Made up of several hundred myosin protein molecules
Protein containing two intertwining alpha-helical tails and two globular heads.
Head contains a site that allows myosin to bind to the thin filament protein actin
Protein hinges on the head allow it to extend out and rock back and forth
Head also contains an ATP binding site
Thin filament structures (3)
A globular protein that polymerizes to form fibers (F-actin). Two F-actin fibers coil around each other to make thin filament core. Contains binding sites for myosin on thick filament
a coiled, a-helical, fibrous protein that threads along the F-actin fibers.It normally blocks the myosin binding sites on actin, unless shifted by troponin
A trimeric globular protein complex that binds the tropomyosin strands at regular intervals. Troponin binds Ca2+.Ca2+ binding stimulates a conformational change that results in troponin moving tropomyosin away from the myosin binding sites on actin.
Muscle contraction activation (3 steps)
Muscle fiber Ca2+ concentrations increase as a result of a nerve signal.
Troponin binds the Ca2+ and undergoes a conformational change.
The troponin conformational change induces a shift in the position of the tropomyosin strand, exposing myosin binding sites on the surface of the actin filament.
Sliding Filament Mechanism of Contraction (4 steps)
Cleavage of ATP to ADP and Pi causes the myosin hinge to open, extending the head to a position further along the F-actin filament—this is a high-energy conformation.
Myosin heads in the high-energy conformation attach to a binding site on F-actin.
Pi is released from myosin, causing the hinge to close.This is called a power stroke and it causes the thick and thin filaments to slide past one another.ADP is also released during the power stroke.
Actin-bound myosin heads bind ATP, which releases the heads from the actin.
Muscle deactivation (4 steps)
Ca2+ levels decrease
Troponin reverts to its original conformation
Tropomyosin also reverts, covering the myosin binding sites on the actin thin filament
Myosin heads are no longer able to bind actin and the thick and thin filaments slide back to their original positions (muscle relaxes)