Video Transcript
In the sliding filament theory of
muscle contraction, what initiates the detachment of the myosin head from the
binding site on the actin filament? (A) A molecule of ADP being
released from the myosin head. (B) The hydrolysis of an ADP
molecule on the actin filament. (C) A molecule of ATP binding to
the myosin head. Or (D) calcium ions binding to
tropomyosin.
To answer this question, we need to
learn some more information about the sliding filament theory of muscle contraction
and the protein filaments that are involved in this process. When a muscle fiber is relaxed, a
protein filament called tropomyosin, shown here in blue, coils around a thin protein
filament called actin, shown here in orange. Tropomyosin blocks sections of the
actin filament that contain binding sites for the globular heads of another protein
filament called myosin, shown here in pink.
When the muscle fiber is relaxed,
each myosin head is bound to a molecule of ADP and an inorganic phosphate. The release of calcium ions from a
specialized organelle in the muscle fiber called the sarcoplasmic reticulum triggers
tropomyosin to pull away from and expose the myosin binding sites on the actin
filament. This allows the myosin heads to
temporarily bind to the actin filament, forming cross bridges, which are otherwise
known as transverse links, between the molecules. The formation of these cross
bridges releases the inorganic phosphate molecule from each myosin head. The myosin head then changes angle,
pulling the whole actin filament along in a process sometimes referred to as the
power stroke.
The power stroke releases the
molecule of ADP from each myosin head. This allows ATP to bind to myosin
instead, leading to the breaking of the cross bridge between the actin and myosin
filaments and causing the myosin heads to detach. The myosin heads can then return to
their original position, further along the actin filament than they were before the
power stroke due to the actin filament being pulled along.
Now we know that the correct answer
to this question asking us what initiates the detachment of the myosin head from the
binding site on the actin filament is (C): a molecule of ATP binding to the myosin
head.
