Question

In the resting muscle fibre, tropomyosin partially covers
A. Calcium-binding sites on troponin
B. Actin binding sites and myosin
C. Myosin binding sites an actin
D. Calcium-binding sites on actin

Answer 1

Hint: There are two forms of filaments in a myofibril. They are Myosin (thick filament) and actin (thin filament). Each actin (thin) filament is made up of two helically wound ‘F' (filamentous) actins. A polymer of monomeric ‘G' (Globular) actions makes up each ‘F' Faction. Tropomyosin, a protein, has two filaments that run close to the ‘F' acting along its length. Troponin, a complex protein, is found at regular intervals on tropomyosin

Complete explanation:
Option A: Troponin-C (TnC) is a muscle protein with four calcium-binding sites arranged as a pair of two-site domains. The C-terminal domain, which contains calcium-binding sites III and IV, is mainly unfolded in the absence of calcium. Calcium binds to these sites.
Hence, option A is incorrect.
Option B: At a binding site on the globular actin protein, myosin binds to actin. Myosin has a second ATP binding site, where enzymatic activity converts ATP to ADP, releasing an inorganic phosphate molecule and energy. Myosin releases actin as it binds to ATP, allowing actin and myosin to separate.
Hence, option B is incorrect.
Option C: Tropomyosin is a protein that is present in muscle actin filaments. Two elongated strands extend along the length of the filament in the molecule. When the muscle is at rest, the tropomyosin molecule protects the actin molecule's binding site, which is where myosin interacts. Tropomyosin is replaced by another protein, troponin, during muscle contraction, allowing actin to interact with myosin.
Hence, option C is correct.
Option D: Troponin on thin filaments, of which the main part is the double-stranded helix of actin, is the target of calcium ions. By combining tropomyosin and troponin, a thin filament model was developed. The troponin arm was discovered to protrude from the calcium-depleted troponin and bind to the carboxyl-terminal region of actin during the process of providing structural evidence for the model.
Hence, option D is incorrect.

Hence, Option C is the correct answer.

Note:
Tropomyosins are a vital element of actin filament that fiddle a pivotal role in governing actin filament action in both muscle and nonmuscle cells. These proteins are rod-shaped coiled-coil hetero- or homo-dimers that bind to most actin filaments in the -helical groove. Dimers align in a head-to-tail manner along the length of the actin filament, causing interaction.