Glycogen Metabolism
Chapter 21: Glycogen Metabolism
Learning Goals
By the end of this chapter, you should be able to:
Identify the steps of glycogen breakdown and the enzymes required.
Distinguish the regulation of glycogen breakdown.
Identify the steps of glycogen synthesis and the enzymes required.
Identify how glycogen breakdown and synthesis are coordinated and regulated.
Distinguish the different roles of glycogen metabolism in liver and muscle.
Chapter 21 Outline
21.1 Glycogen Metabolism Is the Regulated Release and Storage of Glucose in Multiple Tissues
21.2 Glycogen Breakdown Requires the Interplay of Several Enzymes
21.3 Phosphorylase Is Regulated by Allosteric Interactions and Controlled by Reversible Phosphorylation
21.4 Glucagon and Epinephrine Signal the Need for Glycogen Breakdown
21.5 Glycogen Synthesis Requires Several Enzymes and Uridine Diphosphate Glucose
21.6 Glycogen Breakdown and Synthesis Are Reciprocally Controlled by Hormones
Section 21.1: Glycogen Metabolism Is the Regulated Release and Storage of Glucose in Multiple Tissues
Glucose Storage Limitation: Glucose cannot be stored in large amounts due to its osmotic effect, which can disrupt osmotic balance and lead to cell damage or death.
Glycogen Structure: Glycogen is a significantly less osmotically active and highly branched polymer. It can be rapidly broken down to yield glucose molecules when energy is needed.
Present in bacteria, archaea, protists, and animals.
The controlled release of glucose maintains blood glucose levels between specific concentrations, which is essential for health.
Glycogen is a good source of energy for sudden, strenuous activity because it can be metabolized in the absence of high oxygen levels.
Primary Storage Sites for Glycogen
Glycogen is stored primarily in the liver and muscle tissues, appearing in the cytoplasm as granules that consist of multiple glycogen molecules.
Glycogen Composition
Complex Homopolymer: Glycogen is a complex homopolymer of glucose with a protein core.
Individual glycogen molecules approximately contain:
~12 layers of glucose molecules.
Can be as large as 40 nm.
Contain approximately 55,000 glucose residues with glycogenin serving as the core protein.
Fundamental Structure of Glycogen
Polymer Structure: Glycogen is primarily composed of glucose residues linked by $ ext{α-1,4}$ glycosidic bonds.
Branching occurs at about every 12 residues via $ ext{α-1,6}$ glycosidic bonds.
Glycogen Breakdown and Synthesis Overview
Glycogen Degradation: Consists of three main steps that ultimately produce glucose 6-phosphate, which can be further:
Metabolized via glycolysis.
Converted into free glucose in the liver and released into the bloodstream.
Processed by the pentose phosphate pathway to yield ribose and NADPH derivatives.
Glycogen Synthesis: Occurs when glucose is abundant and glycogen stores are depleted.
Glucose 6-Phosphate Metabolic Fates
Derived from Glycogen: Glucose 6-phosphate can be used in several metabolic pathways:
As a fuel for anaerobic or aerobic metabolism, especially in muscle tissues.
Converted into free glucose in the liver for bloodstream release.
Processed by the pentose phosphate pathway to generate NADPH and ribose in various tissues.
Section 21.2: Glycogen Breakdown Requires the Interplay of Several Enzymes
Key Enzyme Overview:
Glycogen Phosphorylase: This enzyme cleaves glycogen by adding orthophosphate (Pi).
Phosphorolysis Definition: Cleavage of a bond by the addition of orthophosphate.
The Phosphorylase Mechanism
Mechanism Details:
Phosphorylase catalyzes the sequential removal of glucosyl residues from the non-reducing ends.
Orthophosphate cleaves the glycosidic linkage between C-1 of the terminal residue and C-4 of the adjacent one, retaining the $ ext{α}$ configuration at C-1.
Conversion: Glucose 1-phosphate can be converted to glucose 6-phosphate by phosphoglucomutase.
Pyridoxal Phosphate in Phosphorolytic Cleavage
Enzyme Details:
Phosphorylase is a dimer of identical subunits folded into an N-terminal domain containing a glycogen-binding site and a C-terminal domain.
Active site excludes water to conserve ATP required to phosphorylate free glucose.
The glycogen-binding site is 30 Å away from the catalytic site but is linked by a narrow crevice that can accommodate 4-5 glucose units.
Pyridoxal phosphate (PLP) is used as a cofactor, forming a Schiff-base linkage with a lysine residue of the phosphorylase.
Glycogen Phosphorylase Structural Features
Homodimer Formation: Each catalytic site has a PLP group linked to lysine 680 of the enzyme. The active site and phosphate-binding site are separate yet connected through a narrow crevice.
Glycogen Remodeling Enzyme Activities
Enzyme Functions: Glycogen remodeling requires three distinct catalytic activities:
Phosphorylase Activity: Cleaves $ ext{α-1,4}$ glycosidic bonds on each branch, leaving 4 residues.
