Glycogen Metabolism: Lecture 8

• The lecture is about Glycogen Metabolism
• Lecture number 8

Key Topics

• Glycogenesis (Glycogen synthesis)
• Glycogenolysis (Glycogen degradation)
• Glycogen metabolism regulation (hormonal, allosteric)
• Glycogen storage diseases

Importance of Glucose

• A constant supply of glucose is essential for human life for different reasons
• The brain needs glucose as its primary energy source
• Cells lacking mitochondria, like RBCs, rely on glucose
• Exercising muscles require glucose as a primary energy source

Sources of Glucose

• Humans obtain glucose from three main sources
• Carbohydrates in diet provide glucose after meals and Excess glucose is stored as glycogen in the liver and skeletal muscles
• Glycogen degradation (Glycogenolysis): Glycogen is broken down to release glucose stored in the liver and skeletal muscles
• In fasting liver glycogen degrades to yield glucose for blood
• During exercise, glycogen is degraded in muscles to supply glucose for energy
• Gluconeogenesis (Glucose Synthesis): Synthesis of glucose from non-carbohydrate sources which occurs in prolonged fasting

Glycogen Structure and Function

• The liver (100 g) and skeletal muscles (400 g) serve as the main storage sites for glycogen.
• Glycogen in muscles provides glucose for ATP and energy
• Glycogen in the liver maintains blood glucose levels
• Storing glucose of glycogen prevents excess glucose as free glucose

Regulation of Glycogen Stores

• Liver glycogen stores increase in a well-fed state and decrease during fasting
• Muscle glycogen is not affected by short periods of fasting
• Muscle glycogen is moderately decreased in prolonged fasting and affected by exercise

Glycogen Structure in Detail

• Glycogen is a highly branched polymer of D-glucose with branching every 8-14 residues
• Chains of glucose have glycosidic links: α(1→4) linkages and α(1→6) linkages where branches occur
• Highly branched structure leads to more solubility and increases rates of degradation/ synthesis
• Glycogen molecules can contain up to 50,000 residues

Glycogenesis

• Glycogenesis is the process of glucose storage as glycogen.
• Glycogenesis is a 4 step process activated by insulin in the liver or skeletal muscle.

Glycogenolysis

• Glycogen is broken down for glucose release, not a simple reversal of steps
• Enzyme phosphorylase splits off a glucose molecule by phosphorylation to form glucose 1-phosphate.
• This process occurs in hepatocytes, therefore muscles can't release glucose.
• Activating enzymes are glucagon (pancreas) and epinephrine (adrenal).

Glycogen Formation

• UDP-glucose pyrophosphorylase is formation of new glycogen or enlargement of existing glycogen particle
• Glycogen synthase (elongation) catalyzes α1-4 linkages
• Branching enzyme (branching) amylo-α(1-4) converts to α(1-6)
• Glycogenin is required in formation of glycogen

Glycogenesis Steps using Enzymes

• Glucose is converted to glucose-6-phosphate by hexokinase in muscles or Glucokinase in liver.
• Glucose-6-phosphate converts to glucose-1-phosphate by Phosphoglucomutase.
• Glucose-1-phosphate is used by UDP-glucose pyrophosphatase to create UDP-glucose
• UDP- glucose is used by Glycogen synthase or Branching enzyme to create glycogen

Glycogenesis Overview

• A sequence of adding UDP-glucose to glycogen's non-reducing ends occurs via sequential addition
• Mechanisms for Chain Elongation happens when glucose α(1,4) joins
• Branch Formation happens when glucose α(1,6) joins
• Branching enzymes creates α(1,6) bonds
• Glycogen synthase creates α(1,4) bonds

Detailed Enzyme Reactions

• Glucose to Glucose-6-phosphate: ΔG =-16.7 kJ/mol
• UDP formation has a "primer" to imitate glycogenesis
• A small fragment of pre-existing glycogen acts as a 'primer’ to begin glycogen synthesis
• Glycogenin accepts a specific protein to accept glucose from UDP Glucose
• The hydroxyl (OH) of the amino acid tyrosine of glycogenin is the site where the initial glucose unit attaches.
• The glycogen chain adds one glucose residue via UDP-glucose + glycogen (n) ⟶ glycogen (n+1) + UDP
• Branching enzymes catalyze glycogen formation branching via amylo-(1→4→1,6)-transglycosylase (also called branching enzyme)
• α-(1→6) linkages occur every 8-12 residues
• The branching enzymes transfer 6- or 7-residue segments nonreducing end.

Glycogenolysis

• Glycogenolysis is the breakdown of stored glycogen in liver and muscle facilitated by enzymes in cytosol
• Glycogen synthesis and degradation are not reversible
• Breaking α-1,4 and α-1,6 Glycosidic bonds degrades glycogen

Glycogen Phosphorylase

• Catalyzes phosphorolysis and requires Pyridoxal phosphate (PLP), a prosthetic group for activity
• Relays a proton from inorganic phosphates and substrate phosphate to help cleave the scissile bond of Glyocogen.
• Glycogen phosphorylase uses inorganic phosphates and glycogen as substrates in a reaction that produces G1P.

Glycogen Phosphorylase Mechanism

• Phosphorylase follows a “random sequential” enzyme mechanism involving PLP (pyridoxyl-5’-phosphate), a B6 derivative.
• Debranching uses glucan transferase and also debranching enzyme (alpha-1,6-glucosidase)
• Branching is hydrolyzed, releasing branched glucose reside as free glucose
• After the branch removal, phosphorylase enzyme is used
• Phosphoglucomutase catalyzes G1P to glucose 6-phosphate (G6P)

Steps of Glycogenolysis

• Glycogen --> Glycogen phosphorylase-->α(1→4)→α(1→4)-glucan transferase--> Amylo-α(1→6)-glucosidase--> Glucose 1-phosphate
• Glucose l-phosphate -->Phosphoglucomutase-->Glucose 6-phosphate
• Glucose 6-phosphate in Muscle
• Phosphatase --> Glucose in Liver

Regulation of Glycogenesis and Glycogenolysis

• Regulated by the enzymes glycogen synthase and Glycogen phosphorylase using three mechanisms
• Allosteric Regulation
• Hormonal Regulation
• Influence of Calcium

Hormonal Regulation

• Hormones like epinephrine and glucagon cause glycogenolysis by acting on glycogen phosphorylase through cAMP.
• Glycogen phosphorylase has two forms: Active ‘a’ form (phosphorylated) and Inactive form ‘b’ (dephosphorylated)
• Glycogen synthase (inactive form) is in inhibited in glycogenesis
• Glycogen phosphorylase is stimulates glycogenolysis in its active form

Regulation of Glycogen Breakdown

• Muscle contraction needs Ca2+-calmodulin complex activation in α1-adrenergic receptors.
• Ca2+-dependent enzymes activate,like e.g., glycogen phosphorylase kinase
• Glycogen phosphorylase kinase phosphorylates which leads to activates calcium and in turn glycogen phosphorylase
• Epinephrine activates glycogen degradation via a rise in cAMP, not Ca2+.

Glycogen Storage Diseases

• Type I:Von Gierke has Deficient Enzyme of Glucose -6- Phosphate
• Type II: Pompe has Deficient Enzyme of Lysosomal α 1,4 glycosidase
• Type III: Cori has Deficient Enzyme of Debranching Enzyme
• Type IV: Anderson has Deficient Enzyme of Branching Enzyme
• Type V: McArdle has Deficient Enzyme of Muscle Glycogen Phosphorylase
• Type VI: Hers has Deficient Enzyme of Hepatic Glycogen Phosphorylase