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M04.05.03 Functional Proteins of the Sarcomere
Sarcomeres are the functional units of striated muscles, composed of thin and thick filaments. Understanding the roles of the functional proteins in these filaments is essential for grasping muscle contraction mechanisms in skeletal and cardiac muscle.
Structure of the Sarcomere
The sarcomere consists of two key filament types:
- Thin filaments: Composed primarily of actin, along with regulatory proteins like tropomyosin and troponin.
- Thick filaments: Composed of myosin, which plays a central role in the contractile process.
Changes in Muscle Contraction
The following outlines the events that occur in resting muscle and how they change following the attachment of calcium to troponin, as depicted in the diagram below:
Resting Muscle
- In the absence of calcium, tropomyosin blocks the myosin-binding sites on actin, preventing the formation of actin-myosin cross-bridges.
- Troponin remains inactive, preventing muscle contraction.
Changes Following Calcium Binding to Troponin
- Calcium binds to troponin-C, causing a conformational change.
- This movement shifts the troponin-tropomyosin complex, exposing the myosin-binding sites on actin.
- Once exposed, myosin can bind to actin, initiating contraction.
Proteins of the Thin Filaments
| Protein | Function |
|---|---|
| Actin | Structural protein forming the core of the thin filament. It has attachment sites for myosin. |
| Tropomyosin | Blocks myosin-binding sites on actin in the resting state. |
| Troponin | Composed of three subunits: troponin-T (binds to tropomyosin), troponin-I (inhibits contraction), and troponin-C (binds calcium). |
Key Points:
- Actin: Structural protein with myosin attachment sites.
- Tropomyosin: Inhibits actin-myosin interaction by covering binding sites.
- Troponin: Regulates contraction through calcium binding and interaction with tropomyosin.
Proteins of the Thick Filaments
Myosin is the primary protein of the thick filaments and has a crucial role in generating the mechanical force required for muscle contraction.
| Protein | Function |
|---|---|
| Myosin | Possesses ATPase activity, which generates the energy needed for muscle contraction. |
Steps in Muscle Contraction:
- ATP hydrolysis: Myosin splits ATP into ADP and Pi, transitioning into a high-energy state.
- Cross-bridge formation: Myosin binds to actin, leading to the conversion of chemical energy into mechanical energy.
- Power stroke: Myosin pulls the actin filament, generating tension.
- Contraction types:
- Isotonic contraction: If the force generated moves the load, the muscle shortens.
- Isometric contraction: If the force generated does not move the load, the muscle does not shorten.
Points to Remember:
- Muscle contraction is regulated by the interplay between calcium, actin, and myosin.
- Tropomyosin and troponin prevent contraction under resting conditions by blocking the actin-binding sites for myosin.
- Myosin’s ATPase activity is key to converting chemical energy into mechanical force during the contraction cycle.
- Calcium binding to troponin-C is the trigger that starts the contraction process.
Bibliography:
- Lodish, H., et al. Molecular Cell Biology. 8th ed., W. H. Freeman, 2016. Link
- Alberts, B., et al. Molecular Biology of the Cell. 6th ed., Garland Science, 2014. Link
Bibliography
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 6th ed. New York: Garland Science; 2014.
- Hall JE. Guyton and Hall Textbook of Medical Physiology. 13th ed. Philadelphia: Elsevier; 2016.
- Marieb EN, Hoehn K. Human Anatomy & Physiology. 10th ed. Boston: Pearson; 2016.
- Silverthorn DU. Human Physiology: An Integrated Approach. 8th ed. Boston: Pearson; 2018.