Glycogen Metabolism

Glycogen Metabolism - Glycogen Metabolism Bryant Miles...

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Glycogen Metabolism Bryant Miles Glycogen is the storage polysaccharide of animals. Glycogen is found primarily in the liver and muscle tissue. A day’s supply of glucose is stored in the liver in the form of glycogen to maintain blood glucose levels during fasting. The muscle tissue stores glycogen for an energy reserve. Glycogen consists of glucose molecules linked together with α (1 ± 4)linkages with α (1 ± 6) branch points occurring every 8 to 12 residues. The purpose of the high branched structure is to have many nonreducing ends so that glucose can be rapidly mobilized in times of metabolic needs. The high degree of branching also increases the solubility of glycogen. O OH CH2OH O CH 2 O O CH2 O CH2OH O O O CH 2 O CH 2 O O CH2OH O O O CH2OH O 2 O O CH2OH O O O CH2OH O The nonreducing ends are shown in red. The residue that starts a branch is shown in green and the reducing end is shown in blue. Glycogen metabolism is important for several reasons. Glycogen stores in the liver are used to maintain a constant blood glucose concentration. Muscles also maintain glycogen stores as a reservoir of glucose for strenuous muscular activity. The synthesis and degradation of glycogen occur by different metabolic pathways allowing for reciprocal regulation. In addition, the enzymes of glycogen metabolism are under hormonal regulation. The biochemical pioneers of glycogen metabolism were the Cori’s, Carl and Gerty, a husband and wife team. They demonstrated the glycogen is broken down by phosphorolysis. Glycogen n + Pi Glucose-1-phosphate + glycogen n-1
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Glycogen phosphorylase catalyzes this reaction. This enzyme catalyzes the sequential phosphorolysis of glucose residues from a nonreducing end. The bond between the C1 carbon atom and the glycosidic oxygen atom is cleaved by inorganic phosphate in such a way that the stereochemistry at the C1 carbon is maintained. This phosphorolytic cleavage is advantageous because the cell is saved the expense of phosphorylating glucose with ATP. In addition the phosphorylated glucose cannot diffuse out of the cell. The retention of configuration at the stereocenter is an important clue into the mechanism of this enzyme. Sn2 substitution reactions proceed with inversion of configuration at the stereocenter. Retention implies a Sn1 like substitution mechanism. Sn1 mechanism begins with the dissociation of the leaving group to form a carbocation.
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Glycogen Metabolism - Glycogen Metabolism Bryant Miles...

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