The interactions of several amino acids in myophosphorylases structure are known. Ser-14 is modified by phosphorylase kinase during activation of the enzyme. Lys-680 is involved in binding the pyridoxal phosphate, which is the active form of vitamin B6, a cofactor required by myophosphorylase. By similarity, other sites have been estimated: Tyr-76 binds AMP, Cys-109 and Cys-143 are involved in subunit association, and Tyr-156 may be involved in allosteric control. Function
Myophosphorylase is the form of the glycogen phosphorylase found in muscle that catalyses the following reaction:((1→4)-alpha-D-glucosyl) (n) + phosphate = ((1→4)-alpha-D-glucosyl) (n-1) + alpha-D-glucose 1-phosphate
Failure of this enzyme ultimately impairs the operation of ATPases. This is due to the lack of normal pH fall during exercise, which impairs the creatine kinase equilibrium and exaggerates the rise of ADP. Pathophysiology
Myophosphorylase is involved in the breakdown of glycogen to glucose for use in muscle. The enzyme removes 1,4 glycosyl residues from outer branches of glycogen and adds inorganic phosphate to form glucose-1-phosphate. Ordinarily, the removal of 1,4 glycosyl residues by myophosphorylase leads to the formation of glucose-1-phosphate during glycogen breakdown and the polar, phosphorylated glucose cannot leave the cell membrane and so is marked for intracellular catabolism. In McArdles Disease, deficiency of myophosphorylase leads to accumulation of intramuscular glycogen and a lack of glucose-1-phosphate for cellular fuel. Myophosphorylase exists in the active form when phosphorylated. The enzyme phosphorylase kinase plays a role in phosphorylating glycogen phosphorylase to activate it and another enzyme, protein phosphatase-1, inactivates glycogen phosphorylase through dephosphorylation.
References
External links

Euromac, an EU-funded consortium of medical and research institutes across Europe which is building a patient registry and raising standards of care for people with McArdle Disease. International Association for Muscle Glycogen Storage Disease (IamGSD). Walking With McArdles - IAMGSD videos
EUROMAC Introduction - Video about McArdle Disease and the EUROMAC Registry of McArdle Disease and other rare glycogenoses patients