The structural basis of muscle contraction

The myosin cross-bridge exists in two conformations, which di¡er in the orientation of a long lever arm. Since the lever arm undergoes a 608 rotation between the two conformations, which would lead to a displacement of the myosin ¢lament of about 11nm, the transition between these two states has been associated with the elementary `power stroke’ of muscle. Moreover, this rotation is coupled with changes in the active site (CLOSED to OPEN), 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 a¡ects both ATP and ADP a¤nity and vice versa. The structural basis of these e¡ects is only partially explained by the presently known conformers of myosin. Therefore, additional states of the myosin cross-bridge should exist. Indeed, cryoelectron microscopy has revealed other angles of the lever arm induced by ADP binding to a smooth muscle actinˆmyosin comple