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Phosphorylase kinase (PhK) 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 (PKA, or cAMP-dependent protein kinase) reduces their respective inhibitory activities. The δ subunit is the ubiquitous eukaryotic protein calmodulin which itself has 4 calcium ion binding sites. When cytosolic Ca2+ levels rise-to as low as 10−7 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.〔 ==Genes== * Alpha: PHKA1, PHKA2 * Beta: PHKB * Gamma: PHKG1, PHKG2 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Phosphorylase kinase」の詳細全文を読む スポンサード リンク
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