To detect structural changes around the reactive Cys707 (SH1) in the myosin heavy chain during the ATPase reaction, the reactivity of SH1 in rabbit skeletal myosin subfragment-1 (S-1) was measured using a fluorescent reagent, 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid, in the presence of various ATP analogs: adenosine 5'-(3-thiotriphosphate) (ATPgammaS), ADP-vanadate (ADP-Vi), ADP-BeFx, and ADP-AlF4. The SH1 reactivities in the S-1 complexes with ATPgammaS and ADP-BeFx, analogs of the E-ATP state, were very high, as well as that in the E-ADP state. In contrast, the SH1 reactivities in the S-1 complexes with ADP-Vi and ADP-AlF4, analogs of the E-ADP-P state, were extremely low. The structural changes around SH1 can be correlated to changes in the structure of the gamma-phosphate of ATP during the ATPase reaction or to the structure of the corresponding part of ATP analogs at the active site of ATPase. This is consistent with the crystal structure of S-1 in which the heavy chain structure around SH1 of S-1-ADP-BeFx is significantly different from those of S-1-ADP-Vi and S-1-ADP-AlF4 [Fisher et al. (1995) Biochemistry 34, 8960-8972; Smith and Rayment (1996) Biochemistry 35, 5404-5417].