Chapter 21

CHAPTER 21

Glycogen Metabolism

Section 21.1 Glycogen Metabolism Is the Regulated Release and Storage of Glucose in Multiple Tissues

  • Glucose cannot be stored because high concentrations of glucose disrupt the osmotic balance of the cell, causing cell damage or death.

  • glycogen = a significantly less osmotically active and highly branched polymer that can be rapidly broken down to yield glucose molecules when energy is needed

    • present in bacteria, archaea, protists, and animals

    • controlled release maintains blood-glucose concentration between meals

    • good source of energy for sudden, strenuous activity as it can be metabolized in the absence of O2

Liver and Muscle Tissue Are the Primary Storage Sites for Glycogen

  • Glycogen is present in the cytoplasm, appearing as granules consisting of multiple glycogen molecules.

Glycogen Is a Complex Homopolymer of Glucose with a Protein Core

  • Individual glycogen molecules:

    • have ~12 layers of glucose molecules.

    • can be as large as 40 nm.

    • contain ~55,000 glucose residues and a single glycogenin protein at the core.

The Fundamental Structure of Glycogen Is a Linear Polymer with Occasional Branches

  • Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds.

  • Branches at about every 12 residues are created by α-1,6-glycosidic bonds.

Glycogen Degradation and Synthesis

  • Glycogen degradation consists of three steps, ultimately producing glucose 6-phosphate which can be:

    • metabolized by glycolysis.

    • converted into free glucose in the liver for release into the bloodstream.

    • processed by the pentose phosphate pathway to yield NADPH and ribose derivatives.

  • Glycogen synthesis also requires several steps.

    • takes place when glucose is abundant and glycogen is depleted

Glucose 6-Phosphate Can Have Many Metabolic Fates

Section 21.2 Glycogen Breakdown Requires the Interplay of Several Enzymes

  • glycogen phosphorylase (phosphorylase) = cleaves glycogen by the addition of orthophosphate (Pi)

  • phosphorolysis = cleavage of a bond by the addition of orthophosphate

The Phosphorylase Mechanism

  • Phosphorylase catalyzes the sequential removal of glucosyl residues from the nonreducing ends.

  • Orthophosphate splits the glycosidic linkage between C-1 of the terminal residue and C-4 of the adjacent one.

    • the α configuration at C-1 is retained

  • Glucose 1-phosphate can be converted to glucose 6- phosphate by phosphoglucomutase.

Mechanism: Pyridoxal Phosphate Participates in the Phosphorolytic Cleavage of Glycogen

  • Phosphorylase is a dimer of identical subunits, each compactly folded into an amino-terminal domain containing a glycogen-binding site and a carboxyl-terminal domain.

  • The active site excludes water to save the ATP required to phosphorylate free glucose.

The Mechanistic Basis of the Phosphorolytic Cleavage of Glycogen

  • Both the glycogen substrate and the glucose 1-phosphate product have an α configuration at C-1.

    • suggests that a carbocation intermediate is formed

  • Glycogen phosphorylase requires pyridoxal phosphate (PLP) as a cofactor.

    • The aldehyde group of PLP forms a Schiff-base linkage with a Lys residue of the phosphorylase.

  • The glycogen-binding site is 30 Å away from the catalytic site but is connected by a narrow crevice able to accommodate four to five glucose units.

    • enables the enzyme to phosphorolyze many residues without having to dissociate and reassociate

Glycogen Phosphorylase Forms a Homodimer with a PLP Group in Each Active Site

PLP Forms a Schiff-Base Linkage with Lysine

  • Phosphate helps hold inorganic phosphate used in binding site

  • one phosphate is attached to PLP and one phosphate is the inorganic one that is going to get used

The Mechanism of Phosphorylase Involves PLP and the Formation of a Carbocation

  • phosphate and glucose held in specific spots so that P has to come in a specific way so that it gives alpha addition

Debranching Enzyme Also Is Needed for the Breakdown of Glycogen

  • Glycogen phosphorylase can only cleave α-1,4-glycosidic bonds.

  • transferase = shifts a small block of three glucosyl residues from one outer branch to another

    • exposes a single glucose residue joined by an α-1,6-glycosidic bond

  • α-1,6-glucosidase = hydrolyzes the α-1,6-glycosidic bond

  • debranching enzyme = bifunctional enzyme in eukaryotes that contains transferase and α-1,6-glucosidase activities

  • hexokinase phosphorylates glucose if the glucose will enter glycolysis or the pentose phosphate pathway

  • can only pull off until there are 4 left, transferase moves the ones from the 4 that can’t be grabbed anymore and add it to the longer chain so that they can get pulled off, transfers all but 1

Glycogen Remodeling Requires Three Distinct Catalytic Activities

Action of α-1,6-Glucosidase

Phosphoglucomutase Converts Glucose 1-Phosphate into Glucose 6-Phosphate

  • A phosphoryl group is transferred from a Ser residue at the active site of phosphoglucomutase to the C-6 hydroxyl group of glucose 1-phosphate, forming glucose 1,6-bisphosphate.