Debranching Enzyme (Bifunctional):
Transferase shifts a block of 3 glucosyl residues from one branch to another.
$ ext{α-1,6}$-glucosidase removes the remaining glucose, yielding a linear chain for further cleavage.
Action of $ ext{α-1,6}$-Glucosidase
This enzyme catalyzes the removal of the remaining glucose, producing a linear chain suitable for action by phosphorylase.
Phosphoglucomutase Activity
Mechanism:
A phosphoryl group is transferred from the active site serine residue of phosphoglucomutase to the C-6 hydroxyl group of glucose 1-phosphate, producing glucose 1,6-bisphosphate.
This is followed by a transfer restoring phosphoglucomutase, leading to glucose 6-phosphate production.
Liver Function in Glucose Homeostasis
Glucose 6-Phosphatase Role:
This enzyme hydrolytically cleaves glucose 6-phosphate, yielding free glucose and orthophosphate.
The free glucose is released into the blood for use by the brain and other tissues.
Absent in most tissues, leading muscle tissues to retain glucose 6-phosphate for ATP generation.
Free glucose is not a major fuel for the liver, which instead focuses on maintaining blood glucose levels.
Section 21.3: Phosphorylase Regulation
Isozymic Forms: Glycogen phosphorylase exists in two forms: liver form and skeletal form.
The enzyme is regulated by two mechanisms:
Allosteric effectors signaling the energy state of the cell.
Reversible phosphorylation, responsive to hormones.
Muscle Phosphorylase Functionality
Dimeric Muscle Phosphorylase:
Exists in both phosphorylated (active) and unphosphorylated (inactive) forms.
Both forms interconvert between active (R) and less active (T) states, with equilibria favoring:
phosphorylase a favoring the R state (active).
phosphorylase b favoring the T state (inactive).
Quaternary States of Phosphorylase
Phosphorylase a: Phosphorylated on serine 14 of each subunit, favoring the active R state, while active sites are partly occluded in the T state.
Phosphorylase Regulation Mechanism
Both phosphorylase forms exist between active R and less active T states. Phosphorylase a is typically more active due to equilibrium favoring R, and b is less active due to equilibrium favoring T. Regulatory structures are indicated in blue and green.
Liver Glycogen Phosphorylase Characteristics
The prevalent mode of liver phosphorylase a is the active form, aiding in maintaining blood glucose levels.
Binding glucose to the active site shifts labeled phosphorylase a from active R to less active T state, highlighting the complex regulation of glycogen breakdown.
Liver Glycogen Phosphorylase Activity
Glucose binding induces allosteric shift towards inactive state while the phosphorylated state remains unchanged (T state).
Regulation of Muscle Phosphorylase
Muscle phosphorylase defaults to the b form, indicating active phosphorylase scenarios.
AMP activates muscle phosphorylase b by stabilizing the R state while ATP and glucose 6-phosphate act as inhibitors stabilizing the T state.
Allosteric Regulation Overview
Allosteric regulation involves energy charge states influencing phosphorylase forms.
Muscle phosphorylase a (activated) contrasts with muscle phosphorylase b (inhibited), informing upon enzyme activity based on energy demands.
Phosphorylation and Glycogen Breakdown Activation
Phosphorylation promotes the conversion from b to a form of phosphorylase, chiefly in response to glucagon and epinephrine.
Such hormonal responses underscore the rapid mobilization capability in times of low glucose levels or energy demands.
Phosphorylase Kinase Activation
Phosphorylase kinase has a subunit structure of (αβγδ)4, with key activation by calcium ions and hormonal signals assisting in glycogen phosphorylase activation.
Control of Glycogen Breakdown via Hormones
Both glucagon and epinephrine act in tissue-specific manners to regulate glycogen breakdown, adjusting energy supply based on physiological demands.
Signal Transduction Mechanisms
When glucagon and epinephrine bind to their specific 7TM receptors, G proteins transduce signals leading to adenylate cyclase activation and cAMP production, which subsequently facilitates phosphorylase kinase and glycogen phosphorylase activation.
Signal Termination Mechanisms
Glycogen breakdown must be rapidly terminated through various signaling pathways, promoting decreased enzyme activity once hormone levels normalize, including:
G protein inactivation.
The role of phosphodiesterases (PDEs) in converting cAMP to AMP.
Protein phosphatase 1 (PP1) activity dephosphorylating phosphorylase and phosphorylase kinase, returning them to inactive states.
Section 21.5: Glycogen Synthesis Pathway
Glycogen synthesis utilizes UDP-glucose as the activated glucose donor, proceeding through establish pathways requiring specific enzymes for generation.
UDP-Glucose Formation
UDP-glucose is created by UDP-glucose pyrophosphorylase, and although the reaction is reversible, hydrolysis of pyrophosphate drives the reaction towards UDP-glucose formation.
