Editing Glycogen (section)

==Metabolism==

===Synthesis===
{{Main|Glycogenesis}}

Glycogen synthesis is, unlike its breakdown, [[endergonic]]—it requires the input of energy. Energy for glycogen synthesis comes from [[uridine triphosphate]] (UTP), which reacts with [[glucose-1-phosphate]], forming [[UDP-glucose]], in a reaction catalysed by [[UTP—glucose-1-phosphate uridylyltransferase]]. Glycogen is synthesized from monomers of [[UDP-glucose]] initially by the protein [[glycogenin]], which has two [[tyrosine]] anchors for the reducing end of glycogen, since glycogenin is a homodimer. After about eight glucose molecules have been added to a tyrosine residue, the enzyme [[glycogen synthase]] progressively lengthens the glycogen chain using UDP-glucose, adding α(1→4)-bonded glucose to the nonreducing end of the glycogen chain.<ref>{{cite book |last=Nelson |first=D. |year=2013 |title=Lehninger Principles of Biochemistry |edition=6th |page=618 |publisher=W.H. Freeman and Company}}</ref>

The [[glycogen branching enzyme]] catalyzes the transfer of a terminal fragment of six or seven glucose residues from a nonreducing end to the C-6&nbsp;hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule. The branching enzyme can act upon only a branch having at least 11&nbsp;residues, and the enzyme may transfer to the same glucose chain or adjacent glucose chains.

===Breakdown===
{{Main|Glycogenolysis}}
Glycogen is cleaved from the nonreducing ends of the chain by the enzyme [[glycogen phosphorylase]] to produce monomers of glucose-1-phosphate:
[[File:Glycogen phosphorylase stereo.png|600px|center|Action of glycogen phosphorylase on glycogen]]

In vivo, phosphorolysis proceeds in the direction of glycogen breakdown because the ratio of phosphate and glucose-1-phosphate is usually greater than 100.<ref>{{cite book |last=Stryer |first=L. |year=1988 |title=Biochemistry |edition=3rd |page=451 |publisher=Freeman}}</ref> Glucose-1-phosphate is then converted to [[G6P|glucose 6&nbsp;phosphate (G6P)]] by [[phosphoglucomutase]]. A special [[glycogen debranching enzyme|debranching enzyme]] is needed to remove the α(1→6)&nbsp;branches in branched glycogen and reshape the chain into a linear polymer. The G6P&nbsp;monomers produced have three possible fates:
* G6P can continue on the [[glycolysis]] pathway and be used as fuel.
* G6P can enter the [[pentose phosphate pathway]] via the enzyme [[glucose-6-phosphate dehydrogenase]] to produce [[NADPH]] and 5&nbsp;carbon sugars.
* In the liver and kidney, G6P can be dephosphorylated back to glucose by the enzyme [[glucose 6-phosphatase]]. This is the final step in the [[gluconeogenesis]] pathway.