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Phosphorylase kinase is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen. PhK phosphorylates glycogen phosphorylase at two serine residues, triggering a conformational shift which favors the more active glycogen phosphorylase “a” form over the less active glycogen phosphorylase b.
The protein is a hexadecameric holoenzyme—that is, a homotetramer in which each subunit is itself a tetramer—arranged in an approximate “butterfly” shape. Each of the subunits is composed of an α, β, γ and δ subunit. The γ subunit is the site of the enzyme's catalytic activity while the other three subunits serve regulatory functions.
When unmodified, the α and β subunits inhibit the enzyme's catalysis, but phosphorylation of both these subunits by protein kinase A reduces their respective inhibitory activities. The δ subunit is the ubiquitous eukaryotic protein calmodulin which itself has 4 calcium ion binding sites. When cytosolic Ca levels rise-to as low as 10 M—the δ subunit undergoes a large conformational change that activates the kinase's activity by binding to a complementary hydrophobic patch on the catalytic γ subunit.