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How Glycogen Metabolism Is Controlled In The Liver And Muscle Notes

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This is an extract of our How Glycogen Metabolism Is Controlled In The Liver And Muscle document, which we sell as part of our Biochemistry Notes collection written by the top tier of Oxford University students.

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Control of glycogen metabolism in the liver and muscle Glycogen catabolism:

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Glycogen is broken down by the following enzymes:

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Glycogen phosphorylase removes one glucosyl residue at A time from the non-reducing ends of glycogen, catalysing the following reaction: o Glycogen(n) + Pi ? glucose-1-phosphate + glycogen(n-1)

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Glycogen phosphorylase can only act on a-1,4 glycosidic linkages until 4 residues away from a branching a-1-6 linkage

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The bifunctional debranching enzyme moves the next 3 residues to another branch (transferase site) and cleaves the a-1,4 linkage to release the last glucose (a1,6 glucosidase site)

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This leaves one unbranched elongated chain for glycogen phosphorylase

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The glucose-1-phosphate released is converted to glucose-6-phosphate by phosphoglucomutase Muscle and liver differences are reflected in glycogen catabolism control:

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Glucose-6-phosphate enters glycolysis in most tissues inc. muscle, but the liver expresses glucose-6-phosphatase (in the lumen of ER) so the glucose-6phosphate can be converted to glucose and released

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Muscle uses glucose to produce ATP for its own energy use whereas liver provides glucose for peripheral tissues; this necessitates different regulation in these tissues

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Control is primarily of glycogen phosphorylase, which is the rate-limiting process of glycogen catabolism and is expressed as different isozymes in liver and muscle

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Both isozymes have a phosphorylase A and a phosphorylase B isoform; phosphorylase A is phosphorylated

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Both isoforms have an active, relaxed (R) and inactive, tense (T) state

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Phosphorylase B's equilibrium strongly favours the T state; phosphorylase A's equilibrium strongly favours the R state In muscle:

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Most phosphorylase is in the B isoform at rest

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High AMP ratios (signalling exercise) allosterically stabilises the R state of phosphorylase B

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ATP competes with AMP for this binding site, so abundant energy promotes the T state

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Glucose-6-phosphate, phosphorylase B's product, also favours the T state

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Phosphorylase A is active independent of AMP, ATP or G6P, so phosphorylase B is converted to phosphorylase A via phosphorylase kinase upon hormonal stimulation

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Adrenaline (signalling impending exercise) initiates a cascade via badrenoreceptors: o ? Gs activated ? adenylate cyclase activated ? [cAMP] increases ? PKA activated ? phosphorylase kinase phosphorylated ? glycogen phosphorylase B phosphorylated ?[glycogen phosphorylase A] increases

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Phosphorylase kinase is also stimulated upon electrical excitation of the cell and upon [Ca2+] increasing (both of which signal contraction)

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Ca2+activates phosphorylase kinase because one of its units is calmodulin

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