Glycogen Synthase Function
Glycogen synthase is critical regulatory enzyme responsible for adding glucosyl units to the growing glycogen chains through $ ext{α-1,4}$ linkages.
Role of Glycogenin in Synthesis
Glycogenin is essential in forming the primer for glycogen synthase initiation, producing initial α-1,4-glucose polymers until a minimum chain length is reached before glycogen synthase can take over.
Branching Enzyme Activity
The branching enzyme facilitates the formation of $ ext{α-1,6}$ linkages, enhancing glycogen solubility and thus storage efficiency through increased branch points and accessibility for enzymatic action.
Importance of Branching in Glycogen Structure
Branching leads to:
Increased solubility of glycogen.
Enhanced mobilization efficiency due to a larger number of terminal residues for phosphorylase and synthase interaction.
Glycogen Synthase Regulation
Glycogen synthase has two forms, with the nonphosphorylated form (active a) contrasting against the phosphorylated form (inactive b).
Activators: Glucose 6-phosphate stabilizes R state of synthase to promote glycogen synthesis.
Inactivators: Glycogen synthase kinase functions under insulin regulation inducing phosphorylation that inhibits enzyme activity.
Efficient Glucose Storage via Glycogen
The requirement for one ATP molecule during glucose incorporation into glycogen reflects the energetic status prerogative in maintaining glucose homeostasis while abundant ATP can favor synthesis over breakdown.
Section 21.6: Reciprocal Regulation of Glycogen Metabolism
Glycogen synthesis is inhibited by signaling pathways activated by glucagon and epinephrine, simultaneously engaging pathways stimulating glycogen breakdown.
Activation of PKA is pivotal in instigating phosphorylase kinase activation whilst concurrently inhibiting glycogen synthase activity.
Hormonal Regulation Mechanisms
Hormone-triggered cascades involving cyclic AMP activate PKA, which subsequently leads to the breakdown of glycogen while inhibiting glycogen synthesis.
Role of Protein Phosphatase 1 in Glycogen Regulation
PP1 counteracts the effects of kinases by dephosphorylating proteins participating in breakdown regulation, effectively enfranchising glycogen storage activities.
Regulatory Aspects of PP1 Activity
PP1 typically binds regulatory subunits which can modify its activity, while PKA can inhibit PP1 through phosphorylation mechanisms fostering dynamic regulation of glycogen metabolism.
Insulin’s Role in Glycogen Storage Promotion
Insulin inactivates glycogen synthase kinase via a signaling cascade, bolstering the synthesis of glycogen by preventing the maintenance of glycogen synthase in its inactive phosphorylated state.
Final Section: Glycogen Storage Diseases
An overview of several glycogen storage diseases, their defective enzymes, affected organs, clinical features, and unique characteristics.
Type I (Von Gierke): Defective glucose 6-phosphatase leads to severe hypoglycemia and liver enlargement.
Type II (Pompe): Deficiency in α-1,4-glucosidase resulting in lysosomal accumulation of glycogen and respiratory failure.
Type III (Cori): Associated with debranching enzyme deficiency with similar yet milder symptoms compared to Type I.
Each disease presents distinct biochemical and clinical implications rooted in the disruptions of glycogen metabolism processes.
Selected Disease Features:
Pompe Disease: Characterized by excessive lysosomal glycogen accumulation, leading to disrupted myofibril function and ultimately rupture.
Von Gierke Disease: Manifested through severe hypoglycemia and abdominal enlargement due to unregulated glycogen storage in liver tissues.
Conclusion
The detailed regulation and interplay of enzymes involved in glycogen metabolism emphasize the importance of both glycogen synthesis and breakdown in maintaining glucose homeostasis, with implications for understanding metabolic disorders and energy management in biological systems.
Learning Goals
By the end of this chapter, you should be able to:
Identify the steps of glycogen breakdown and the enzymes required.
Distinguish the regulation of glycogen breakdown.
Identify the steps of glycogen synthesis and the enzymes required.
Identify how glycogen breakdown and synthesis are coordinated and regulated.
Distinguish the different roles of glycogen metabolism in liver and muscle.
Chapter 21 Outline
21.1 Glycogen Metabolism Is the Regulated Release and Storage of Glucose in Multiple Tissues
21.2 Glycogen Breakdown Requires the Interplay of Several Enzymes
21.3 Phosphorylase Is Regulated by Allosteric Interactions and Controlled by Reversible Phosphorylation
21.4 Glucagon and Epinephrine Signal the Need for Glycogen Breakdown
21.5 Glycogen Synthesis Requires Several Enzymes and Uridine Diphosphate Glucose
21.6 Glycogen Breakdown and Synthesis Are Reciprocally Controlled by Hormones
Section 21.1: Glycogen Metabolism Is the Regulated Release and Storage of Glucose in Multiple Tissues
Glucose Storage Limitation: Glucose cannot be stored in large amounts due to its significant osmotic effect. High intracellular concentrations of glucose would draw significant amounts of water into the cell, disrupting osmotic balance, leading to cell swelling, lysis, or death. Therefore, glucose is efficiently converted into glycogen.