  • To restore phosphoglucomutase, the C-1 phosphoryl group of glucose 1,6-bisphosphate is transferred to the Ser residue.

    • forms glucose 6-phosphate

Phosphoglucomutase Transfers a Phosphoryl Group to the Substrate

(it’s not just serine, this is simplified)

The Liver Contains Glucose 6-Phosphatase, a Hydrolytic Enzyme Absent from Muscle

  • Glucose 6-phosphatase hydrolytically cleaves the phosphoester linkage of glucose 6-phosphate in liver, yielding glucose and orthophosphate.

  • Glucose 6-phosphatase is absent from most other tissues.

    • Muscle tissues retain glucose 6-phosphate for ATP generation.

    • glucose is not a major fuel for the liver

Free Glucose Is Released from the Liver

  • The free glucose is released into the blood for use by the brain, red blood cells, and other tissues.

  • Glucose 6-phosphatase is located on the lumenal side of the smooth ER membrane.

    • Glucose 6-phosphate is transported into the ER.

    • Glucose and orthophosphate are shuttled back into the cytoplasm.

Section 21.3 Phosphorylase Is Regulated by Allosteric Interactions and Controlled by Reversible Phosphorylation

  • Glycogen phosphorylase is regulated by:

    • allosteric effectors that signal the energy state of the cell.

    • reversible phosphorylation, which is responsive to hormones.

  • Glycogen phosphorylase has two isozymic forms: a liver one and a skeletal muscle one.

Liver Phosphorylase Produces Glucose for Use by Other Tissues

  • Dimeric phosphorylase exists in two forms:

    • a usually active phosphorylated a form.

    • a usually inactive unphosphorylated b form.

  • Both forms exist in equilibrium between an active relaxed (R) and less active tense (T) state.

    • Equilibrium for phosphorylase a favors the R state.

    • Equilibrium for phosphorylase b favors the T state.

Phosphorylase Exists in Two Distinct Quaternary States

Phosphorylase Is Regulated by Phosphorylation


The a Form Is the Default State of Liver Phosphorylase

  • The role of glycogen degradation in liver is to maintain adequate blood glucose levels.

    • results in the a form being the default state

  • The liver phosphorylase a form exhibits the most responsive R-to-T transition.

  • Binding of glucose to the active site shifts the a form from the active R state to the less-active T state.

Liver Phosphorylase Is Allosterically Inhibited by Glucose

a → more active

phosphorylated → R state

Muscle Phosphorylase Is Regulated by the Intracellular Energy Charge

  • The default form of muscle phosphorylase is the b form.

  • Phosphorylase is active during muscle contraction.

  • AMP activates muscle phosphorylase b by binding to a nucleotide-binding site and stabilizing the R state.

  • ATP and glucose 6-phosphate allosterically inhibit muscle phosphorylase and stabilize the T state.

    • ATP competes with AMP.

    • Glucose 6-phosphate binds at the ATP-binding site and stabilizes phosphorylase b.

Allosteric Regulation of Muscle Phosphorylase

not phosphorylated: b

Biochemical Characteristics of Muscle Fiber Types Differ

  • Skeletal muscle consists of three fiber types:

    • Type I (slow-twitch) fibers = fibers that use cellular respiration, powered by fatty acid degradation, to generate ATP

    • Type IIb (fast-twitch) fibers = fibers that use glycogen as their main fuel

    • Type IIa fibers = fibers that possess properties intermediate between the other two fiber types

Biochemical Characteristics of Muscle Fiber Types

TABLE 21.1 Biochemical characteristics of muscle fiber types

Phosphorylation Promotes the Conversion of Phosphorylase b to Phosphorylase a

  • glucagon = peptide hormone released in response to low blood glucose levels

  • epinephrine (adrenaline) = hormone released during strong emotions (e.g., fear, excitement of exercise)

Phosphorylase Kinase

  • phosphorylase kinase = regulatory enzyme that catalyzes the phosphorylation of a single Ser residue in each subunit of phosphorylase to yield phosphorylase a

    • catalyzed in response to glucagon or epinephrine

    • both liver and muscle phosphorylase can be covalently modified

  • Phosphorylation moves a peptide loop out of the active site of the b form, rendering the enzyme more active.

