Abstract Pyruvate kinase catalyzes the last step of glycolysis which is important for generating adenosine triphosphate (ATP). Mammals express four major pyruvate kinase (PK) isozymes: muscle (M1), liver (L), erythrocyte (R), and the ubiquitous M2 types. All but the M1 isozyme exhibit positive cooperative kinetic behaviors upon binding the allosteric activator fructose-1,6-diphosphate and in the presence of increasing phosphoenolpyruvate (PEP) concentrations. All four isozymes are inhibited by phenylalanine. The l-PK isozyme activity is also inhibited by phosphorylation in response to glucagon. Consistent with its important role in regulating glucose metabolism and fat synthesis in liver, l-PK is also extensively regulated at the level of transcription. l-PK transcription increases in response to glucose and insulin and decreases in response to glucagon and high fat diets.