Q. 20 - Biology

Question

In relaxed muscle, the myosin-binding site on actin is blocked by ________.  

1. titin 
2. troponin 
3. myoglobin 
4. tropomyosin 

Step-by-Step Solution

Answer

The correct answer is option (d)

Tropomyosin blocks the myosin-binding site on actin in relaxed muscle. 

Tropomyosin

• Tropomyosin, which resemble rod-shaped coiled coil dimers, form continuous polymers along the main groove of the majority of actin filaments. 
• Through its interactions with actin, myosin, and the troponin complex, it is a crucial thin filament protein that controls muscle contraction and relaxation.

Step: 1 Explanation for Correct answer

• The troponin-tropomyosin complex inhibits myosin head attachment to the actin microfilament active sites in a relaxed muscle. 
• A binding site for Ca++ ions is present in troponin as well. 
• Together, these two control proteins react to calcium and "regulate" the contraction of sarcomeres. Thus, the option d is correct.

Step: 2 Explanation for incorrect answer

• Titin increases the rigidity of muscles by binding calcium at certain places, and cross-bridge attachment to actin is hypothesized to free up titin binding sites on actin. So, option a is wrong. 
• When calcium and troponin separate, relaxation happens. TnI blocks actomyosin ATPase when a muscle is at ease. So, option b is wrong as well.
• Exercises that focus on aerobic fitness and endurance result in significant metabolic changes in muscle tissue, such as an increase in mitochondria and the protein myoglobin. Hence, option c is wrong too.