Muscle Contraction- Contractile Proteins and their Functions - Practice Questions & MCQ

• The smallest unit of the skeletal muscle is called the sarcomere.
• It is the structural and functional unit of the myofibril
• The sarcomere consists of the thick and thin filament. 
• The thin filament is made up of the globular protein called actin.
• The thick filament is made up of the protein called myosin.
• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.
• Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. 
• In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it.
• The thin filaments are firmly attached to the ‘Z’ line.
• The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line. 
• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. 
• The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere. The length of a sarcomere is about 2 -3 micron
• In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments.
• This central part of thick filament, not overlapped by thin filaments is called the ‘H’ zone.

Actin Filament:

• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. 
• Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. 
• Mg brings about conversion of G into F Actin
• Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. 
• A complex protein Troponin is distributed at regular intervals on the tropomyosin (which bears actin-binding sites)
• Troponin is a trimeric protein. The three subunits are TpC, TpI and TpT
• In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.
• Actin,Troponin and Tropomyosin together forms the Thin filament

Myosin Filament:

• Each myosin (thick) filament is also a polymerised protein. 
• Many monomeric proteins called Meromyosins constitute one thick filament.
• 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, the light meromyosin (LMM). 
• The HMM component, i.e.; the head and short arm projects outwards at a regular distance and angle from each other from the surface of a polymerised myosin filament and is known as a cross arm. 
• The globular head is an active ATPase enzyme and has binding sites for ATP and actin binding sites (ABS)