Question

What are the four component proteins of muscle cells, which compose the sarcomere? Describe how they interact with one another and allow for muscle contractions.

Answer 1

• mainly there are two filaments which are involved in muscle contraction- thick filament or thin filament.
• th thick filament myosin (globular protein) contains two polymerised portions- heavy meromyosin and light meromyosin. of which the two heavy chains coils, forming globular structure called myosin head at one end.
• myosin filaments are linked to Z lines by titin- which is a elastic protein.
• The thin filament contains mainly three proteins - actin, tropomyosin and troponin.
• Actin- they forms the backbone of thin filament. It is a double helix structure. Basic unit is F actin molecule. G-actin molecules form each strand of these F actin strands.
• It is te function of G actin to provide active binding site to myosin head.
• Tropomyosin- Thread like proteins which are observed along the groove of actin spiral. it covers the actin sites thatt binds with cross bridges during the resting stage . And thus prevent the binding between myosin head and binding site.
• Troponin- it is a globular molecule, which bnds to a particular site on the tropomyosin. They have an important role in causing muscle contraction. They are composed of 3 polypeptide units. 1) Troponin T- it has site for attachment with tropomyosin. It has high affinity for tropomyosin. 2) Troponin- I - they inhibit actin activated myosin ATPase activity during resting stage. they can readily bind with actin ad tropomyosin. 3) Troponin- C- one molecule can bind with 4 Ca2+ ions. tey have got igh Ca 2+ affinity. they triggers the intiation of muscle contraction by combining of Ca2+ ions with troponin,
• With the stimulation, a high level of Ca2+ is released from cisternaes. Which causes the Ca2+ ions readily to combine with tropnin C of troponin- tropomyosin complex (which during resting stage covers the G actin for binding with myosin). Thus conformational change occurs, making tropomyosin to go deeper into the groove. thus the G-actins are exposed to form cross bridges with myosin head. thus intiating muscle contraction.