Question

Actin binding sites are located over
A. Troponin
B. Tropomyosin
C. Meromyosin
D. Both B and C

Answer 1

Hint: Myosin is a polymerized protein. Each monomeric protein has two parts: The head or heavy meromyosin and tail or light meromyosin. There are two binding sites on myosin head. The one site binds with actin and the other binds to ATP.

Complete answer:
 In this question, we have asked the location of the actin binding site. As we know, the actin binding site is present on the myosin. So, first we'll learn a little about myosin. Myosin or the thick filament is a polymerized protein. The monomeric unit of myosin is called Meromyosins, which constitute one thick filament. The meromyosin has two parts, a globular head with a short arm and a tail, the head is called the heavy meromyosin (HMM) and the tail is called the light meromyosin (LMM). The globular head is an active ATPase enzyme. As we already mentioned that the head of meromyosin has two binding sites for ATP and active sites for actin.
So, our first option is Troponin. Troponin is found on thin filament or on actin. It is a calcium-regulatory protein for the calcium regulation in muscle contraction. The next option is Tropomyosin. Tropomyosin is also found on actin or thin filament. When skeletal muscle is relaxed, it covers myosin-binding sites on actin molecules, and prevents binding of myosin to actin. Option C is meromyosin. As we already know, that meromyosin is a monomer of myosin and an active binding site is present on the head of meromyosin.

So, the correct option is C. Meromyosin.

Note:
Myosin has another binding site, at which ATP binds. When ATP binds, by enzymatic activity ATP hydrolysis to ADP, releases an inorganic phosphate molecule and energy. ATP binding causes myosin to release actin and allow actin to detach from myosin.