Troponin C

Troponin C, or TnC, is the calcium-binding component of the troponin complex, a group of proteins that regulates contraction in skeletal and cardiac muscle. The complex lies on the actin-containing thin filaments of the sarcomere and consists of troponin C, troponin I, and troponin T. Troponin C acts as the complex's calcium sensor .^1,2

When a muscle cell is activated, the concentration of calcium in its cytoplasm rises. Calcium binds to regulatory sites on troponin C, causing changes in the structure and interactions of the troponin complex. This reduces the inhibitory effect of troponin I on the thin filament and allows tropomyosin to shift so that myosin can interact more readily with actin. When cytoplasmic calcium falls again, this activation is reversed and the thin filament returns toward its relaxed state .^1,2

Humans have two main troponin C isoforms. TNNC1 encodes the form expressed in cardiac muscle and slow-twitch skeletal muscle, while TNNC2 encodes the form found in fast-twitch skeletal muscle. The regulatory region of cardiac and slow-skeletal troponin C has one functional calcium-binding site, whereas fast-skeletal troponin C has two regulatory calcium-binding sites .¹

Troponin C should not be confused with cardiac troponin I or cardiac troponin T, which are commonly measured as blood biomarkers of heart-muscle injury. The troponin C isoform expressed in cardiac muscle is also expressed in slow skeletal muscle, so it does not provide the same cardiac tissue specificity .¹