Question

Contractile proteins include
A) Troponin
B) Tropomyosin
C) Actin and Myosin
D) All the three

Answer 1

Contractile proteins are the main proteins that bring about contraction and relaxation. Regulatory proteins help in the process.

Complete Answer:
Skeletal Muscles are made up of many Muscle fibres. These fibres are arranged in bundles known as fasciculi. Each muscle fibre inturn consists of many myofibrils. Each myofibril has sarcomere that is the basic contraction and relaxation unit.

Sarcomere in striated muscles consists of dark and light bands that are arranged alternately. It lies between two Z lines. Dark bands are in the centre and light bands are bisected by the Z line.
Dark bands are also known as A bands or Anisotropic bands. Light bands are I bands or Isotropic bands.

Dark bands consist of Myosin protein. This is a polymer made up of small units known as Meromyosin.
Meromyosin is a tadpole in appearance. These have a head and tail. Head shows Actin binding site.
Meromyosin

Light bands contain three proteins; Actin, Troponin and Tropomyosin. Actin is a double stranded structure. Troponin is a trimeric structure that consists of three small proteins (TpC, TpT and TpI). Tropomyosin is wrapped around it in such a way that during relaxation it masks the Myosin binding sites on actin and during contraction it uncovers this region. Troponin helps Tropomyosin in this process.
Light band Components

Thus when Actin binding sites on Myosin and Myosin binding sites on Actin are attached, contraction takes place due to cross bridge formation and when they are separated , relaxation occurs. Thus Actin and Myosin are the main proteins that bring about the muscle movement. Thus these are Contractile proteins. The troponin and tropomyosin help in contraction and hence are Regulatory proteins.

Hence the correct answer is option ‘C’.

Note: Calcium and ATP are main substances that initiate the contraction. Ca binds with Troponin and this causes structural changes in Tropomyosin and thus it unmasks the Myosin binding region of Actin. At the same time, ATP causes movement of Myosin towards Actin. Thus cross bridge formation takes place.