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Catalytic domain of the Serine/Threonine Kinase, Phosphorylase kinase Gamma subunit STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. Phosphorylase kinase (PhK) catalyzes the phosphorylation of inactive phosphorylase b to form the active phosphorylase a. It coordinates hormonal, metabolic, and neuronal signals to initiate the breakdown of glycogen stores, which enables the maintenance of blood-glucose homeostasis during fasting, and is also used as a source of energy for muscle contraction. PhK is one of the largest and most complex protein kinases, composed of a heterotetramer containing four molecules each of four subunit types: one catalytic (gamma) and three regulatory (alpha, beta, and delta). Each subunit has tissue-specific isoforms or splice variants. Vertebrates contain two isoforms of the gamma subunit (gamma 1 and gamma 2). The gamma subunit, when isolated, is constitutively active and does not require phosphorylation of the A-loop for activity. The regulatory subunits restrain this kinase activity until signals are received to relieve this inhibition. For example, the kinase is activated in response to hormonal stimulation, after autophosphorylation or phosphorylation by cAMP-dependent kinase of the alpha and beta subunits. The high-affinity binding of ADP to the beta subunit also stimulates kinase activity, whereas calcium relieves inhibition by binding to the delta (calmodulin) subunit. The PhKG subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",