Crystallographic and mutagenesis studies have unravelled the general features of the allosteric transition mechanism in pyruvate kinase. The enzyme displays a dramatic conformational change in going from the T‐ to the R‐state. All three domains forming each subunit of the tetrameric enzyme undergo simultaneous and concerted rotations, in such a way that all subunit and domain interfaces are modified. This mechanism is unpreceDAnted since in all tetrameric allosteric enzymes, characterised at atomic resolution, at least one of the domain or subunit interfaces remains unchanged on the T‐ to R‐state transition. The molecular mechanism of allosteric regulation here proposed proviDAs a rationale for the effect of single site mutations observed in the human erythrocyte pyruvate kinase associated with a congenital anaemia.