Structural mechanism of muscle contraction

X-ray crystallography shows the myosin cross-bridge to exist in two conform ations, the beginning and end of the "power stroke." A long lever-arm under goes a 60 degrees to 70 degrees rotation between the two states. This rotat ion is coupled with changes in the active site (OPEN to CLOSED) and phospha te release. Actin binding mediates the transition from CLOSED to OPEN. Kine tics shows that the binding of myosin to actin is a two-step process which affects ATP and ADP affinity. The structural basis of these effects is not explained by the presently known conformers of myosin. Therefore, other sta tes of the myosin cross-bridge must exist. Moreover, cryoelectronmicroscopy has revealed other angles of the cross-bridge lever arm induced by ADP bin ding. These structural states are presently being characterized by site-dir ected mutagenesis coupled with kinetic analysis.