Troponin

Ribbon diagram of troponin

Troponin is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle. Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue.

Structure

Troponin is composed of three subunits:

• Troponin C (TnC): This subunit binds calcium ions and is responsible for the conformational change that allows muscle contraction to occur. It is similar to calmodulin in structure and function.

• Troponin I (TnI): This subunit inhibits actin-myosin interactions, thus preventing muscle contraction in the absence of calcium.

• Troponin T (TnT): This subunit binds the troponin complex to tropomyosin, anchoring it to the actin filament.

Function

Troponin plays a crucial role in the regulation of muscle contraction. When calcium ions are released from the sarcoplasmic reticulum, they bind to troponin C, causing a conformational change in the troponin complex. This change moves tropomyosin away from the actin binding sites, allowing myosin heads to bind to actin and initiate contraction.

Clinical Significance

Troponin levels are measured in the blood to diagnose myocardial infarction (heart attack). Elevated levels of cardiac-specific troponins (cTnI and cTnT) indicate damage to the heart muscle. These biomarkers are more specific and sensitive than previous markers such as creatine kinase (CK-MB).

Related Pages

• Muscle contraction
• Calcium signaling
• Myocardial infarction
• Tropomyosin

References

• Marieb, E. N., & Hoehn, K. (2018). Human Anatomy & Physiology. Pearson Education.
• "Troponin." (2023). In Wikipedia, The Free Encyclopedia. Retrieved from https://en.wikipedia.org/wiki/Troponin