Glycogen Structure: Glycogen is a significantly less osmotically active and highly branched polymer of glucose, commonly referred to as "animal starch." Its branched structure allows for rapid breakdown to yield glucose molecules when energy is needed. Glycogen granules, visible in the cytoplasm, range from 10 to 40 nm in diameter. It is present in bacteria, archaea, protists, and animals.
The controlled release of glucose maintains blood glucose levels within specific concentrations (typically 70-120 \text{ mg/dL} or 3.9-6.7 \text{ mM}), which is essential for proper physiological function, especially for the brain and red blood cells.
Glycogen is a good source of energy for sudden, strenuous activity because it can be metabolized rapidly in the absence of high oxygen levels (anaerobic glycolysis).
Primary Storage Sites for Glycogen
Glycogen is stored primarily in the liver and muscle tissues, appearing in the cytoplasm as granules that consist of multiple glycogen molecules and the enzymes required for its synthesis and degradation.
Liver Glycogen (up to 10% of liver weight): Primarily functions to maintain blood glucose homeostasis for the entire organism. It acts as a glucose reservoir, releasing glucose into the bloodstream to supply other tissues, especially the brain.
Muscle Glycogen (up to 1-2% of muscle weight): Serves as an immediate and readily available source of glucose for the muscle's own energetic needs, particularly during muscle contraction.
Glycogen Composition
Complex Homopolymer: Glycogen is a complex homopolymer of glucose units with a protein core, glycogenin.
Individual glycogen molecules approximately contain:
~12 layers of glucose molecules.
Can be as large as 40 nm.
Contain approximately 55,000 glucose residues with glycogenin serving as the core protein, initiating glycogen synthesis.
Fundamental Structure of Glycogen
Polymer Structure: Glycogen is primarily composed of glucose residues linked by \text{α-1,4} glycosidic bonds, forming linear chains.
Branching occurs at about every 8-12 residues via \text{α-1,6} glycosidic bonds. These branches create a compact structure and provide numerous non-reducing ends, which are crucial for rapid synthesis and degradation.
Enzymes act on these non-reducing ends, allowing multiple enzymes to work simultaneously, thereby greatly accelerating glucose release or storage.
Glycogen Breakdown and Synthesis Overview
Glycogen Degradation: Consists of three main steps that ultimately produce glucose 6-phosphate, which can be further:
Metabolized via glycolysis for energy production.
Converted into free glucose in the liver and released into the bloodstream to maintain blood glucose levels.
Processed by the pentose phosphate pathway to yield ribose (for nucleotide synthesis) and NADPH derivatives (for reductive biosynthesis and antioxidant defense).
Glycogen Synthesis: Occurs when glucose is abundant and glycogen stores are depleted, ensuring efficient storage of excess glucose for later use.
Glucose 6-Phosphate Metabolic Fates
Derived from Glycogen: Once glycogen is broken down to glucose 6-phosphate, it can be used in several distinct metabolic pathways depending on the tissue:
As a fuel for anaerobic or aerobic metabolism directly within the cell, especially in muscle tissues, to generate ATP. Muscle cells lack glucose 6-phosphatase, so they cannot release free glucose.
Converted into free glucose in the liver by glucose 6-phosphatase and released into the blood for systemic distribution.
Processed by the pentose phosphate pathway to generate NADPH and ribose in various tissues, supporting anabolic reactions and protecting against oxidative stress.
Section 21.2: Glycogen Breakdown Requires the Interplay of Several Enzymes
Key Enzyme Overview:
Glycogen Phosphorylase: This enzyme is the primary control point for glycogen breakdown. It catalyzes the sequential removal of glucose residues from the non-reducing ends of glycogen by adding orthophosphate (Pi).
Phosphorolysis Definition: Cleavage of a bond by the addition of orthophosphate, as opposed to hydrolysis (cleavage by water).
The Phosphorylase Mechanism
Mechanism Details:
Phosphorylase catalyzes the sequential removal of glucosyl residues from the non-reducing ends of glycogen until it reaches a point approximately four glucose residues away from an \text{α-1,6} branch point.
Orthophosphate cleaves the \text{α-1,4} glycosidic linkage between C-1 of the terminal residue and C-4 of the adjacent one, producing glucose 1-phosphate (G1P). The reaction retains the \text{α} configuration at C-1 of the glucose 1-phosphate.
Conversion: Glucose 1-phosphate is then converted to glucose 6-phosphate (G6P) by phosphoglucomutase, making it available for glycolysis or other pathways.
