Glycogen Metabolism: Synthesis, Storage, and Function

Glycogen Metabolism Overview

• Glycogen synthesis, degradation, regulation, and biomedical importance are key aspects.

Glycogen Definition and Importance

• Glycogen is the primary carbohydrate storage form in animals, akin to starch in plants.
• It is a branched polymer composed of α-d-glucose molecules.
• Glycogen is essential for maintaining blood glucose levels.

Glycogen Storage Sites

• Predominantly found in the liver and muscle tissues.
• Smaller quantities exist in the brain.
• Within cells, glycogen is present as glycogen particles located in the cytoplasm.

Glycogen Function in Liver and Muscle

• In the liver, glycogen synthesis and breakdown are regulated to maintain blood glucose levels.
• In muscle, these processes are controlled to meet the energy demands of muscle cells.

Biomedical Significance of Glycogen

• The liver stores more glycogen, but muscles have more overall because of greater mass.
• Muscle glycogen fuels glycolysis within the muscle.
• Liver glycogen serves as a glucose reserve during fasting.
• Liver glycogen is virtually depleted after 12-18 hours of fasting.
• Muscle glycogen does not directly yield free glucose due to the absence of glucose-6-phosphatase
• Pyruvate can be converted to alanine and used in gluconeogenesis in the liver.
• Glycogen storage diseases are also an important consideration.

Glycogen Biosynthesis and UDP-Glucose

• Glucose is phosphorylated to glucose-6-phosphate, catalyzed by hexokinase in muscle and glucokinase in the liver, similar to glycolysis.
• Glucose-6-phosphate is isomerized to glucose-1-phosphate via phosphoglucomutase.
• Phosphoglucomutase is phosphorylated, with the phosphate group participating in a reversible reaction involving glucose 1,6-bisphosphate.

Glycogen Synthesis Process

• Glucose-1-phosphate reacts with uridine triphosphate (UTP) to produce uridine diphosphate glucose (UDPGlc) and pyrophosphate, catalyzed by UDPGlc pyrophosphorylase.
• The reaction favors UDPGlc formation due to pyrophosphatase-catalyzed hydrolysis of pyrophosphate into 2 x phosphate, removing a reaction hindrance.

UDP-glucose Role

• Uridine diphosphate glucose (UDP-glucose) serves as the immediate precursor molecule for glycogen synthesis.
• Residues are added to glycogen
• UDP-glucose acts as the substrate, which releases UDP as a reaction byproduct.
• Nucleotide diphosphate sugars are precursors to complex carbohydrates, such as oligosaccharide chains in glycoproteins.

Glycogenin's Role in Glycogen Synthesis

• Glycogenin initiates glycogen synthesis.
• Glycogenin is a protein that undergoes glucosylation at a specific tyrosine residue by UDPGlc.
• It functions as an enzyme that catalyzes the attachment of a glucose molecule to its own tyrosine residues.
• It exists as a dimer, with each copy of the enzyme glucosidating the other.

Glycogen Primer Formation

• Glycogenin catalyzes the transfer of seven glucose residues from UDPGlc
• A 1 → 4 linkage is formed, which results in a glycogen primer.
• The glycogen primer becomes the substrate for glycogen synthase.
• Glycogenin remains at the core of the glycogen granule.

Glycogen Synthase Function

• Glycogen synthase catalyzes the formation of a glycoside bond between C-1 of UDPGlc glucose and C-4 of a terminal glucose residue of glycogen, releasing UDP.
• Addition occurs at the outer end of the glycogen molecule, thus creating successive 1-4 linkages resulting in an elongation of the molecule.
• Overall, Glycogen Synthase catalyzes elongation of glycogen chains initiated by Glycogenin.

Glycosidic Bond Formation

• A glycosidic bond forms between the C1 of UDP-glucose-derived glucose moiety and Glycogenin's tyrosine side chain's hydroxyl oxygen.

