A: Contractile proteins:
i. Thin filament: Each thin filament is formed by two filamentous actin
(F - actin) molecules, which are wound around each other. Each F - actin formed by many globular actin (G - actin) molecules. Running close to the
F - actin molecules, there are two filaments of protein molecules known as tropomyosin. Another protein, namely, troponin is distributed on the tropomyosin
at regular intervals. Troponin has three units namely,
Troponin T(Tn - T), Troponin I (Tn - I) and Troponin C (Tn - C). Tn - T is
attached to the tropomyosin. Tn - I inhibits the attachment of myosin and actin and
Tn - C binds with Ca++ ions. In the absence of Ca++, Tn - C, stabilizes tropomyosin
in its blocking position over the active sites of actin.
Troponin and tropomyosin are often called regulatory proteins because they
play major role in masking and unmasking the active sites of actin.
ii. Thick filament: Thick filaments are formed by motor proteins called myosin.
It is able to convert the chemical energy (ATP) into mechanical energy. Myosin
is a polymer formed by many monomers called meromyosins. Each
meromyosin has a head, a neck and a tail.
Head and neck are formed by heavy meromyosin and tail by light meromyosin. Athick filament contains 200 - 300 myosin molecules. The heads are directed towards Z - lines and tails towards M - line. The head and neck project outwards at regular intervals and angles from each other from the surface of a thick filament is known as cross arm.
Each head has two binding sites - one for ATP and the other for an active site
of thin filament.
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