Question

Actin binding sites are located over
(A) Troponin
(B) Tropomyosin
(C) Meromyosin
(D) Creatinine
(E) Both A and B

Answer 1

Each myofibril of the muscle poses an alternate dark and light bands on it, which is due to the presence of two contractile proteins actin and myosin. Muscle contraction is basically due to the sliding of the skinny filaments (actin) over the thick filaments (myosin). Based on this, it is effortless to understand that actin-binding web sites are placed on myosin (thick filament).

Complete answer:
Each muscle fiber contains and constitutes a massive range of parallelly arranged filaments known as myofibrils or myofilaments. Each myofibril has alternate darkish and light bands on it. This is due to the presence of two contractile proteins actin (light band) and myosin (dark band). The mild band consists of actin and is referred to as an Isotropic band and the darkish band consists of myosin and is known as an anisotropic band.
Each actin (thin) filament consists of three proteins, particularly actin, tropomyosin, and troponin. Whereas, every myosin (thick) filament is made up of monomeric proteins known as meromyosin. Each meromyosin has two essential parts, a globular head with a quick arm and a tail, the former being termed to be as the Heavy meromyosin (HMM) and the latter is called the Light meromyosin (LMM). The globular head constitutes active ATPase enzymes and binding sites for actin and ATP respectively.

Understanding the options one by way of one,

(A) Troponin :
Troponin is a part of actin filament and helps in covering the active binding web sites for myosin on the actin filaments in the resting state. Therefore, this is not the correct answer.

(B) Tropomyosin: Tropomyosin is also a section of actin filament and therefore this is now not the correct answer.

(C) Meromyosin : Meromyosin is a monomeric protein current in myosin filament. The globular head of the meromyosin consists of the actin-binding site. Hence, this is the right answer.

(D) Both (B) and (C) : This is no longer the correct reply because tropomyosin is a part of actin filament and meromyosin is a section of myosin filament.
Therefore, Option (C) Meromyosin, is correct.

Note: Muscle contraction result because of the sliding filament theory which involves sliding of the thin filaments over the thick filaments. Hence it's easy to understand that the actin-binding site is located on the myosin filament and the myosin binding site is present on the actin filament.