Pyruvate kinase (PK): Large allosteric enzyme that regulates glycolysis through binding of the substrate, phosphoenolpyruvate, and one or more allosteric effectors. Like other allosteric enzymes, PK has a high substrate affinity R state and a low affinity T state. PK exists as several different isozymes, depending on organism and tissue type. In mammals, there are four PK isozymes: R, found in red blood cells, L, found in liver, M1, found in skeletal muscle, and M2, found in kidney, adipose tissue, and lung. PK forms a homotetramer, with each subunit containing three domains. The T state to R state transition of PK is more complex than in most allosteric enzymes, involving a concerted rotation of all 3 domains of each monomer in the homotetramer.