Two isoenzyme forms of pyruvate kinase exist.

In liver and in other gluconeogenic tissues, the so-called L form of pyruvate kinase predominates, whereas the M form is found largely in muscle. The L form is inhibited both by ATP and by some amino acids, particularly alanine, the major gluconeogenic precursor among the amino acids.

This relationship allows inhibition of glycolysis, with consequent activation of gluconeogenesis, specifically in gluconeogenic tissues, when ample energy and substrates are available. Control at the pyruvate kinase step allows conservation of high-energy phosphate in the phosphoenolpyruvate molecule.