Question

Which of the following is the contractile protein of a muscle?
A. Tubulin
B. Myosin
C. Tropomyosin
D. All the above

Answer 1

The contractile proteins are the proteins that mediate sliding of contractile fibres of a cell's cytoskeleton. Myosin is the main component of thick myofilaments, whereas actin is the main component of the thin myofilaments.

Complete step by step answer: Myosin is formed of the myofibril, which is a very fine contractile protein of a muscle. Myofibrils are the group of fibres, which extend in parallel columns along with the length of striated muscle fibres. These bind to the actin and lead to the shortening of the sarcomeres. It hydrolyzes the ATP to form ADP, which is used for the muscle contraction. The myosin is the contractile proteins of muscles. Most of the primary myofilaments are made up of this protein. Each of the myosin filaments is a polymerized protein made of many monomeric proteins called meromyosins. The secondary myofilaments are composed of the protein actin, having with it two regulatory proteins such as the tropomyosin and the troponin. The contractile proteins are embedded in the cytoplasm of muscle fibres that consist of the myofilaments, myosins and the actin. ATP binds to the myosin, moving the myosin to its high-energy state, releasing the myosin head from the active site. Then, the ATP attaches to the myosin, which allows the cross-bridge cycle to start again; further, the muscle contraction can occur. The protein has many important roles in our body.
i. It helps to repair and to build our body tissues, allows metabolic reactions to take place and coordinates bodily functions.
ii. In addition to providing our body with a structural framework, proteins also maintain proper pH and the fluid balance.
Hence, the correct answer is option B.

Note: The process of the muscular contraction includes:
i. The depolarization and the release of calcium ions.
ii. The actin and myosin cross-bridge formation.
iii. The sliding mechanism of the actin and the myosin filaments.