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Tropomyosin is the third of
the four contractile proteins. It wraps the actin and covers the active
sites. Troponin, in turn, wraps around both the actin and tropomyosin. Tropomyosin
is the fourth contractile protein.
Troponin has three
molecular subunits, and each one binds to something different.
One subunit will bind to
actin.
One subunit will bind to
tropomyosin.
One subunit will bind to
calcium.
When skeletal muscle is in
a state of relaxation, two of the three subunits are activated; The actin
subunit is active and the tropomyosin subunit is active. During relaxation,
the calcium concentration in the sarcoplasm is minimal, therefore the
calcium subunit is not active. When calcium is released from the
sarcoplasmic reticulum, the third molecular subunit of troponin will be
activated, causing tropomyosin to move out of the way and the actin active
sites to be exposed. As a result, skeletal muscle contraction is made
possible.
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