3. Glycogenolysis, Gluconeogenesis and the Cori Cycle PDF

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The third in a lecture series of 7 focusing on metabolism. Talking about glycogenolysis, gluconeogenesis and the Cori cycle...

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3. Glycogenolysis, Gluconeogenesis and the Cori Cycle Objectives  Understand the regulation of glycogen metabolism  Understand gluconeogenesis in liver  Good examples of the co-ordinated regulation of pathways by both protein phosphorylation and allosteric control Glycogen  Storage of glucose o Reduces osmotic potential of glucose which would damage cells o Avoids glycosylation of proteins (does happen in diabetes though)  Mainly joined at  (1->4) glucose  Branch points every 10 glucose units when branching is (1->6) glucose  The structure exposes a large no. of glucose units o Glycogenolysis can occur very rapidly during 'fight or flight' situations.  Reducing end attached to the protein glycogenin  Size of glycogen particles varies with feeding o Aroung 10nm between meals o >40nm after feeding  70g in liver, 200g in muscle  McArdle's syndrome o Glycogen storage disease o Exercise intolerance with muscle pain, early fatigue, painful cramps and myoglobin in the urine  Synthesis and degradation o Feeding- citrate and ATP produced  Act as allosteric inhibitors of glycolysis, preventing breakdown of glucose to pyruvate and allowing the conversion of glucose to glycogen

3. Glycogenolysis, Gluconeogenesis and the Cori Cycle

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UTP= ATP with uricle instead of adenine  Why use UTP?  Glucose-1P not powerful enough a glucose donor to form a gluc-gluc bond so it requires an energy input form UTP  G1P +UTP -> UDPG +PPi  PPi + H2O -> Pi + Pi  UDPG is powerful enough to glycosylate glycogen Regulation of glucogen metabolism o Hormonal and electrical stimulation o During exercise glycogen phosphorylase is active and glycogen synthase is inactive- glycogen breakdown occuring

3. Glycogenolysis, Gluconeogenesis and the Cori Cycle

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Adrenaline stimulates glycogen metabolism  Binds to a receptor which activates adenylate cyclase to make cAMP.  cAMP is a secondary messenger that is broken down by cAMP phosphodiesterase to make AMP  Stimulated by insulin and inhibited by caffeine  Negligible amounts of AMP produced, doesn't impact metabolism  cAMP activates Protein Kinase A.  Protein Kinase A activates phosphorylase kinase and inhibits glycogen synthase.  Phosphorylase kinase activates glycogen phosphorylase b to make the active form, glycogen phosphorylase a.  This provides a large amplification in the overall process o AMP allosterically stimulates glycogen phosphorylase  ATP overcomes this stimulation o Ca2+ activates phosphorylase kinase  Phosphorylase kinase is made up of 4 subunits-   Ca2+ bind to the Subunit, calmodulin o Glycogen formation in the well feed state  Need to turn off the signal to break down glycogen  cAMP is hydrolyses to 5'AMP and protein phosphatase remove phosphates from proteins.  Insulin opposes the action of adrenaline and glucagon  Inhibits glycogen synthase kinase 3 (GSK3) and turns on glycogen synthase and so glycogen is formed o Glucose sensors  Decrease in measured [O2] proportional to [glucose]  First generation of glucose monitor- Glucose oxidase entrapped at a Clark oxygen electrode using a dialysis membrane  Latest generation devices use less than 1 microlitre of blood, give results in seconds and can store lots of data Gluconeogenesis- occurs in the cytosol o Important- maintaining normal function in the brain.  Glucose is the primary fuel of the brain.  When stores are depleted lactate generated by other organs is converted into glucose to fuel the brain  Negligible quantities of glucose are produced by fat in most mammals  The brain can use ketone bodies

