25.17:
Actin and Myosin in Muscle Contraction
Sarcomeres, the smallest muscle contractile units, comprise parallel overlapping troponin-tropomyosin bound actin filaments and bundles of myosin II filaments.
During muscle contraction, calcium ions bind the troponin proteins attached to the tropomyosin. This binding causes a conformational change in the tropomyosin, exposing myosin-binding sites present in the actin filaments.
The myosin head with an ADP and loosely attached phosphates from the previous cycle of ATP hydrolysis releases the bound phosphate, allowing the globular myosin head to bind the actin filament and form a cross-bridge.
As the ADP dissociates from the two myosin filaments, the energy released pulls the actin filaments towards the center, shortening the sarcomere.
ATP molecules quickly occupy the vacated nucleotide-binding site in the myosin head, stopping the sliding movement and separating the actin and myosin filaments.
The ATPase in the globular head hydrolyzes the ATP to ADP and phosphate for the next round of sliding of actin filaments.
25.17:
Actin and Myosin in Muscle Contraction
Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded. Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark the border of units called sarcomeres, the functional skeletal muscle units.
Sarcomere comprises of two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. The tail of a myosin connects with other myosin molecules to form the central region of a thick filament near the M line. In contrast, the heads align on either side of the thick filament where the thin filaments overlap. The thin filaments consist of actin and two other accessory proteins, tropomyosin and troponin. The actin protein has myosin-binding sites for attachment. Strands of tropomyosin block the binding sites and prevent actin-myosin interactions when the muscles are at rest. Troponin consists of three globular subunits. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds to Ca2+ ions. The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which ATP provides. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy.
This text is adapted from Openstax, Biology 2e, Section 38.4: Muscle Contraction and Locomotion