The structural basis of muscle contraction

The myosin cross-bridge exists in two conformations, which differ in the or ientation of a long lever arm. Since the lever arm undergoes a 60 degrees r otation between the two conformations, which would lead to a displacement o f the myosin filament of about 11 nm, the transition between these two stat es has been associated with the elementary 'power stroke' of muscle. Moreov er, this rotation is coupled with changes in the active site (CLOSED to OPE N), which probably enable phosphate release. The transition CLOSED to OPEN appears to be brought about by actin binding. However, kinetics shows that the binding of myosin to actin is a two-step process which affects both ATP and ADP affinity and trice versa. The structural basis of these effects is only partially explained by the presently known conformers of myosin. Ther efore, additional states of the myosin cross-bridge should exist. Indeed, c ryoelectron microscopy has revealed other angles of the lever arm induced b y ADP binding to a smooth muscle actin-myosin complex.