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Chapter 1: Q. 20 (page 7)

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

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

Short Answer

Expert verified

The correct answer is option (d)

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

Step by step solution

01

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.
02

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.
03

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.

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