Final answer:
Myofibrils are composed of six proteins: myosin, actin, troponin, tropomyosin, nebulin, and titin, with myosin being responsible for the power stroke in muscle contraction. Myosin heads bind to actin, hydrolyze ATP, and pull the actin filaments to shorten the muscle during the power stroke.
Step-by-step explanation:
Six proteins that make up the myofibrils are myosin, actin, troponin, tropomyosin, nebulin, and titin. Myofibrils are essential structures within muscle fibers responsible for muscle contraction. These myofibrils consist of repeating units called sarcomeres, which contain both thick and thin myofilaments made primarily from the contractile proteins myosin and actin.
The protein that creates the power stroke for contraction is myosin. During muscle contraction, the myosin heads attach to actin forming cross-bridges. With the hydrolysis of ATP, the myosin heads pivot pulling the actin filaments toward the center of the sarcomere, which results in muscle shortening and the generation of force.
Troponin and tropomyosin are the regulatory proteins that facilitate the binding of myosin to actin by shifting their positions when calcium ions bind to troponin, thus exposing the binding sites on actin filaments for myosin to attach and create the power stroke.