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During long term starvation the body must convert proteins into glucose via amino acids and the citric acid cycle Formation of glucose from lactate (via pyruvate) Lactate not the only substrate for gluconeogenesis  Pyruvate  Oxaloacetate  Glycerol (via glycerol-P) (glycerol kinase only found in liver, not in adipose tissue)  Alanine (via pyruvate) Mostly reverse reactions of glycolysis. Exceptions: Pyruvate + CO2 + ATP -> oxaloacetate + ADP + Pi  Pyruvate carboxylase Oxaloacetate + GTP -> phosphoenolpyruvate + GDP +CO2  Phosphoenolpyruvate carboxykinase Fructose-1,6-bisphosphatase -> fructose-6-phosphate  Fructose-1,6-bisphosphatase Glucose-6-phosphate -> glucose  Glucose-6-phosphatase

Pyruvate -> Oxaloacetate-> Phosphoenolpyruvate

3. Glycogenolysis, Gluconeogenesis and the Cori Cycle

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Pyruvate kinase is the enzyme that forms pyruvate from oxaloacetate  This 'forward' reaction yields an ATP  2 ATP are required for the reaction to 'go backwards' during gluconeogenesis  Step 1- Pyruvate, ATP and bicarbonate react to form oxaloacetate.  The ATP is hydrolysed to ADP and Pi.  Pyruvate carboxylase  Step 2- Oxaloacetate then forms phosphoenolpyruvate  GTP->GDP  PEP carboxykinase  PEP cannot form Pyruvate as it is inhibited in the liver  The steps as far as fructose-1,6-bisphosphate are reversible  Fructose-1,6-bisphosphate -> fructose-6-phosphate  Hydrolysis of a phosphate group by fructose-1,6-phosphatase  Glucose-6-phosphate -> glucose  Hydrolysis by glucose-6-phosphatase Regulation of gluconeogenesis  Pyruvate kinase has a lower activity in the liver to favour gluconeogenesis  Inhibited by alanine- a major gluconeogenic precursor  Substrate cycle between fructose 6-phosphate and fructose-1,6bisphosphatase

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Left unchecked would just consume ATP

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Used to control flux through glycolysis and gluconeogenesis In the liver fructose-2,6-bisphosphate controls both of the enzymesFructose-1,6-bisphosphatase and Phosphofructokinase (PFK)  PFK- glycolysis  Fructose-1,6-bisphosphatase- gluconeogenesis

Fructose-2,6- bisphosphate inhibits fructose-1,6-bisphosphate and activates PFK-1  PFK-2 converts fructose-1,6-biphosphate to fructose-2,6biphosphate  More fructose-2,6-biphosphate favours glycolysis  PFK-2 is under hormonal control  In the liver glucagon acts when [glucose] is low  Glucagon activates protein kinase A which phosphorylates the bifunctional enzyme  [PFK-2] decreases, [Fru-2,6-bisPase] increases  Fall in [Fru-2,6-P2] favours gluconeogenesis over glycolysis  In muscle there is no gluconeogenesis Fru-2,6-bisP activator system works differently as there are different isoenzymes of the bifunctional PFK-2/Fru-2,6-bisPase  Cardiac- adrenalin causes phorsphorlation of PFK-2 on different residues, increasing its fate  [Fru-2,6-bisP] increases, glycolysis increases  Skeletal- PFK-2 isoform is not phosphorylated, it just responds to changes in [Fru-6-P], reinforcing the effects of AMP  Heart and skeletal muscle is different as the heart requires a steady output The Cori Cycle between muscle and liver tissue  The interplay between anaerobic glycolysis in muscle tissue and gluconeogenesis in liver tissue.  Muscle tissue generates lactate during exercise o Acidosis of the muscle tissue would occur if the lactate wasn't taken away in the blood o Lactate still contains energy so is converted back into glucose via gluconeogenesis o After exercise the glucose is transported back to muscle tissue and stored as glycogen, ready for more exercise

3. Glycogenolysis, Gluconeogenesis and the Cori Cycle

Gluconeogenesis and Type 2 diabetes  During type 2 diabetes an excess of lactate, alanine and glycerol is produced by the adipose tissue and skeletal muscle o Substrates for gluconeogenesis with energy needed for ATP coming from oxidation of fatty acids.  Normally gluconeogenesis is controlled by the expression of PEPCK which negatively regulated by insulin. o Expression of PEPCK rises and more glucose produced= hyperglycaemia o Metformin is a fist line treatment for type 2 diabetes and supresses liver gluconeogenesis...