The reaction is: (\text{Glycogen}){n} + \text{Pi} \rightleftharpoons (\text{Glycogen}){n-1} + \text{Glucose 1-phosphate}
Pyridoxal Phosphate in Phosphorolytic Cleavage
Enzyme Details:
Phosphorylase is a dimer of identical subunits, each folded into an N-terminal domain containing a glycogen-binding site and a C-terminal domain.
The active site of phosphorylase excludes water to prevent hydrolysis and conserve the ATP that would be required to phosphorylate free glucose if it were formed by hydrolysis.
The glycogen-binding site is approximately 30 Å away from the catalytic site but is linked by a narrow crevice that can accommodate 4-5 glucose units, guiding the substrate to the active site.
Pyridoxal phosphate (PLP), a derivative of vitamin \text{B}_{6}, is used as a cofactor. It forms a Schiff-base linkage with a specific lysine residue (Lys680) of the phosphorylase. The phosphate group of PLP acts as a general acid-base catalyst, facilitating the attack of orthophosphate on the glycosidic bond.
Glycogen Phosphorylase Structural Features
Homodimer Formation: Each catalytic site has a PLP group covalently linked to lysine 680 of the enzyme. The active site and phosphate-binding site are separate yet connected through a narrow channel, allowing substrate specific interaction.
The enzyme undergoes significant conformational changes during its regulation, transitioning between active (R) and inactive (T) states.
Glycogen Remodeling Enzyme Activities
Glycogen remodeling, crucial for complete breakdown, requires three distinct catalytic activities to handle the branched structure:
Phosphorylase Activity: Cleaves \text{α-1,4} glycosidic bonds from the non-reducing ends of each branch, leaving 4 residues on each branch until it can no longer proceed due to the proximity of the \text{α-1,6} branch point.
Debranching Enzyme (Bifunctional): This single enzyme possesses two distinct catalytic activities:
Transferase Activity (\text{α-1,4}-glucanotransferase): It shifts a block of 3 glucosyl residues from one branch to another, transferring them from a limit dextrin to a nearby non-reducing end. This exposes the \text{α-1,6} branch point.
\text{α-1,6} -Glucosidase Activity: This hydrolytic enzyme removes the single remaining glucose residue that was linked by an \text{α-1,6} bond at the branch point, releasing it as free glucose. This yields a linear chain for further cleavage by glycogen phosphorylase.
Action of \text{α-1,6}-Glucosidase
This enzyme catalyzes the hydrolysis of the \text{α-1,6} glycosidic bond at the branch point, releasing the glucose molecule as free glucose (not glucose 1-phosphate). This is a critical step to allow phosphorylase to continue its action on the now linear chain.
Phosphoglucomutase Activity
Mechanism:
Phosphoglucomutase contains a phosphoryl group attached to an active site serine residue (e.g., \text{Ser}^{108}).
The enzyme catalyzes the interconversion of glucose 1-phosphate to glucose 6-phosphate through a glucose 1,6-bisphosphate intermediate.
A phosphoryl group is transferred from the active site serine residue of phosphoglucomutase to the C-6 hydroxyl group of glucose 1-phosphate, producing glucose 1,6-bisphosphate.
This is followed by a transfer of the phosphoryl group from the C-1 position of glucose 1,6-bisphosphate back to the enzyme's active site serine, restoring the enzyme and leading to glucose 6-phosphate production.
The net reaction: \text{Glucose 1-phosphate} \rightleftharpoons \text{Glucose 6-phosphate}
Liver Function in Glucose Homeostasis
Glucose 6-Phosphatase Role:
This enzyme hydrolytically cleaves glucose 6-phosphate, yielding free glucose and orthophosphate.
It is localized to the endoplasmic reticulum membrane, and its active site faces the ER lumen. Both glucose 6-phosphate and the resulting glucose and Pi are transported across the ER membrane.
The free glucose is then transported out of the liver cells into the blood for use by the brain and other tissues, maintaining systemic glucose homeostasis.
This enzyme is virtually absent in most other tissues, including muscle. This crucial difference means muscle tissues retain glucose 6-phosphate for their own ATP generation via glycolysis, while the liver acts as the primary supplier of glucose to the rest of the body.
Free glucose is not a major fuel for the liver itself, which instead relies heavily on fatty acid oxidation for its energy needs, thereby sparing glucose for other organs.
Section 21.3: Phosphorylase Regulation
Isozymic Forms: Glycogen phosphorylase exists in different isozymic forms tuned to the specific needs of the tissue:
Liver Phosphorylase: Primarily responsible for maintaining blood glucose levels. Its activity is sensitive to glucose concentration and hormonal signals (glucagon).
Skeletal Muscle Phosphorylase: Provides immediate energy for muscle contraction. Its activity is sensitive to the cell's energy state (AMP, ATP, glucose 6-phosphate) and nervous/hormonal signals (epinephrine).
