Glycogen phosphorylase is regulated by two mechanisms - allosteric control and phosphorylation. Allosteric effectors cause structural changes in enzymes and, when combined with phosphorylation, allow access to the suppressed catalytic site. Phosphorylation transforms glycogen phosphorylase from a disordered state to an ordered one, 

Glycogen phosphorylase can also be regulated by hormones such as insulin, epinephrine, and glucagon, which work using second messenger amplification systems linked to G proteins.