The five proteins of the myofilaments are the following:
<span>1.
</span>Myosin, shaped like a golf club, with two
polypeptides intertwined to form a shaftlike tail and a double globular head,
or cross-bridge, projecting from it at an angle.
<span>2. </span>Fibrous actin is like a bead necklace—a string
of subunits called globular (G) actin. Each G actin has an active site that can
bind to the head of a myosin molecule.
<span>3. </span>Tropomyosin. It blocks the active sites of six
or seven G actins and prevents myosin cross-bridges from binding to them when a
muscle fiber is relaxed.
<span>4. </span>Troponin a smaller calcium-binding protein bound
to each tropomyosin molecule.
<span>5. </span>Titin (connectin), run through the core of a
thick filament, emerge from the end of it, and connect it to a structure called
the Z disc.
