Myofibrils

---------- Forwarded message ----------
From: Text Books <rijuebook@gmail.com>
Date: Wed, 5 Mar 2008 21:50:48 +0530
Subject: Myofibrils
To: rijuebook.bscnursing@blogger.com
Myofilaments
Thin filaments
Thin filaments are composed of F-actin, tropomyosin, troponin, and
associated proteins.
F-actin
It is a polymer of G-actin monomers arranged in a double helix.
• Each monomer possesses an active site that can interact with mYOSIn.
• F-actin is present as filaments (with a diameter of5 nm) that
exhibit polarity, having a (+) and (-) end.
Tropomyosin
Tropomyosin molecules are 41 nm in length. They bind head-to-tail,
forming filaments that are located in the grooves of the F -actin
helix.
Troponin
Troponin is associated with each tropomyosin molecule and is composed of:
• Troponin T (TnT), which forms the tail of the molecule and functions
in binding the troponin complex to tropomyosin.
• Troponin C (TnC), which possesses four binding sites for calcium. It
may be related to calmodulin.
• Troponin I (TnI), which binds to actin, inhibiting interaction of
myosin and actin.
Nebulin
Nebulin is a long inelastic protein. Two nebulin molecules wrap around
each thin filament and assist in anchoring it to the Z disk.
• Each nebulin molecule is embedded in the Z disk by its carboxy
terminal, but does not span the entire Z disk.
• The amino terminal of each nebulin molecule ends in the A band, at
or near the free end of its thin filament.
• Nebulin in skeletal muscle is thought to determine the length of its
associated thin filament, although in cardiac muscle it extends only
one quarter of the length of the thin filament.
Thick filaments
Thick filaments each contain about 250 myosin molecules arranged in an
anti parallel fashion and three associated proteins-myomesin, titin,
and C protein.
Myosin
Myosin is composed of two identical heavy chains and two pairs of
light chains. The myosin molecule resembles a double-headed golf club.
• Myosin heavy chains consist of a long rod-like "tail" and a globular
"head." The tails of the heavy chains wind around each other in an
a-helical configuration.
o Tails function in the self-assembly of myosin molecules into bipolar
thick filaments.
o Actin-binding sites of the heads function in contraction.
• Myosin light chains are of two types; one molecule of each type is
associated with the globular head of each heavy chain.