Phosphorylase kinase is a hexadecameric enzyme that is comprised of four copies of four subunits that are encoded by four separate genes: PHKA, PHKB, PHKG, and PHKD. This serine/threonine specific kinase converts glycogen phosphorylase b to glycogen phosphorylase a, resulting in the release of glucose-1-phophate from glycogen. PHKB (Phosphorylase b kinase regulatory subunit beta) is a 1093 amino acid subunit of phosphorylase kinase that, along with PHKA, has regulatory functions controlled by phosphorylation. Defects in the gene encoding PHKB are the cause of glycogen storage disease type 9B, which is also known as phosphorylase kinase deficiency of liver and muscle. This disease is characterized by a mild phenotype of hepatomegaly with only slightly elevated transaminase and plasma lipids, no clinical muscle involvement, and generally is correlated with a gradual improvement with increasing age. There are four isoforms of PHKB that are produced as a result of alternative splicing events.
Phosphorylase B; Phosphorylase b kinase regulatory subunit beta; phosphorylase kinase beta subunit; phosphorylase kinase beta-subunit; Phosphorylase kinase subunit beta; phosphorylase kinase, beta