Glycogen phosphorylase (GP; EC 18.104.22.168) is the enzyme responsible for controlling the rate of glycogen degradation, which involves catalyzing the phosphorylytic cleavage of α1-4 glycosidic bonds found within macro-glycogen molecules, thus producing glucose-1-phosphate monomers (Glc-1-P), a process referred to as glycogenolysis. Therefore, both glycogen synthesis and glucose liberation are intimately linked, the relationship being controlled by hormonal stimulation (insulin, glucagon, adrenaline, epinephrine). Allosteric effectors binding in specific and localized sites tightly regulate this catalytic activity. Three isoforms of GP have been identified and are located within metabolically active tissues in the human body; the brain (bGP), liver (lGP) and skeletal muscle (mGP). Activation/deactivation of GP is a controlled process sensitive to intra- and extracellular signals. GP is an archetypal control enzyme and fine regulation is made possible through four major sites present on each monomer: catalytic site (C-site), glycogen site (G-site), nucleotide binding site (adenosine monophosphate (AMP)-site) and phosphorylation site (P-site). Interestingly, important regulation steps are performed outside the catalytic cavity making GP a case of study for allosteric interactions. Two states of GP prevail symbolizing its activity state: inactive T state (Tense state) and active R state (Relaxed state). The binding of specific effectors assures the transition between the two states. The inhibition of GP has been proposed as one method for treating type 2 diabetes.
 N. Gaboriaud-Kolar, A.L. Skaltsounis. Glycogen phosphorylase inhibitors: a patent review (2008-2012). Exp. Opin. Ther. Pat. 2013, 23, 1017-1032.
 D.J. Baker, P.L. Greenhaff, J.A. Timmons. Glycogen phosphorylase inhibition as a therapeutic target: a review of the recent patent literature. Exp. Opin. Ther. Pat. 2006, 16, 459-466.