Question

In resting stage, binding sites for myosin on actin filaments are masked by
a. Troponin
b. Light meromyosin
c. Heavy meromyosin
d. Calcium ions

Answer 1

Hint: In the resting stage, binding sites for myosin on actin filaments are masked by a complex protein that is found on the tropomyosin at regular intervals and is found within the slots between actin filaments in muscle tissue. This protein is necessary for muscle contraction.

Complete answer:
• Muscle fibers contain a large number of myofibrils arranged in parallel in the sarcoplasm. These myofibrils have thin filaments of actin protein and thick filament of myosin protein. These two are arranged parallel to each other.
• Each actin filament has two filamentous actin spirally wound to each other. Two filaments of tropomyosin run close to this filamentous actin throughout its length.
• A complex protein called troponin is found on tropomyosin at regular intervals. During the resting stage, this protein masks the active binding sites of actin.
• When the calcium ions get released into the sarcoplasm it binds with the troponin and causes the active sites of actin to open for myosin.
• Myosin is composed of many monomeric proteins known as meromyosin which has two parts: heavy meromyosin (a globular head) and light meromyosin (tail part).

Hence, the correct answer is option (A).

Note: Troponin and Tropomyosin are regulatory proteins that are found associated with actin. Due to the release of calcium ions and its binding to troponin, the unmasking of an active site of actin occurs which leads further to the contraction of the muscle. When calcium ions return to the sarcoplasmic reticulum it results in the masking of active sites of actin.