Muscle Proteins

 

Images courtesy of Mike Ferenczi - http://npbsn41.nimr.mrc.ac.uk

The two major muscle proteins are myosin (the thick filament - located to the left) and actin (the thin filament - seen below).  Together they make up 85% of the muscle's proteins.            Structurally, myosin filaments have long tails that run parallel to each other and twist together.  Branching off of the tails are globular heads (also called cross bridges) that have binding sites on them.  The two most important binding sites are for actin and ATP (to see the binding sites in action, click here).            The thin filament formation is a double chain of globular proteins that are arranged in a coiled, helix like pattern.  Actin also contains two other crucial proteins: troponin and tropomyosin.

 
 

The Troponin-Tropomyosin Complex Tropomyosin filaments are intertwined along the actin filaments.  The troponin protein is connected to the tropomyosin forming the troponin-tropomyosin complex.  Together these two proteins act as a gating mechanism for actin.  In a resting muscle, the tropomyosin blocks the myosin binding sites that are located on the actin filaments, which prevents muscle contractions of any kind.  To initiate a contraction, calcium ions must bind to their receptor site on the troponin.  The binding of calcium ions cause a conformational change in the complex.  At this point, the troponin-tropomyosin complex opens the myosin binding sites on the thin filaments and a muscle contraction occurs.  To see this in action - click here!

 
 
 

Image courtesy of Mike Ferenczi - http://npbsn41.nimr.mrc.ac.uk

This image simply illustrates the interaction between actin and the two specific components of myosin - the crossbridge and the filament.

 
 
 

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