synthesis of glycogen by inactivating glycogen synthase kinase.
  • Insulin first binds to its receptor on the plasma membrane, a dimeric receptor tyrosine kinase.
  • The activated insulin receptor then phosphorylates “Insulin Receptor Substrates”, which eventually activate protein kinases that phosphorylate and inactivate GSK. At this point, the synthase may be dephosphorylated and activated by PP1.

Coordinated Regulation of Glycogen Metabolism

  • Protein phosphatase 1 (PP1) shifts glycogen metabolism from the degradation mode to the synthesis mode.
  • PP1 removes phosphoryl groups from glycogen synthase b, converting it into the more active a form.
  • Once intracellular glucose levels rise, both phosphorylase kinase and glycogen phosphorylase are dephosphorylated by PP1, glycogen breakdown is stopped.
  • Glucagon and epinephrine activate glycogen breakdown and shut off glycogen synthase by activating:
    • Phosphorylase kinase
    • Phosphorylase
    • Glycogen synthase

PP1 and Glycogen Metabolism Regulation

  • PP1 inhibits glycogen breakdown and stimulates glycogen synthesis.

Liver Glycogen Metabolism and Blood Glucose Levels

  • After a rich meal, blood glucose levels rise, stimulating glycogen synthesis in the liver. While insulin is the primary signal, the concentration of blood glucose itself also affects liver function.
  • The amount of liver phosphorylase a drops rapidly when glucose is abundant. After a brief lag period, the levels of glycogen synthase a increase to allow for storage to take place. Thus, phosphorylase a is the glucose sensor in the liver.
  • The T state is the substrate of PP1.

Glucose Regulation of Liver Glycogen Metabolism

  • GL: the regulatory subunit of PP1 in the liver.
  • PP1 binds tightly to phosphorylase a only when the phosphorylase is in the R state, but PP1 is inactive when bound.
  • Glucose binding to phosphorylase a leads to its transition to the T state, which dissociates from GL and frees PP1 for catalytic activity.
  • PP1 dephosphorylates phosphorylase a and glycogen synthase b, turning off glycogen degradation and turning on synthesis.