Phosphorylase b kinase catalyzes the phosphorylation of serine in certain substrates, including troponin I. The beta chain acts as a regulatory unit and modulates the activity of the holoenzyme in response to phosphorylation. Defects in PHKB are the cause of glycogen storage disease type 9B (GSD9B); also known as phosphorylase kinase deficiency of liver and muscle (PKD). GSD9B is a metabolic disorder characterized by hepathomegaly, only slightly elevated transaminases and plasma lipids, clinical improvement with increasing age, and remarkably no clinical muscle involvement. Biochemical observations suggest that this mild phenotype is caused by an incomplete holoenzyme that lacks the beta subunit, but that may possess residual activity. Belongs to the phosphorylase b kinase regulatory chain family. 4 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.