Glycogen phosphorylase with a mass of 97.434 kDa in muscle cells. While the enzyme can exist as an inactive monomer or tetramer, it is biologically active as a dimer of two identical subunits. In mammals, the major isozymes of glycogen phosphorylase are found in muscle, liver, and brain. The brain type is predominant in adult brain and embryonic tissues, whereas the liver and muscle types are predominant in adult liver and skeletal muscle, respectively. The glycogen phosphorylase dimer has many regions of biological significance, including catalytic sites, glycogen binding sites, allosteric sites, and a reversibly phosphorylated serine residue. First, the catalytic sites
Glycogen synthase and the pyrophosphate of UDP itself. The control of glycogen synthase is a key step in regulating glycogen metabolism and glucose storage. Glycogen synthase is directly regulated by glycogen synthase kinase 3 (GSK-3), AMPK, protein kinase A (PKA), and casein kinase 2 (CK2). Each of these protein kinases lead to phosphorylated and catalytically inactive glycogen synthase. The phosphorylation sites of glycogen synthase are summarized below. For enzymes in the GT3 family, these regulatory kinases inactivate glycogen synthase by phosphorylating it at the N-terminal of the 25th residue and the C-terminal of the 120th residue. Glycogen synthase is also regulated by