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How Glycogen Metabolism Is Controlled In The Liver And Muscle Notes

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


Glycogen is broken down by the following enzymes:


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)


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


The bifunctional debranching enzyme moves the next 3 residues to another branch (transferase site) and cleaves the α-1,4 linkage to release the last glucose (α1,6 glucosidase site)


This leaves one unbranched elongated chain for glycogen phosphorylase


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


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


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


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


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


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


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


Most phosphorylase is in the B isoform at rest


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


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


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


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


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


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


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

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