In the energy transducion of muscle contraction, it is important to know the nature and extent of conformational changes of the head portion of the myosin molecules. In the presence of magnesium adenosine triphosphate (MgATP), fairly large conformational changes of the myosin head [subfragment-1 (S1)] in solution were observed by small-angel x-ray scattering with the use of synchrotron radiation as an intense and stable x-ray source. The presence of MgATP reduced the radius of gyration of the molecule by about 3 angstrom units and the maximum chord length by about 10 angstroms, showing that the shape of S1 becomes more compact or round during hydrolysis of MgATP. Comparison with various nucleotide-bound S1 complexes that correspond to the known intermediate states during ATP hydrolysis indicates that the shape of S1 in a key intermediate state, S1-bound adenosine diphosphate (ADP) and phosphate [S1¨.ADP.P] differs significantly from the shape in the other intermediate states of the S1 adenosine triphosphatase cycle as well as that of nucleotide-free S1.
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