We have tested the effect of all 20 proteinogenic amino acids on the activity of the M2 isoenzyme of pyruvate kinase (M2PYK) and show that, within physiologically relevant concentrations, phenylalanine, alanine, tryptophan, methionine, valine, and proline act as inhibitors, while histidine and serine act as activators. Size exclusion chromatography has been used to show that all amino acids, whether activators or inhibitors, stabilise the tetrameric form of M2PYK. In the absence of amino-acid ligands an apparent tetramer–monomer dissociation Kd is estimated to be ∼0.9 µM with a slow dissociation rate (t1/2 ∼ 15 min). X-ray structures of M2PYK complexes with alanine, phenylalanine, and tryptophan show the M2PYK locked in an inactive T-state conformation, while activators lock the M2PYK tetramer in the active R-state conformation. Amino-acid binding in the allosteric pocket triggers rigid body rotations (11°) stabilising either T or R states. The opposing inhibitory and activating effects of the non-essential amino acids serine and alanine suggest that M2PYK could act as a rapid-response nutrient sensor to rebalance cellular metabolism. This competition at a single allosteric site between activators and inhibitors provides a novel regulatory mechanism by which M2PYK activity is finely tuned by the relative (but not absolute) concentrations of activator and inhibitor amino acids. Such ‘allostatic’ regulation may be important in metabolic reprogramming and influencing cell fate.