Phosphorylase b kinase catalyzes the phosphorylation of serine in certain substrates, including troponin I. The alpha chain may bind calmodulin. Defects in PHKA2 are the cause of glycogen storage disease type 9A (GSD9A); also known as X-linked liver glycogenosis (XLG). GSD9A is a metabolic disorder resulting in a mild glycogenosis with clinical symptoms that include hepatomegaly, growth retardation, muscle weakness, elevation of glutamate-pyruvate transaminase and glutamate-oxaloacetate transaminase, hypercholesterolemia, hypertriglyceridemia, and fasting hyperketosis. Two subtypes are known: type 1 or classic type with no phosphorylase kinase activity in liver or erythrocytes, and type 2 or variant type with no phosphorylase kinase activity in liver, but normal activity in erythrocytes. Unlike other glycogenosis diseases, glycogen storage disease type 9A is generally a benign condition. Patients improve with age and are often asymptomatic as adults. Accurate diagnosis is therefore also of prognostic interest. Belongs to the phosphorylase b kinase regulatory chain family. Note: This description may include information from UniProtKB.