Phosphorylase Kinase Is Activated by Phosphorylation and Calcium Ions

  • Phosphorylase kinase has a subunit composition of (αβγδ)4.

    • The α and β subunits are phosphorylation targets.

    • The γ subunit contains the active site.

    • The δ subunit is the Ca2+ binding protein calmodulin.

  • Activation of phosphorylase kinase is initiated when Ca2+ binds to the δ subunit.

  • Maximal activation occurs with the phosphorylation of the β and α subunits by protein kinase A.

Phosphorylase Kinase Is Activated by Calcium Ions and Hormonally Controlled Phosphorylation

Section 21.4 Glucagon and Epinephrine Signal the Need for Glycogen Breakdown

  • Protein kinase A (PKA) activates phosphorylase kinase, which in turn activates glycogen phosphorylase.

Glycogen Breakdown Is Controlled by the Combined Effects of Multiple Hormones in a Tissue-Specific Manner

G Proteins Transmit the Signal for the Initiation of Glycogen Breakdown

  • Step 1: Glucagon and epinephrine bind to specific 7TM receptors in the plasma membranes of target cells, activating the Gs protein.

    • Epinephrine binds to the β-adrenergic receptor in muscle.

    • Glucagon binds to the glucagon receptor in the liver.

  • Step 2: The GTP-bound subunit of Gs activates adenylate cyclase which catalyzes the formation of cAMP from ATP.

  • Step 3: cAMP activates PKA.

  • Step 4: PKA phosphorylates phosphorylase kinase which activates glycogen phosphorylase.

Glycogen Degradation Is Stimulated by Hormone Binding to 7TM Receptors

The Signal-Transduction Processes in the Liver

  • Epinephrine binds to the β-adrenergic receptor and the 7TMα-adrenergic receptor.

  • Binding to the 7TMα-adrenergic receptor initiates the phosphoinositide cascade that induces the release of Ca2+ from ER stores.

  • Binding of Ca2+ to the δ subunit of phosphorylase kinase partially activates phosphorylase kinase.

  • Stimulation by both glucagon and epinephrine leads to maximal mobilization of liver glycogen.

Glycogen Breakdown Must Be Rapidly Turned Off When Necessary

  • Glycogen breakdown is turned off by several means:

    • Signal-transduction pathways are shut down when hormone secretion ceases.

    • G proteins have inherent GTPase activity.

    • Phosphodiesterases convert cAMP to AMP, which does not stimulate PKA.

    • Protein phosphatase 1 (PP1) removes the phosphoryl groups from phosphorylase kinase and glycogen phosphorylase, inactivating the enzymes.

Section 21.5 Glycogen Synthesis Requires Several Enzymes and Uridine Diphosphate Glucose

  • Glycogen is synthesized by a pathway that utilizes uridine diphosphate glucose (UDP-glucose) as the activated glucose donor.

UDP-Glucose Is an Activated Form of Glucose

  • UDP-glucose is synthesized by UDP-glucose pyrophosphorylase in a reaction that produces a pyrophosphate (PPi).

  • The reaction is readily reversible, but hydrolysis of PPi drives the synthesis of UDP-glucose.

Synthesis of UDP-Glucose

  • The reaction is readily reversible, but hydrolysis of PPi drives the synthesis of UDP-glucose.

    • Biosynthetic reactions being driven by the hydrolysis of PPi is a recurring theme in biochemistry.

Glycogen Synthase Catalyzes the Transfer of Glucose from UDP-Glucose to a Growing Chain

  • glycogen synthase = key regulatory enzyme in glycogen synthesis that adds new glucosyl units to the nonreducing terminal residues of glycogen

    • forms an α-1,4-glycosidic linkage

Glycogenin

  • Glycogen synthase requires a primer because it can only add to a chain containing 4+ residues.

  • glycogenin = a dimer of two identical subunits, each of which catalyzes the formation of α-1,4-glucose polymers until a primer of 10–20 glucosyl units is formed

  • Once the primer forms, glycogen synthase takes over.

  • Every glycogen molecule has a glycogenin monomer covalently attached at its core.

A Branching Enzyme Forms 𝛂-1,6 Linkages

  • Glycogen synthase can only synthesize α-1,4-linkages.

  • Branching enzyme generates branches by cleaving an α-1,4-linkage, transferring a block of ~7 residues, and reattaching the block with an α-1,6 linkage.

