Troponin I (Trop I) is a key protein component of the regulatory system for muscle contraction in both cardiac (heart) and skeletal muscle. It is one of three subunits—Troponin I, Troponin T, and Troponin C—that form the troponin complex, which is situated along the thin actin filaments of the muscle fiber. The "I" in its name stands for its primary function: Inhibition of muscle contraction.
While present in both cardiac and skeletal muscle, the form found in the heart, known as cardiac troponin I (cTnI), is structurally distinct, particularly due to an N-terminal extension.This cardiac-specific nature makes cTnI an exceptionally valuable biomarker in clinical diagnostics, especially for detecting heart muscle injury.
The function of Trop I, particularly cTnI, is also finely tuned by posttranslational modifications, such as phosphorylation, which can affect its affinity for calcium and its interaction with other proteins.This modulation is part of the body's homeostatic mechanism to control heart function, for instance, in response to adrenergic stimulation.
The most critical clinical application of Troponin I is as a highly specific and sensitive biomarker for myocardial injury (heart muscle damage), most commonly due to a myocardial infarction (heart attack).
Because cardiac troponin I (20$\text{cTnI}$) is found almost exclusively in the heart muscle, its presence in the bloodstream at elevated levels strongly indicates that heart muscle cells (cardiomyocytes) have been damaged or died, leaking the protein into the blood.21
