Last Updated on November 3, 2020 by Sagar Aryal
Glycogen is stored polysaccharide in vertebrate and some micro-organism, whereas starch in case of the plant.
Breakdown of glycogen takes place through the three enzymatic reactions, glycogen phosphorylase, glycogen debranching enzyme, and phosphoglucomutase. Three steps are as follow –
Glycogen phosphorylase action
Glycogen phosphorylase removes glucose residue as alpha-D glucose 1 -phosphate from the non-reducing end, with the breaking of alpha 1-4 glycosidic bond by inorganic phosphate attack, this process is repeating until it reaches four glucose from the branch point. Further breakdown takes place after the action of the debranching enzyme.
Glycogen debranching enzyme action
The debranching enzyme catalyzes two successive reactions for transferring the branch.
Transferase activity of the debranching enzyme – In this reaction debranching enzyme transfer oligosaccharides from branch to linear.
Alpha 1-6 glycosidase activity of the debranching enzyme – In this step debranching enzyme breakdown alpha 1-6 glycoside bond librating one glucose.
Phosphoglucomutase enzyme action
It converts librated glucose 1-phosphate to glucose 6-phosphate, which can enter glycolysis.
Biosynthesis of Glycogen
Glycogen biosynthesis takes place some-how in all cells of the animal body but mainly takes place in the liver and skeletal muscles. Like glycolysis, it also starts with glucose 6-phosphate, condensed into glycogen through the action of four enzymes – like phosphoglucomutase, UDP-glucose pyrophosphorylase, glycogen synthase, amylo (1-4) to (1-6) transglycosylase.
Here glucose 6-phosphate is converted into glucose 1-phosphate by the action of phosphoglucomutase.
UDP-glucose pyrophosphorylase action
Now glucose 1-phosphate is converted into UDP-glucose with the use of UTP and liberating PPi. UDP-glucose, the sugar-nucleotide donates glucose for glycogen synthesis. This reaction is irreversible making the irreversible synthetic pathway.
Glycogen synthase action
It catalyzes the transfer of glucose residue from UDP glucose to the non-reducing end of glycogen, this enzyme can synthesis glycogen without primer here previous glycogen molecules act as a primer, and enzyme transfer glucose to the non-reducing end of primer glycogen, increasing 1 glucose residue in each cycle. When there are no previous glycogens available, then glycogenin (a protein) acts as a primer. Glycogen synthase can’t synthesize (1-6)bond, found at branch point so here need of another enzyme.
Amylo (1-4) to (1-6) transglycosylase (glycogen branching enzyme) action
Catalyzes the transfer of 6-7 glucose residue long fragment from non-reducing end to the hydroxyl group of C-6 of interior glucose residue creating branch point with (1-6) glycosidic bond, now branch point also act as a primer.
Coordinate regulation of glycogen synthesis and breakdown
Glycogen biosynthesis and breakdown are regulated by coordination. Glycogen phosphorylase is allosterically and hormonally regulated, and glycogen synthase is regulated by phosphorylation and dephosphorylation.
Regulation of glycogen phosphorylase
Carl and Gerty Cori found that glycogen phosphorylase is found in two interconvertible forms glycogen phosphorylase a catalytically active and glycogen phosphorylase b catalytically less active. Phosphorylase b predominantly found in resting muscles during vigorous muscular activity epinephrine leads to phosphorylation of specific ser residue, convert it into more active form Phosphorylase a. And when muscles undergo resting phase then phosphorylase phosphatase removes phosphate from phosphorylase a converting it into less active form phosphorylase b. The process is regulated in the response to glucose.
Regulation of glycogen synthase by phosphorylation and dephosphorylation
Glycogen synthase also can exist in an active and inactive form. Here active form glycogen synthase-a is in unphosphorylated form, its phosphorylation by glycogen synthase kinase 3 (GSK-3) lead to into inactive form glycogen synthase-b. Glycogen synthase kinase adds phosphoryl group to three ser residue near carboxyl terminus, it simply does not phosphorylate until another protein kinase casein kinase II phosphorylate glycogen on a nearby residue process called priming. Enzyme PP1 causes dephosphorylation by phosphatase action making it active. Here the process can be regulated in the response to glucose 6-phosphate.
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