Mechanism of Muscle Action at the Molecular Level
The animation begins with the myosin head bound tightly to an actin monomer in the actin filament. This tight binding requires that the nucleotide binding site on left side of myosin head be empty. An ATP molecule binds at the nucleotide binding site and induces a conformational change in the myosin head group such that the myosin head group can no longer bind to the microfilament. The myosin head is now free to move relative the microfilament. As the myosin head moves ATP is hydrolyzed to ADP + Pi. Both the ADP and Pi remain tightly bound to the myosin head group. Interactions between the myosin head and microfilament cause the Pi to be released. The Pi release is accompanied by a conformational change in the myosin head group which opens an actin binding site. The myosin head group with its still bound ADP, binds to the next actin monomer of the microfilament. As the myosin head binds to the microfilament, the power stroke is triggered. The myosin head group snaps into its tight binding conformation, expelling the bound ADP in the process. The net effect of these steps is to move the myosin head group one actin monomer relative to the microfilament. This process must be repeated many, many times to generate the movement and force characteristic of a macroscopic muscle contraction.
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