Glucose is first enzymically phosphorylated, then reacted with uridine triphosphate to give uridine diphosphate glucose (UDP-glucose). The enzyme glycogen synthetase fits this onto a preexisting glycogen chain (primer and/or protein backbone), splitting off the UDP. The glucose residue added by 1®4 linkages is attached at the outer end of the molecule so that the branches of the glycogen tree become elongated. After 11 glucose residues, another enzyme called the "branching enzyme" (amylol (1®4) ® (1®6) transglucosidase) transfers part of the 1®4 chain of glucose residues (>6) to a neighboring chain to form a 1®6 link and create a branch point. Glycogen grows by further 1®4 and 1®6 additions. As the number increases the total number of reactive sites in the molecule also increases, which hastens both glycogenesis and glycogenolysis. Breakdown of glycogen is mediated by three enzymes: (a) phosphorylase acting at the 1®4 linkages; (b) a(1®4) ® a(1®4) glucan transferase, which transfers a trisaccharide unit from one branch to another, exposing 1®6 branch points; and (c) the "debranching enzyme" (amylo(1®6)-glucosidase), which splits the linkage. Removal of the branch allows further action by the phosphorylase. In the liver and kidney (but not in muscle), a specific enzyme, glucose-6-phosphatase, dephosphorylates the glucose so that it can diffuse from the cells into the bloodstream. The enzymes controlling glycogen metabolism are regulated by a complex series of phosphorylations and dephosphorylations and by allosteric mechanisms under hormonal influence (see insulin and glucagon).
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