The enzyme is regulated through two primary mechanisms:
Allosteric effectors: Small molecules that bind to sites other than the active site, signaling the energy state of the cell.
Reversible phosphorylation: Covalent modification responsive to hormonal signals, leading to significant changes in enzyme activity.
Muscle Phosphorylase Functionality
Dimeric Muscle Phosphorylase:
Exists in both a less active, unphosphorylated form (phosphorylase b) and a more active, phosphorylated form (phosphorylase a).
Both forms exist in a dynamic equilibrium between an active (R, relaxed) state and a less active (T, tense) state, with equilibria favoring:
Phosphorylase a: Typically favors the R state, making it highly active even in the absence of allosteric activators. Phosphorylation at Serine 14 stabilizes the R state.
Phosphorylase b: Typically favors the T state, making it largely inactive unless allosterically activated by AMP.
Quaternary States of Phosphorylase
Phosphorylase a: Phosphorylated on serine 14 of each subunit by phosphorylase kinase. This phosphorylation induces a significant conformational change, shifting the enzyme to favor the active R state, where the active sites are fully accessible. In the T state, active sites are partly occluded by regulatory structures.
Phosphorylase Regulation Mechanism
Both phosphorylase forms (a and b) exist between an active R state and a less active T state. Phosphorylase a is typically more active due to equilibrium favoring the R state, while phosphorylase b is less active due to equilibrium favoring the T state. Regulatory structures involved in these conformational changes are indicated in specific regions (e.g., N-terminal segments, active site loops) and are crucial for allosteric and phosphorylation-mediated control.
Liver Glycogen Phosphorylase Characteristics
The prevalent mode of liver phosphorylase a is the active form, even in the non-phosphorylated state, aiding in maintaining constant blood glucose levels. Unlike muscle phosphorylase, liver phosphorylase is less sensitive to cellular energy charge.
Binding of glucose to the active site of liver phosphorylase a acts as an allosteric inhibitor, shifting the enzyme from the active R state to the less active T state. This serves as a negative feedback mechanism: when blood glucose is high, there is no need for liver glycogen breakdown.
Liver Glycogen Phosphorylase Activity
Glucose binding specifically induces an allosteric shift towards the inactive T state in liver phosphorylase a, while its phosphorylated state remains unchanged. This is a critical distinction from muscle phosphorylase, where glucose 6-phosphate is an inhibitor signaling cellular energy sufficiency.
Regulation of Muscle Phosphorylase
Muscle phosphorylase defaults to the b form (unphosphorylated) but can be rapidly activated by AMP.
AMP: Acts as a powerful allosteric activator of muscle phosphorylase b by binding to a nucleotide-binding site and stabilizing the active R state, signaling low energy charge. This facilitates rapid glucose mobilization during intense muscular activity.
ATP and Glucose 6-phosphate: Act as allosteric inhibitors of muscle phosphorylase b by binding to the same site as AMP or a distinct site, stabilizing the inactive T state. They signal high energy charge and glucose availability, respectively, thus preventing unnecessary glycogen breakdown.
Allosteric Regulation Overview
Allosteric regulation involves energy charge states influencing phosphorylase forms, ensuring that glycogen is only broken down when energy is needed. High ATP and G6P inhibit, while high AMP activates.
Muscle phosphorylase a (activated by phosphorylation) contrasts with muscle phosphorylase b (inhibited by ATP/G6P, activated by AMP), dynamically informing enzyme activity based on cellular energy demands.
Phosphorylation and Glycogen Breakdown Activation
Phosphorylation promotes the conversion from the less active b form to the highly active a form of phosphorylase, chiefly in response to hormonal signals like glucagon (liver) and epinephrine (muscle and liver).
Such hormonal responses underscore the rapid mobilization capability of glucose in times of low blood glucose levels (glucagon) or impending energy demands/stress (epinephrine).
Phosphorylase Kinase Activation
Phosphorylase kinase is a complex enzyme with a subunit structure of (\alpha\beta\gamma\delta)_{4} . Its activity is tightly regulated:
\text{Ca}^{2+} ions: Act as an allosteric activator by binding to the \text{δ} subunit (calmodulin) of phosphorylase kinase, sensing muscle contraction signals.
Hormonal signals (PKA): Triggered by glucagon or epinephrine, protein kinase A (PKA) phosphorylates the \text{β} and \text{α} subunits of phosphorylase kinase, increasing its activity. This dual regulation allows for both rapid neuronal/muscle contraction signals and slower hormonal signals to activate glycogen breakdown.
Control of Glycogen Breakdown via Hormones
Both glucagon and epinephrine act in tissue-specific manners to regulate glycogen breakdown, ensuring that energy supply is adjusted precisely based on physiological demands.
Glucagon: Primarily acts on the liver, signaling low blood glucose levels.
Epinephrine (Adrenaline): Acts on both liver and muscle, signaling immediate energy needs (e.g., fight-or-flight response).
