Which of the following muscle proteins is NOT present in the thin filament?
tropomyosin
actin
troponin
myosin
The correct answer and explanation is:
The correct answer is myosin.
Muscle fibers are made up of myofibrils, which are organized into repeating units called sarcomeres. A sarcomere consists of thick and thin filaments, and the proteins that form these filaments play crucial roles in muscle contraction. The thin filaments primarily consist of three proteins: actin, tropomyosin, and troponin. Myosin, however, is not part of the thin filament; it is the protein that forms the thick filament.
- Actin: This is the primary protein in the thin filament. Actin molecules are arranged in a double helix structure that forms the backbone of the thin filament. The actin filaments are where myosin binds during muscle contraction.
- Tropomyosin: Tropomyosin is a regulatory protein that wraps around the actin filaments, blocking the binding sites for myosin when the muscle is relaxed. This prevents muscle contraction until the muscle is activated.
- Troponin: Troponin is another regulatory protein that works alongside tropomyosin. It binds to both actin and tropomyosin and has binding sites for calcium ions. When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing for muscle contraction to occur.
- Myosin: Myosin is a motor protein that forms the thick filament. The myosin heads have a “walk along” function, binding to the actin filaments during muscle contraction. This process is powered by ATP hydrolysis and results in the sliding of the actin filaments over the myosin, shortening the sarcomere and causing the muscle to contract.
In summary, while tropomyosin, actin, and troponin are essential components of the thin filament involved in muscle contraction, myosin is part of the thick filament. Therefore, myosin is not present in the thin filament.