Biochem glycogen metabolism

Glycogen review

Glycogen is a storage polysaccharide comprised of glucose minimizes bound through a(1→4) glycosidic bonds with branches of a(1-6) glycosidic bonds every 8-12 residues

-stores energy in organisms mainly in the liver and skeletal muscle

Glycogen metabolism

Comprises two processes: glycogenesis and glycogenolysis

Glycogenesis: anabolic reaction that builds new glycogen molecules, activated by insulin

Glycogenolysis: catabolic reaction that breaks down glycogen to supply glucose, activated by glucagon/epinephrine

-both reactions occur at the nonreducing end if molecule (no part of glycosidic bond)

-both reactions are reversible and occur in the cytosol, both runs mediated by 4 enzymes

Glycogenolysis: degrading glycogen

Occurs during fasting or exercise in 4 steps, net reaction: glycogen+ → glucose

1) Glycogen(n) → Glycogen (n-1) + glucose-1-phosphate (G1P)

2) Debranching step- 2 enzymes in enzyme complex (4-alpha-glucanotransferase activity and amylo-1,6-glucosidase activity)

3) G1P → Glucose-6-phosphate (G6P)

4) Liver: G6P → glucose or muscle: G6P → glycolysis

Step 1 of glycogenolysis

Reaction: Glycogen(n) → glycogen (n-1) + glucose-1-phosphate/G1P (n= total number of glucose residues in glycogen)

-mediated by: Glycogen phosphorylase (GP) enzyme

-requires: inorganic phosphate (Pi) and pyridoxal phosphate (PLP) coenzyme (active form of vitamins B6)

Mechanism: catalyzes phosphorolysis of a(1→4) bonds at no reducing ends of glycogen

• releases G1P from non-reducing ends of glycogen by breaking a(1→4) bonds without using ATP

Regulation:

• activated by phosphorylation (via phosphorylase kinase → activated by glucagon or epinephrine via PKA)

• Deactivated by dephosphorylation (via protein phosphatase 1, activated by insulin)

• Allosterically activated by AMP (muscle only) and inhibited by ATP and glucose-6-phosphate

Step 2 of glycogenolysis

Debranching step- two activities, one enzyme complex

-mediated by: Debranching Enzyme Complex

• 4-a-glucanotransferase activity: transfers a block of 3 glucose units to another linear chain (a(1→4) transfer)

• Amylo-1,6-glucosidase activity: cleaves the remaining a(1→6) linked glucose at the branch point

Requirement: no energy or coenzyme required

Mechanism: releases free glucose

• transferase activity: moves a block of 3 glucose residues from a branch to a nearby linear chain

• Glucosidase activity: hydrolyzes the a(1→6) bond at the branch point, releasing free glucose

Regulation: not highly regulated

Step 3 of glycogenolysis

Reaction: G1P → Glucose-6-phosphate (G6P)

Mediated by: phosphoglucomutase enzyme

Requirements: no energy required, substrate level phosphorylation

Mechanism: G1P is phosphorylated by phosphoserine active site to G6P through a G-1,6-biphosphate intermediate

Regulation: not highly regulated

Step 4 of glycogenolysis (liver)

Reaction: G6P → glucose

Mediated by: Glucose-6-phosphatase enzyme

-allows free glucose to be exported into the bloodstream

Requirements: no ATP used

Mechanism: hydrolyzes G6P → glucose + Pi allows free glucose to be exported into the blood to maintain blood glucose levels

Regulation: controlled indirectly by hormonal signals (glucagon → cAMP → transcriptional regulation)

• not present in muscle, so muscle cannot export free glucose

Step 4 of glycogenolysis (muscle)

Reaction: G6P → Glycolysis

Mediated by: glycolytic enzymes

Requirement: no additional energy input for this step

Mechanism: G6P produced from glycogen enters glycolysis to be used for ATP generation locally

• allows muscle to rapidly produce energy for contraction, especially in anaerobic conditions

Regulation: indirectly influenced by energy demand:

• high AMP activates glycogen phosphorylase and glycolysis

• high ATP or G6P inhibits glycogen breakdown

Glycogenesis: Building Glycogen

5 steps:

1) Glucose → G6P

2) G6P → G1P

3) G1P + UTP → UDP-glucose + PPi

4) UDP-glucose + glycogen(n) → glycogen(n+1) + UDP

5) branching enzyme adds a(1→6) branches

-occurs in fed state, stimulated by insulin release

-maintains blood glucose between meals

• liver: regulates blood glucose

• muscle: local energy during exercise

-short term energy storage form of glucose

Net reaction: Glucose + ATP + UTP → glycogen + ADP + UDP + 2Pi

Initiation of glycogenesis

-conversion of G1P to G6P by phosphoglucomutase, initiates glycogenesis

-UDP-glucose pyrophosphorylase catalyzes the formation of UDP-glucose

Step 1 of glycogenesis

Reaction: Glucose → G6P

Mediated by: Hexokinase (muscle) and glucokinase (liver)

