Question

Can you provide some details about the different contractile, structural and regulatory proteins found within skeletal muscle cells?

Answer 1

Answer)

CONTRACTILE PROTEINS -

Contractile proteins are actin and myosin.

Each actin(thin) filament is made up of two 'F' (filamentous) actin molecule helically turn around each other.

Thin filament is responsible for the formation of

'I '(isotropic) band.

Two filaments of another protein, called tropomyosin also run close to the 'F' actin molecules, throughout their length.

A complex protein called troponin is distributed at regular intervals on the tropomyosin.

Myosin is also known as thick filament.

Thick filament is responsible for the formation of 'A' (anisotropic) band

Myosin molecule is composed of 2 large polypeptide heavy chains and four smaller light chains.

These polypeptides combine to form a molecule that consists of two globular heads (containing heavy and light chains) and a long tail formed by the two twisted heavy chains.

Function :Each globular head has two important sites -

a) Actin binding site, where myosin comes in contact with actin

b) ATPase (catalytic) site, that hydrolyzes (breaks) ATP.

The myosin molecules aggregate with their heads pointed in one direction along one half of the filament and in the opposite direction along the other half. The head serves as the cross bridges.

STRUCTURAL PROTEINS -

Structural proteins are actinin, titin and desmin.

Actinin binds the Z lines to actin.

Titin connects Z lines to the M line.

Desmin binds the Z lines to the cell membrane.

These proteins together forms the cytoskeleton of the muscle cell.

REGULATORY PROTEINS -

Regulatory proteins are tropopin and tropomyosin.

Tropomyosin molecules are long filaments located in the groove between two chains in the actin.

It covers the binding site of actin where myosin head comes in contact with actin i.e prevents the interaction between the actin and myosin filaments.

Troponin molecules are small globular units located at intervals along the tropomyosin molecules.

It is made up of 3 subunits -

a) troponin T - binds the other troponin components to tropomyosin.

b) troponin I - inhibits the interaction of myosin with actin.

c) troponin C - contains binding site for calcium that initiates muscle contraction.