Question

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

Answer 1

Hint:Each myofibril of the muscle has 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 thin filaments (actin) over the thick filaments (myosin). Based on this, it is easy to understand that actin-binding sites are located on myosin (thick filament).

Complete answer:Each muscle fiber contains a large number of parallelly arranged filaments called myofibrils or myofilaments. Each myofibril has alternate dark and light bands on it. This is due to the presence of two contractile proteins actin (light band) and myosin (dark band). The light band contains actin and is called an Isotropic band and the dark band contains myosin and is called an anisotropic band.
Each actin (thin) filament consists of three proteins, namely actin, tropomyosin, and troponin. Whereas, each myosin (thick) filament consists of monomeric proteins called meromyosin. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the Heavy meromyosin (HMM) and the latter is called the Light meromyosin (LMM). The globular head contains active ATPase enzymes and binding sites for actin and ATP respectively.
Muscle contraction is due to 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 present on the myosin filament and the myosin binding site is present on the actin filament.

Understanding the options one by one,
(A) Troponin
Troponin is a part of actin filament and helps in masking the active binding sites for myosin on the actin filaments in the resting state. Therefore, this is not the correct answer.
(B) Tropomyosin
Tropomyosin is also a part of actin filament and therefore this is not the correct answer.
(C) Meromyosin
Meromyosin is a monomeric protein present in myosin filament. The globular head of the meromyosin contains the actin-binding site. Hence, this is the correct answer.
(D) Both (B) and (C)
This is not the correct answer because tropomyosin is a part of actin filament and meromyosin is a part of myosin filament.
Therefore, Option (C) Meromyosin, is correct.

Note:The easy way to remember this is by knowing the mechanism of contraction. Muscle contraction is due to 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 present on the myosin filament and the myosin binding site is present on the actin filament. Moreover, it is important to note that actin filament is made up of three proteins (actin, troponin, and tropomyosin) and myosin filament is made up of only one protein (meromyosin).