Requirements: uses 1 ATP → ADP

Mechanism: phosphorylates glucose at the 6 position to trap it inside of the cell

Regulation:

• hexokinase: inhibited by G6P (product inhibition)

• Glucokinase: regulated by insulin and by translocation between cytoplasm and nucleus (via glucokinase regulatory protein)

Step 2 of glycogenesis

Reaction: G6P → G1P

Mediated by: phosphoglucomutase enzyme

Requirements: no direct energy input

Mechanism: transfers the phosphate group from C6 to C1 via G-1,6-biphosphate intermediate

Regulation: not highly regulated

Step 3 of glycogenesis

Reaction: G1P + UTP → UDP-glucose + PPi

Mediated: UDP-glucose pyrophosphorylase enzyme

Requirements: Uses 1 UTP → UDP + PPi (PPi is rapidly hydrolyzed to 2 Pi, driving the rxn forward)

Mechanism: activates glucose by linking it to UDP, forming UDP-glucose, a high energy sugar donor

Regulated by: not a major regulation point, but product availability (UDP-glucose) can influence downstream reactions

Step 4 of glycogenesis

Reaction: UDP-glucose + glycogen(n) → glycogen(n+1) + UDP

Mediated by: glycogen synthase enzyme

Requirements: no ATP used in this step, but uses high energy UDP-glucose. ATP is required to reproduce UTP

Mechanism: transfers the glucose unit from UDP-glucose to the non-reducing end of glycogen through an oxonium ion intermediate, forming an a(1→4) glycosidic bond

Regulated by:

• GS activated by insulin

• GS inactivated by phosphorylation

• GS activated by dephosphorylation

• GS allosterically activated by G6P

Step 5 of glycogenesis

Reaction: Glycogen → branched glycogen

Mediated by: branching enzyme (Amylo-(1,4→1,6)-transglycosylase)

Requirement: no energy required

Mechanism: transfers a block of 6-7 glucose residues from a linear a(1→4) chain to a more interior position, creating an a(1→6) branch point

Regulated by: not strongly regulated, but activity ensures solubility and rapid mobilization of glycogen

Glycogen Synthase is Regulated by Covalent Modification

-Glycogen synthase exists in 2 distinct forms (active, dephosphorylated R state and less active phosphorylated T state)

-the phosphorylated form is allosterically activated by G6P

-4 different protein kinases are involved:

-Dephosphorylation is carried out by phosphoprotein phosphatase 1 (PP1)

-PP1 inactivates glycogen phosphorylase and activates glycogen synthase

-the metabolic effects of insulin are mediated through protein phosphorylation and second messenger modulation

Hormonal control of Glycogen Metabolism

1) Insulin: Activates glycogenesis and inhibits glycogenolysis

2) Glucagon (liver): inhibits glycogenesis and activates glycogenolysis

3) Epinephrine (liver and muscle): activates glycogenolysis in both liver and muscle

4) Glucocorticoids: cortisol- activates glycogenesis

Insulin and Glycogen Metabolism

-Insulin triggers glycogen synthesis when blood glucose rises (blood glucose rises after a meal and then drops after a few hours)

-Insulin is secreted from the pancreas (to liver) in response to an increase in blood glucose

-Insukin acts to lower blood glucose rapidly, stimulating glycogen synthesis and inhibiting glycogen breakdown

-binding of insulin to plasma membrane receptors in the liver and muscles triggers protein kinases cascades that stimulate glycogen synthesis

-Insulin’s effect include: lipid synthesis or breakdown, glycogen synthesis, protein synthesis, glycolysis, inhibition of gluconeogenesis

-glucose uptake provides substrate for glycogen synthesis and G6P which allosterically activates the otherwise inactive form of glycogen synthase

Glucagon and Epinephrine on glycogen metabolism

-glucagon and epinephrine stimulates glycogen breakdown (opposite effect of insulin)

-glucagon (29 residues peptide) is also secreted by pancreas and acts on liver and adipose tissue only

-epinephrine (adrenaline) is released from adrenal glands and acts on liver and muscles

-when either hormone binds to its receptor on the outside surface if the cell membrane, a protein kinase cascade amplifies the signal

-both are glycogenolysis in liver but for different reasons

-epinephrine is the fight or flight hormone (rapidly mobilizes large amounts of energy)

-glucagon is for long term maintenance of steady state levels of glucose in the blood (activates glycogen breakdown and activates liver gluconeogenesis)

Cortisol and glycogen metabolism

-Glucocorticoids are steroid hormones that exert distinct effects on liver, skeletal muscle, and adipose tissue

-cortisol is primarily catabolic- it promotes protein breakdown and decreases protein synthesis in skeletal muscle

-in the liver, it stimulates gluconeogenesis and increases glycogen synthesis by:

• stimulating expression of genes for gluconeogenic enzymes

• Activating enzymes of amino acid metabolism

• Stimulating the urea cycle