Role of calcium in striated Muscle contraction

Starting point

• thin filaments made up of actin, dark spot represents the myosin binding site on the actin

• can see other thin filament proteins which include tropomyosin which is the gold thread

• the tropomyosin covers the myosin biding site on the thin filaments

• proteins troponin which there are three of
  - troponin regulates the position of tropomyosin

• in red, those are the myosin heads

• in this position the myosin can not form a cross bridge with the thin filaments because tropomyosin covers up the myosin binding site on the actin
  - the muscle is relaxed

cross section through sarcomere

• rotate the thing filaments 90 degrees (thin filaments are coming out of the screen)

• see the actin which is the main portion of the thin filament with the myosin binding sites in these areas

• tropomyosin and troponin are present

• tropomyosin is really important because that can inhibit the binding of actin and myosin

Troponin complex

Proteins that make this up:
- troponin T
- troponin C ** this is the one that would regulate
- troponin I

• if we alter the shape of troponin C we also alter the shape of troponin I
  - when this happens, tropomyosin gets moved either towards or away from the myosin binding site on the actin molecules

• troponin complex: can move tropomyosin to allow contraction or to allow relaxation

• tropomyosin (regulatory protein) which covers up the myosin binding site on the actin is an inhibitory protein
  - it prevents muscle contraction

how do we cause troponin C to change shape and move tropomyosin

• calcium

• troponin C has a high bond to calcium
  - if we release calcium from the sarcoplasmic reticulum into the cytosol the calcium can bind to troponin C

• this binding of calcium to the troponin C molecule causes troponin C to change shape and pulls on the other troponin molecules which then move tropomyosin and pulls it away from the myosin binding site on the thin filaments

• C) when we expose the myosin binding site on the thin filaments, myosin can now form a cross bridge and we can begin cross bridge cycling

• D) calcium is the molecule that stimulates the movement of tropomyosin allowing for cross bridges to form and allow for cross bridge cycling to occur

What happens if calcium is pumped out?

• myosin cant form any cross bridges again because troponin C changed shape and allowed tropomyosin to go back and cover up the binding site on the thin filaments

What is the role of calcium in muscle contraction?

• to alleviate the inhibition of contraction