- Myosin moves actin during the contraction of a muscle.
- The contraction of a myofibril occurs as a result of the movement of actin filaments sliding over filaments of myosin.
- + Myosin filaments have globular heads that are hinged so they can move back and forth
- + Each myosin head has a binding site for actin and a binding site for ATP
- + Actin filaments have binding sites for myosin heads called actin-myosin binding sites.
- + Two other proteins called troponin and tropomyosin are found between actin filaments.
- + These proteins are attached to each other and form a blocking complex
When the muscle is relaxed, the myosin heads are held away from the myosin binding sites on the actin filaments. The actin-myosin binding site is blocked by tropomyosin, which is held in place by troponin.
The myofilaments can't slide past each other because the myosin heads can't bind to the actin-myosin binding sites on the actin filaments.
When muscle is contracted, the myosin heads move out from their resting position and link to the myosin binding sites on the actin filaments, forming actin-myosin cross bridges.