by combining G1P and UTP; hydrolysis of Ppi by the inorganic pyrophosphatase renders the overall reaction exergonic. The initial priming of glycogen is carried out by the self-glucosylating protein glycogenin (Gyg). Glycogen is then extended and branched by glycogen synthase (Gys) and the glycogen branching enzyme (Gbe1), respectively. Glycogen synthase catalyzes the transfer of glucosyl groups from UDP-glucose to glycogen via alpha1-4 linkage; when the elongated glycogen has at least 11 residues, the branching enzyme transfers a chain of ~seven glucosyl groups to another chain via alpha1-6 linkage. G6P is an allosteric regulator of glycogen synthase as well as glycogen phosphorylase, a key component of the glycogen degradation pathway. Note that the two essential enzymes of glycogen metabolism are subject to covalent and allosteric regulation, aspects of which cross paths in opposite directions. Glycogen synthase is inactivated and activated by phosphorylation/dephosphorylation, respectively. The allosteric activation by G6P involves unfolding of the enzyme which overrides the effect of phosphorylation and conformational changes that prime it for dephosphorylation. Several kinases, including glycogen synthase kinase-3 (Gsk3) phosphorylate the synthase; dephosphorylation is primarily mediated by protein phosphatase-1 (Pp-1). The phosphatase is a complex between a catalytic subunit encoded by three genes in mammals and any of a host of regulatory subunits that allow for the formation of a variety of distinct multimeric holoenzymes that regulate many cellular processes. In the case of glycogen metabolism, the glycogen or G-targeting subunits are part of the Pp-1 complex anchored to the glycogen particles to which the key enzymes of glycogen anabolism and catabolism are bound. Pp-1 also modifies and inactivates glycogen phosphorylase and phosphorylase kinase; the first, as mentioned, is a key enzyme of glycogen degradation and the second is its activator. Thus, the two arms of glycogen metabolism are closely regulated at the level of the key enzymes involved in them. To see the ontology report for annotations, GViewer and download click here [click to see the ontology report for associated GO term - GO:0005978 and entry at Reactome - REACT_1736.2]...(less)