Final answer:
Myosin is the structure composed of six polypeptide chains, with two heavy chains and four light chains. It is involved in muscle contraction by interacting with actin, which is regulated by tropomyosin and troponin.
Step-by-step explanation:
The structure that is composed of six polypeptide chains, two heavy chains, and four light chains is referred to as myosin. This is a myosin molecule which is a primary component of thick filaments in muscle fibers. Myosin's two heavy chains twist together to form a long tail domain, while the four light chains are associated with the head domains. The myosin heads are responsible for interacting with actin during muscle contraction.Actin is the main component of thin filaments and works together with myosin for muscular contraction. Thin filaments also include tropomyosin and troponin, which are regulatory proteins. Tropomyosin obstructs the myosin-binding sites on actin when the muscle is relaxed, while troponin holds the tropomyosin in place and binds calcium ions to assist in regulating muscle contraction.