Question

5. Discuss how myosin II walks along an actin filament. Be sure to include the details of the following steps: a) Attached b) Released c) Cocked d) Force-generating e) Attached

Answer 1

Actin is a preserved structural protein ubiquitously expressed in eukaryotic cell. It occurs in three isoforms. Myosin II   is a dimeric molecule which is having a long rod like tail domain that assembles into a bipolar and thick filament which helps in muscle contraction.

The myosin are motor proteins which will interact with the actin filaments. An ATP hydrolysis undergoes conformational changes which will result in the relative movement of myosin and an actin filament.

The process of movement of the myosin starts with attachment of the myosin head to the actin filament, then the bending of the head which is followed by the subsequent detachment in a cyclical ATP- dependent process. In this cycle the myosin moves 5 to 25nm with the hydrolysis of one ATP.

The main steps involved are;

Step 1 – In the start of the new cycle the myosin head lacks a bound ATP so it is attached to the actin filament by a rigor conformational change.

Step 2 – Then the ATP undergoes a small conformational shift in the actin binding site which will reduce the affinity for actin and indeed the myosin head will be release the actin filament.

Step 3 – The ATP binding creates a large conformational shift in the lever arm of the myosin so the myosin head bends and moves more along the filament. Then the ATP is hydrolyzed by leaving inorganic phosphate and ADP.

Step 4 – The weak contact of the myosin with the actin filament will make small conformational change on myosin and the inorganic phosphate is released.

Step 5 – The reinforcement created by the release of inorganic phosphate helps the binding interaction between the myosin and the actin and thereby trigger the power stroke. This is the key force useful for generating myosin motor protein. A force is created on the actin filament and the myosin protein get reverted back to its original conformation.

Step 6 – After regaining the original conformation, the ADP is released and but the myosin remain bound to the filament to the original position from where it has started thereby bringing the cycle back to the beginning.