Glycogen Chain Extension

• Glycogenin promotes glucosylation at C4 of the attached glucose via UDP-glucose.
• This glucosylation forms an O-linked disaccharide with α(1→4) glycosidic linkage.
• This process repeats, building a short linear glucose polymer with α(1→4) glycosidic linkages on Glycogenin.

Glycogen Synthesis Steps

• Glycogen synthesis involves the formation of a primer by glycogenin and the addition of glycosyl residues by glycogen synthase.

Glycogen Branching Process

• Branching occurs when at least 11 glucose residues exist on a growing chain.
• Branching enzyme transfers a segment from the 1 → 4 chain (at least 6 glucose residues) to a neighboring chain.
• The transfer creates a 1 → 6 linkage, creating a branch point.
• The branches enlarge via repeated addition of 1 → 4-glucosyl units along with further branching.
• Alpha-1,4 bonds are broken by a branching enzyme.
• This removes a 7 residue block and creates a 1,6-linkage
• Branching increases the solubility of glycogen.
• Branching increases rates of both synthesis and degradation.
• Glycogen synthase is inactivated by phosphorylation.

Glycogen Structure

• Glycogen is a polymer of glucose residues.
• Residues are mainly linked by a(1→4) glycosidic bonds along with a(1→6) glycosidic bonds at branch points.
• Glucose is stored predominantly as glycogen in the liver and muscle cells.

Regulation of Glycogen Metabolism

• If both synthesis and breakdown of glycogen occurred in a cell at the same time, it would be a futile cycle.
• Glycogen synthase and glycogen phosphorylase are reciprocally regulated through allosteric effectors and phosphorylation to prevent it.

Glycogenolysis Definition

• Glycogenolysis is the catabolic breakdown of glycogen into glucose.
• Glycogen is stored in liver and muscle cytosol granules.

Glycogenolysis Process

• Glycogenolysis isn't the reverse of glycogenesis but a separate pathway.
• Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis.
• This step involves cleavage, thus breaking the 1 → 4 linkages which yield glucose 1-phosphate.
• Isoenzymes of glycogen phosphorylase occur in the liver, muscle, and brain.
• Glycogen phosphorylase requires pyridoxal phosphate as its coenzyme.
• The terminal glucosyl residues on the outermost chains are sequentially removed until four glucose residues remain near a 1 → 6 branch.

Debranching Enzyme

• Debranching enzyme contains two active catalytic sites in a polypeptide chain.
• One site acts as a glucan transferase, transferring a trisaccharide unit off of one branch to an end and exposing the 1 → 6 branch point.
• The other site is a 1,6-glycosidase, catalyzing the hydrolysis of the 1 → 6 glycoside bond, thus releasing free glucose.
• Phosphorylase action proceeds, and through the combined actions of phosphorylase and other enzymes, then glycogen is completely broken down.

Linear Molecule Activity

• Phosphorylase acts specifically for the α-1,4 linkage, requiring two further additional enzymes.
• It forms a linear molecule.
• Shifts 3 glycosyl units towards the core.
• Hydrolyzes a single 1,6 glucose unit releasing free glucose.

Glucose-6-Phosphate in the Liver

• Phosphoglucomutase action is reversible, so glucose-6-phosphate is formed from glucose 1-phosphate.
• Glucose-6-phosphate can enter glycolysis, or mainly within the liver, is dephosphorylated thus being released into the blood.
• Glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate, releasing glucose.
• Glucose is then released, increasing blood glucose concentration.
• Most other tissues will lack this enzyme.

Hormonal Control of Glycogen Breakdown

• Hormones such as glucagon and epinephrine both stimulate glycogen breakdown.
• The cAMP cascade in the liver by glucagon or epinephrine acts with an opposite effect on glycogen synthesis.
• Glycogen Synthase is phosphorylated by both Protein Kinase A and Phosphorylase Kinase.
• Phosphorylation of Glycogen Synthase results in a less active conformation.
• Overall the cAMP cascade inhibits glycogen synthesis.
• Rather than being converted to glycogen, glucose-1-P can be converted to glucose-6-P and then dephosphorylated and released into the blood.

