In this stage of the contraction cycle, ATP binds to the myosin head and induces conformational changes of the actin-binding site. This change reduces the affinity of the myosin head for the actin molecule of the thin filament, causing the myosin head to uncouple from the thin filament.
STAGE 2: RELEASE
Bending is the third stage of the cycle; the myosin head, as a result of hydrolysis of ATP, advances a short distance in relation to the thin filament.
The ATP-binding site on the myosin head undergoes further conformational changes, causing the myosin head to bend. This movement is initiated by the breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate; both products, however, remain bound to the myosin head. In this stage of the cycle, the linear displacement of the myosin head relative to the thin filament is approximately 5 nm.
STAGE 3: BENDING
Force generation is the fourth stage of the cycle; the myosin head releases inorganic phosphate and the power stroke occurs.
The myosin head binds weakly to its new binding site on the neighboring actin molecule of the thin filament, causing release of the inorganic phosphate. This release has two effects. First, the binding affinity between the myosin head and its new attachment site increases. Second, the myosin head generates a force as it returns to its original unbent position. Thus, as the myosin head straightens, it forces movement of the thin filament along the thick filament. This is the “power stroke” of the cycle. During this stage, ADP is lost from the myosin head.
STAGE 4:
Reattachment is the fifth and last stage of the cycle; the myosin head binds tightly to a new actin molecule.
The myosin head is again tightly bound to a new actin molecule of the thin filament (rigor configuration), and the cycle can repeat.
Although an individual myosin head may detach from the thin filament during the cycle, other myosin heads in the same thick filament will attach to actin molecules, thereby resulting in movement. Because the myosin heads are arranged as mirror images on either side of the H band (antiparallel arrangement), this action pulls the thin filaments into the A band, thus shortening the sarcomere.
Regulation of contraction involves Ca2, sarcoplasmic reticulum, and the transverse tubular system.
Ca2 must be available for the reaction between actin and myosin. After contraction, Ca2 must be removed. This rapid delivery and removal of Ca2 is accomplished by the combined work of the sarcoplasmic reticulum and the transverse tubular system.
The sarcoplasmic reticulum is arranged as a repeating series of networks around the myofibrils. Each network of the reticulum extends from one A–I junction to the next A–I junction within a sarcomere. The adjacent network of sarcoplasmic reticulum continues from the A–I junction to the next A–I junction of the neighboring sarcomere. Therefore, one network of sarcoplasmic reticulum surrounds the A band, and the adjacent network surrounds the I band (Fig. 11.8). Where the two networks meet, at the junction between A and I bands, the sarcoplasmic reticulum forms a slightly more regular ringlike channel called the terminal cisterna. The terminal cisternae serve as reservoirs for Ca2. To release Ca2 into the sarcoplasm, the plasma membrane of the terminal cisternae contains an abundance of gated Ca2-release channels. Also located around the myofibrils in association with the sarcoplasmic reticulum are large numbers of mitochondria and glycogen granules, both of which are involved in providing the energy necessary for the reactions involved in contraction.