Signal Transduction Mechanisms
When glucagon and epinephrine bind to their specific 7TM receptors, they activate G proteins, which then transduce signals leading to adenylate cyclase activation and subsequent cAMP production. Elevated cAMP levels activate protein kinase A (PKA), which in turn phosphorylates and activates phosphorylase kinase, ultimately leading to glycogen phosphorylase activation and increased glycogen breakdown.
Signal Termination Mechanisms
Glycogen breakdown must be rapidly terminated through various signaling pathways once the stimulus is removed, promoting decreased enzyme activity as hormone levels normalize. Key mechanisms include:
G protein inactivation: Stimulated G proteins (Gα-GTP) possess intrinsic GTPase activity, which hydrolyzes GTP to GDP, leading to their inactivation and dissociation from adenylate cyclase.
The role of phosphodiesterases (PDEs): These enzymes convert cAMP to AMP, thereby reducing the concentration of the second messenger and decreasing PKA activity.
Protein phosphatase 1 (PP1) activity: PP1 plays a crucial role in reversing the effects of PKA. It dephosphorylates phosphorylase kinase and glycogen phosphorylase a, returning them to their inactive forms (phosphorylase b and inactive phosphorylase kinase), thus promoting glycogen synthesis and inhibiting breakdown.
Section 21.5: Glycogen Synthesis Pathway
Glycogen synthesis utilizes UDP-glucose as the activated glucose donor, proceeding through established pathways requiring specific enzymes for its generation. This is an energy-consuming process that allows for efficient storage of glucose.
UDP-Glucose Formation
UDP-glucose is created by UDP-glucose pyrophosphorylase in a two-step reaction from glucose 1-phosphate and UTP: \text{Glucose 1-phosphate + UTP} \rightleftharpoons \text{UDP-glucose + PPi}
Although the immediate reaction is reversible, the subsequent hydrolysis of pyrophosphate (PPi) by inorganic pyrophosphatase to 2 Pi, which is a highly exergonic reaction, effectively drives the overall reaction towards UDP-glucose formation, making it irreversible under physiological conditions.
Glycogen Synthase Function
Glycogen synthase is the critical regulatory enzyme responsible for adding glucosyl units to the growing glycogen chains. It catalyzes the formation of new \text{α-1,4} linkages, transferring glucose from UDP-glucose to the non-reducing end of a glycogen primer.
Role of Glycogenin in Synthesis
Glycogenin is essential in forming the primer for glycogen synthase initiation. It is a protein that acts as both a primer and an enzyme, catalyzing the addition of the first few glucose molecules to itself (auto-glucosylation) via \text{α-1,4} linkages, typically up to 8 glucose residues. This initial short, linear glucose polymer (primer) provides the necessary substrate for glycogen synthase to take over and extend the glycogen chain.
Branching Enzyme Activity
The branching enzyme (\text{amylo-(1,4 \rightarrow 1,6)-transglycosylase}) facilitates the formation of new \text{α-1,6} linkages. It hydrolyzes an \text{α-1,4} bond in a linear chain and transfers a block of approximately 7 glucose residues (from a chain of at least 11 residues) to an internal glucose residue via an \text{α-1,6} linkage.
This process enhances glycogen solubility and storage efficiency through increased branch points and a larger number of non-reducing ends, which are crucial for rapid synthesis and degradation by multiple enzymes simultaneously.
Importance of Branching in Glycogen Structure
Branching leads to:
Increased solubility of glycogen: The highly branched structure prevents the formation of large, insoluble aggregates.
Enhanced mobilization efficiency: Due to a larger number of terminal residues for phosphorylase and synthase interaction, both breakdown and synthesis can occur more rapidly.
Glycogen Synthase Regulation
Glycogen synthase, like phosphorylase, exists in two interconvertible forms, with its activity regulated by phosphorylation:
Nonphosphorylated form (active a): This form is generally active, especially in the presence of glucose 6-phosphate. It contrasts against the phosphorylated form (inactive b).
Phosphorylated form (inactive b): This form is much less active or inactive, requiring allosteric activation by high levels of glucose 6-phosphate for activity.
Activators: Glucose 6-phosphate acts as a powerful allosteric activator for the inactive (b) form of glycogen synthase, stabilizing its active R state to promote glycogen synthesis when glucose is abundant.
Inactivators: Glycogen synthase kinase (GSK3) and PKA function under insulin and glucagon/epinephrine regulation, respectively, inducing phosphorylation at multiple sites that inhibit enzyme activity.
Efficient Glucose Storage via Glycogen
The requirement for one ATP molecule (which includes the UTP equivalent for UDP-glucose synthesis) for each glucose residue incorporated into glycogen reflects the energetic status prerogative in maintaining glucose homeostasis. When ATP is abundant, it can favor synthesis over breakdown, ensuring energy storage.
