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23 Cards in this Set
- Front
- Back
Glycogen
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Carbohydrate store in animals (glucose) - muscle and liver. Remobilised to provide energy production or allow lover to regulate blood glucose. Highly branched polymer - alpha 1,4 links (chains) and alpha 1,6 links (branches). One unit of glucose can be released at a time due to branches). Stored as granules, with metabolic enzymes |
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The breakdown of glycogen
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Glycogen phosphorylase (G1P) - remove glucose from the branches - up to 4 units from end of a branch. Each unit released as glucose 1 phosphate units. Transferase -- moves the three glucose units to another branch to be broken down by G1P. One residue left - broken down b debranching enzyme in form of glucose |
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Products of glycogen breakdown
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Glucose (debranching enzyme) - minor product Glucose-1-phosphate - phosphorylase and transferase enzyme. Converted to glucose-6-phosphate by phosphogucomytase to enter glycolysis pathways. Can be converted to glucose by glucose-6-phosphate. |
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Uses of glycogen breakdown products in liver and muscle
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Liver - glucose 6 phosphate converted into glucose and released into blood. Presence of glucose-6-phosphatase Muscle - glucose 6 phosphate powers glycolysis directly, glucose-6-phosphatase not present |
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Glycogen synthesis
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Glucose-6-phosphate converted to Glucose-1-phosphate by phosphoglucamatase Converted to UDP-glucose by UDP-glucose pyrophosphorylase. Pyrophosphate released. Glycogen unit added to produce glycogen using enzyme glycogen synthase. UDP released. |
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The enzymes controlling breakdown and synthesis of glycogen
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Breakdown - glycogen phosphorylase Synthesis - glycogen synthase Control mediated by metabolites and hormones |
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Glycogen breakdown in the liver
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Glucagon released by pancreas in times of fasting, detected by specific membrane receptors. Binds to receptors which activates adenylyl cyclase converting ATP to cAMP. cAMP activates protein kinase A by binding to it. Active kinase A can add phosphate to inactive phosphorylase kinase-b forming active phosphorylase kinase - a (P). This adds phosphate to phosphorylase kinase - b forming phoshorylase - a (P). Catalyses conversion of glycogen to glucose -1-phosphate |
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Glycogen breakdown in muscle
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Hormone used: adrenaline - adrenaline receptors on muscle. Same phosphorylation cascade occurs. In both tissues - reversible covalent modification of enzyme occurs, covalent attachment of phosphate group. Enzymes switched on by phosphorylation - amplification occurs. |
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How is the pathway switched off?
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Protein phosphatase 1 - inactivates the newly activated phosphorylase-a (P) enzyme. Reforms phosphorylase-b. Occurs in both muscle and liver |
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Metabolite and hormonal control of glycogen breakdown
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Phosphorylase kinase-a (P) activated by calcium ion, which signal for muscle contraction AMP - activates phosphorylase-b. AMP rises as muscle contract as ATP used. |
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Metabolite and hormonal control of glycogen synthesis
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Protein kinase a directly phosphorylates glycogen synthase (active enzyme) to form inactive glycogen synthase-P enzyme. Same hormone can switch on glycogen breakdown and switch off glycogen synthesis - allows for perfect regulation. |
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Control of metabolism in muscle contraction
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Exercise releases adrenaline, glycogen breakdown needed for glycolysis Calcium ions (stimulate phosphorylase kinase) and AMP (stimulase phosphorylase -b) released from muscle. Synchronises muscle contraction with glycogen breakdown |
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Control of metabolism - blood glucose
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Low blood glucose - glucagon secreted, glycogen broken down in liver to release glucose In times of plenty - phosphorylase a-(p) binds glucose in liver, inhibiting breakdown. Insulin levels high - promote uptake of glucose by GLUT4 and activated glycogen synthase. Glycogen phosphorylase activity low |
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Glycolysis
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Degradation of 6C glucose to 3C pyruvate to produce ATP Anaerobic, catabolic (6C to 3C) but also anabolic (biosynthetic intermediates to produce larger molecules). Control - glucose transporters and control of enzymes |
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The stages of glycolysis and enzymes used
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Glucose -> Glucose-6-phosphate by hexokinase -> fructose-6-phosphate -> fructose-1,6-bisphosphate by phosphofructokinase. Split into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde-3-phosphate converted to 1,3diphosphoglycerate, then to phosphoenolpyruvate. This converted to pyruvate by pyruvate kinase |
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Inhibitors of Phosphofructokinase
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Fructose-6-phosphate to fructose-1,6-bisphosphate Inhibited by ATP, allosteric binding and lowering affinity for F6P. High energy - so switch off glycolysis. Opposed by AMP (ATP decreases so glycolysis needs to increase). Inhibited by citrate, enhances ATP effects. In TCA cycle, signals abundance of biosynthetic precursors. |
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Stimulators of phosphofructokinase
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Fructose 2,6-bisphosphate - allosteric. In liver glucagon acts to reduce levels of F26BP and switch off glycolysis (blood glucose released into blood). Made by enzyme phosphofructokinase 2: F6P + ATP -> F26P2 + ADP |
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Fructose-2,6,-bisphosphate
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Liver enzyme - inhibited by cAMP, dependent on phosphorylation. Heart muscle - adrenaline stimulates production of fructose-2,6-bisphosphate and stimulates glycolysis to power muscle contraction. Stimulated by cAMP |
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Hexokinase
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Glucose to glucose-6-phosphate Inhibited: glucose 6 phosphate (own product). If phosphofructokinase inhibited then G6P rises, prevents unnecessary conversion of glucose Isoenzymes - different affinities for substrates (km values). 3 have low Km values but isoenzyme IV (glucokinase) has high Km value (found in liver). Not inhibited by G6P |
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Pyruvate kinase
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Converts phosphoenolpyruvate to pyruvate Allosteric - ATP inhibits (abundance of energy). F1,6BP stimulates, keeping glycolysis occurring. If F1,6BP increases it inhibits phosphofructokinase, feeding forward to pyruvate kinase so glycolysis happens quicker. Hormonal - glucagon stimulates cAMP production, phosphorylates pyruvate kinase and inhibit it. |
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Chronic (longer term control of glycolysis)
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Change in permanent diet - changes in pathways or production of certain enzymes Insulin promotes synthesis of some glycolytic enzymes Glucagon promotes synthesis of some gluconeogenic enzymes (create glucose). |
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Pyruvate dehydrogenase
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Pyruvate + NAD + CoA -> acetylCoA + NADH + CO2 (acetyl CoA for TCA cycle). Decarboxylation, not reversible, multiple controls. Reversible covalent modification - cAMP independent Exists in an active and inactive form |
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Regulation of pyruvate dehydrogenase
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Inactivation - phosphorylated by kinase, forming PDH-P. Kinase enzyme regulated by acetyl coA, NADH and CO2 Activated - dephosphorylation by phosphatase. Calcium ions released in muscle contraction activates phosphatase - more pyruvate converted in the TCA cycle. |