PHK

Phosphorylase kinase (PhK) is a Ca²⁺/calmodulin-regulated serine/threonine protein kinase that occupies a central position in glycogen metabolism by phosphorylating glycogen phosphorylase and promoting glycogen breakdown into glucose-1-phosphate^[1][2]. Mechanistically, PhK is a large (αβγδ)₄ hexadecameric complex in which the γ subunit provides catalytic activity, whereas the α, β, and δ subunits function primarily in regulation^[1][2]. The δ subunit is an endogenous calmodulin molecule that couples intracellular Ca²⁺ signals to kinase activation, and Ca²⁺-dependent movement of this subunit contributes to relief of γ-subunit inhibition and enhanced catalytic function^[1]. In parallel, phosphorylation of the α and β regulatory subunits by cAMP-dependent protein kinase (PKA) synergizes with Ca²⁺/calmodulin signaling to activate PhK, thereby integrating hormonal and calcium-dependent control of glycogenolysis^[1][2]. This regulatory architecture distinguishes PhK from other Ca²⁺/calmodulin-dependent protein kinases because calmodulin serves as an intrinsic holoenzyme subunit rather than a transient regulatory factor^[2]. Disease relevance is demonstrated by mutations in PHKA, PHKB, and PHKG genes, which impair PhK function and cause glycogen storage disease type IX, affecting liver and/or skeletal muscle glycogen metabolism^[1][3]. Recent structural studies further showed that phosphorylation-dependent conformational compaction and Ca²⁺-driven calmodulin-mediated rearrangements cooperate to achieve full enzyme activation, providing a framework for mechanistic studies of glycogen metabolic regulation and disease-associated variants^[1].