    • The block must include the nonreducing terminus.

    • The block must come from a chain at least 11 residues long.

    • The new branch must be at least four residues away from existing branches.

Branching Enzyme Removes Approximately Seven Residues from the Nonreducing End and Reattaches Them with a α-1,6 Linkage

A Cross Section of a Glycogen Molecule Reveals the Basic Branching Structure

  • Branching:

    • increases the solubility of glycogen.

    • creates a large number of terminal residues, the site of glycogen phosphorylase and synthase.

Glycogen Synthase Is the Key Regulatory Enzyme in Glycogen Synthesis

  • Glycogen synthase exists in two forms:

    • an active nonphosphorylated a form

    • a usually inactive phosphorylated b form

  • Glycogen synthase is phosphorylated by:

    • glycogen synthase kinase, which is under the control of insulin.

    • PKA.

  • Glucose 6-phosphate is a powerful activator of glycogen synthase b, stabilizing the R state of the enzyme relative to the T state.

Glycogen Is an Efficient Storage Form of Glucose

  • One ATP is required to incorporate glucose 6-phosphate into glycogen.

  • One molecule of ATP is used to phosphorylate branch residues, which are hydrolytically cleaved, to glucose 6-phosphate.

Section 21.6 Glycogen Breakdown and Synthesis Are Reciprocally Controlled by Hormones

  • Glycogen synthesis is inhibited by the same glucagon and epinephrine signaling pathways that stimulate glycogen breakdown.

  • PKA phosphorylates phosphorylase kinase, activating the enzyme and initiating glycogen breakdown.

  • Glycogen synthase kinase and PKA phosphorylate glycogen synthase, decreasing enzyme activity and inhibiting glycogen synthesis.

Glycogen Metabolism Is Regulated in Part by Hormone-Triggered Cyclic AMP Cascades

Protein Phosphatase 1 Reverses the Regulatory Effects of Kinases on Glycogen Metabolism

  • Protein phosphatase 1 (PP1) dephosphorylates proteins to decrease the rate of glycogen breakdown.

    • inactivates phosphorylase a

    • inactivates phosphorylase kinase

    • converts glycogen synthase b to the more active glycogen synthase a

Glycogen Synthesis Is Regulated by Protein Phosphatase 1

Protein Phosphatase 1 Regulatory Subunits

  • The catalytic subunit of PP1 is usually bound to one of a family of regulatory subunits.

    • GM in skeletal muscle and heart

    • GL in liver

  • The regulatory subunits act as scaffolds to bring together the phosphatase with:

    • glycogen.

    • the catalytic subunit.

    • target enzymes.

Phosphorylation of Inhibitors

  • The cAMP cascade that activates PKA reduces the activity of PP1 by two mechanisms:

    • in muscle, phosphorylation of GM leads to dissociation of the catalytic subunit from glycogen, leading to a decrease in phosphatase activity

    • almost all tissues contain small proteins that, when phosphorylated, bind to the catalytic subunit of PP1 and inhibit it

PP1 Is Regulated by Hormonally Controlled Cascades

Insulin Stimulates Glycogen Synthesis by Inactivating Glycogen Synthase Kinase

  • When blood-glucose concentration is high, insulin inactivates glycogen synthase kinase through a tyrosine kinase signal-transduction pathway.

    • stimulates synthesis of glycogen

    • the inactive kinase cannot maintain glycogen synthase in its phosphorylated, inactive state

  • PP1 dephosphorylates glycogen synthase, activating it and restoring glycogen reserves.

  • Insulin also increases the number of glucose transporters in the membrane.;

Insulin Inactivates Glycogen Synthase Kinase

Glycogen Metabolism in the Liver Regulates the Blood-Glucose Concentration

  • The amount of liver phosphorylase a decreases rapidly when glucose is infused.

    • Glucose binding shifts phosphorylase a from the active R form to the inactive T form and PP1 dissociates, converting it to phosphorylase b

  • After a lag period, the amount of glycogen synthase a increases, resulting in glycogen synthesis.

  • Released PP1 dephosphorylates glycogen synthase, converting glycogen synthase b to the more active glycogen synthase a.

Blood Glucose Regulates Liver-Glycogen Metabolism

The Lag Between Termination of Glycogen Degradation and the Beginning of Glycogen Synthesis

  • There are ~10 phosphorylase a molecules per molecule of PP1.

  • The activity of glycogen synthase begins to increase only after most of phosphorylase a is converted into b.

  • The lag prevents the two pathways from operating simultaneously.

Glucose Regulation of Liver-Glycogen Metabolism