The phosphorylase B kinase enzyme is a polymer made up of 16 subunits; 4 each of the alpha, beta, gamma and delta subunits. The alpha and beta subunits have regulatory functions, the gamma subunit contains the active site and the delta subunit is calmodulin. It is a serine phosphorylase found primarily in the liver and muscles. The enzyme is responsible for converting glycogen phosphorylase B into its active form of glycogen phosphorylase A. By carrying out its function, it plays an important role in glycogenosis, the glycogen metabolism pathway that releases glucose to cells. The PHKA1 gene encodes the alpha subunit of the muscle-specific phosphorylase B kinase.
Mutations in the PHKA1 gene create a non-functioning phosphorylase kinase enzyme and lead to the accumulation of glycogen in muscle cells. This results in Glycogen Storage Disease, Type IXd (GSD9D), a condition characterized by exercise-induced muscle stiffness and pain.