Section 21.6: Reciprocal Regulation of Glycogen Metabolism
Glycogen synthesis is inhibited by signaling pathways activated by glucagon and epinephrine, simultaneously engaging pathways stimulating glycogen breakdown. This reciprocal regulation ensures that synthesis and breakdown do not occur simultaneously, preventing a futile cycle.
Activation of PKA (by cAMP) is pivotal in instigating phosphorylase kinase activation whilst concurrently inhibiting glycogen synthase activity through phosphorylation. PKA phosphorylates both phosphorylase kinase (activating it) and glycogen synthase (inactivating it).
Hormonal Regulation Mechanisms
Hormone-triggered cascades involving cyclic AMP (cAMP) activate PKA, which subsequently leads to the breakdown of glycogen (via phosphorylase activation) while inhibiting glycogen synthesis (via glycogen synthase inactivation). This coordinated control maximizes efficiency during times of demand.
Role of Protein Phosphatase 1 in Glycogen Regulation
PP1 counteracts the effects of kinases by dephosphorylating key proteins participating in glycogen metabolism, effectively promoting glycogen storage activities and inhibiting breakdown.
PP1 dephosphorylates:
Glycogen phosphorylase a to glycogen phosphorylase b (inactivates breakdown).
Phosphorylase kinase to its inactive form (inactivates breakdown).
Glycogen synthase b to glycogen synthase a (activates synthesis).
Regulatory Aspects of PP1 Activity
PP1 typically binds regulatory subunits (e.g., GM in muscle, GL in liver) which can modify its activity and subcellular localization, allowing it to target specific substrates efficiently. PKA can inhibit PP1 through phosphorylation mechanisms, either by directly phosphorylating PP1 inhibitors (like Inhibitor 1) or by phosphorylating the regulatory subunits, fostering dynamic regulation of glycogen metabolism. For instance, PKA phosphorylation of Inhibitor 1 activates it, so it can bind to and inhibit PP1.
Insulin’s Role in Glycogen Storage Promotion
Insulin, a hormone signaling high blood glucose, promotes glycogen synthesis.
It activates a signaling cascade (via the insulin receptor tyrosine kinase) that ultimately inactivates glycogen synthase kinase 3 (GSK3) by phosphorylating it. By preventing GSK3 from phosphorylating and inhibiting glycogen synthase, insulin effectively bolsters the synthesis of glycogen by maintaining glycogen synthase in its active, dephosphorylated (a) state.
Insulin also activates PP1, further promoting dephosphorylation of glycogen synthase to its active form and dephosphorylation of phosphorylase and phosphorylase kinase to their inactive forms.
Final Section: Glycogen Storage Diseases
An overview of several glycogen storage diseases (GSDs), their defective enzymes, affected organs, clinical features, and unique characteristics.
Type I (Von Gierke Disease): Defective glucose 6-phosphatase (liver, kidney, intestine) leads to severe hypoglycemia (due to inability to release free glucose), hepatomegaly (liver enlargement), nephromegaly, lactic acidosis, and hyperlipidemia.
Type II (Pompe Disease): Deficiency in lysosomal \text{α-1,4}-glucosidase (acid maltase) resulting in lysosomal accumulation of glycogen in all organs, particularly muscle and heart, leading to cardiorespiratory failure and muscle weakness. It's the only GSD that is a lysosomal storage disorder.
Type III (Cori Disease or Forbes Disease): Associated with debranching enzyme deficiency (liver, muscle, heart) with similar yet milder symptoms compared to Type I, including hypoglycemia and hepatomegaly, but muscle weakness is also prominent.
Type IV (Andersen Disease): Deficiency in branching enzyme (liver, spleen, muscle) leads to the accumulation of glycogen with abnormally long outer chains and fewer branch points. This abnormal glycogen is less soluble and leads to cirrhosis and early death.
Type V (McArdle Disease): Deficiency in muscle glycogen phosphorylase (muscle) results in muscle cramps and weakness on exertion, and myoglobinuria, but no hypoglycemia as liver glycogen is unaffected.
Each disease presents distinct biochemical and clinical implications rooted in the disruptions of specific enzymes involved in glycogen metabolism processes.
Selected Disease Features:
Pompe Disease: Characterized by excessive lysosomal glycogen accumulation, leading to disrupted myofibril function, enlargement of lysosomes, and ultimately rupture, causing cellular damage, particularly in cardiac and skeletal muscle.
Von Gierke Disease: Manifested through severe hypoglycemia (especially fasting hypoglycemia) and abdominal enlargement secondary to unregulated glycogen storage in liver and kidney tissues. Patients also exhibit metabolic acidosis.
Conclusion
The detailed regulation and interplay of enzymes involved in glycogen metabolism emphasize the importance of both glycogen synthesis and breakdown in maintaining glucose homeostasis, with implications for understanding metabolic disorders and energy management in biological systems.