Cyclic AMP Integration

• Key enzymes glycogen phosphorylase and glycogen synthase, are inversely regulated through reversible covalent modifications and allosteric mechanisms in response to hormone actions.
• Phosphorylation of glycogen phosphorylase increases activity and glycogen synthase phosphorylation reduces activity.
• Phosphorylation is increased in response to cyclic AMP release which is formed by hormones binding to adenylyl cyclase, such as epinephrine, norepinephrine and glucagon.
• Cyclic AMP gets hydrolyzed by phosphodiesterase, doing so and ending hormone action.
• Insulin in the liver enhances phosphodiesterase activity.

Glycogen Phosphorylase Regulation

• In the liver, glycogen releases free glucose helping maintain blood concentration.
• In muscles, glycogen releases free glucose-6-phosphate, fueling glycolysis for muscle contraction.
• In tissues, phosphorylase is activated by phosphorylation via phosphorylase kinase, which produces phosphorylase A.
• It is inactivated by dephosphorylation via phosphoprotein phosphatase which creates phosphorylase B, influenced by hormone signals.
• Active phosphorylase A is allosterically inhibited by ATP and glucose-6-phosphate in all tissues.
• Muscle phosphorylase varies with liver in that having a binding site for 5' AMP, which acts as an allosteric activator.
• 5' AMP is a important signal of muscle cell energy, produced when ADP rises indicating greater needs for ATP.
• Low blood sugar leads to both hormones in response.
• Glucagon is created in pancreatic a-cells and activates cAMP in liver.
• Epinephrine then triggers cAMP in muscle.

Activities of Glycogen Synthase and Phosphorylase Regulation

• Glycogen synthase isoenzymes are present in liver, muscle, and brain.
• Glycogen synthase, exists in phosphorylated and nonphosphorylated forms.
• Phosphorylation is the reverse of what's seen in phosphorylase.
• Active glycogen synthase A gets dephosphorylated and inactive glycogen synthase B gets phosphorylated.
• Meaning that glycogen phosphorylase has an opposite effect from phosphorylation
• As phosphorylase activates from raising concentrations of cAMP, glycogen synthase is converted into it's inactive state via the cAMP-dependent protein kinase.
• Overall, glycogenolysis and net glycogenesis inhibit each other.
• Dephosphorylation of phosphorylase A, glycogen synthase B, and phosphorylase kinase, are catalyzed by a broad specificity, single-enzyme protein phosphatase-1.
• Protein phosphatase-1 is inhibited by cAMP-dependent protein kinase by the inhibitor-1 protein.

Blood Glucose Balance

• Blood glucose levels rise due to blood carbohydrate ingestion, thus shifting into glycogen synthesis.
• Insulin produced due to high blood glucose results in activating phosphoprotein phosphatase.
• This phosphatase removes regulatory phosphate residues from glycogen synthase, phosphorylase kinase and phosphorylase.
• Insulin goes against effects of cAMP activated by glucagon and epinephrine.

Calcium Regulation

• Glycogen breakdown is additionally regulated by Calcium ions(Ca++).
• Calcium is released from sarcoplasmic reticulum to promote actin and myosin interaction through stimulation of contraction.
• Phosphorylase kinase is activated through calcium release, the enzyme, the d subunit contains calmodulin.
• Calcium binds the calmodulin subunit, partly activating the pathway.

Genetic Defects

• A genetic liver isoform defect leads to glucose elevations after eating which leads to increased lactate/lipid levels and lowered, ketones when fasting.
• A defect is present in Glycogen Synthase resulting in non-storage during feeding resulting in glycolysis.
• Lactate is produced, including ketone bodies in fasting due to lack of glycogen stores.