The high-resolution crystal structures of three homologous adenylate kinases with zero, one and both ( = 2-substrate mimicking inhibitor) bound substrates have been compared. The comparisons are meaningful, because all structures occur in two or three different crystal contact environments indicating that they represent intrinsically stable conformations in solution. Molecular superimpositions revealed that two domains comprising 30 and 38 residues undergo large movements on substrate binding, which can be approximated by rigid-body rotations over 39 degrees and 92 degrees, respectively. Moreover, these movements can be subdivided into two steps: first, a change on binding substrate AMP, which involves only the 30 residue domain (C alpha shifts up to 8.2 A), and second, a change on additional binding of substrate ATP, which again involves the 30 residue domain (C alpha shifts up to 7.6 A) but also the 38 residue domain (C alpha shifts up to 32.3 A). Taken together, these observations yield a three-picture "moving film" of the induced-fit.