Troponin
Overview
Troponin is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Bundles of filaments, made up of actin and myosin, create this contraction. It is a component of thin filaments (along with actin and tropomyosin), and is integral for muscle contraction.
Structure
Troponin proteins reside on the actin filament of the thin filaments in the sarcomeres of cardiac and skeletal muscle. The troponin complex is composed of three subunits: troponin C (TnC), troponin T (TnT), and troponin I (TnI). Each of these subunits has a unique function and structure.
Troponin C
Troponin C (TnC) is the calcium binding component of the troponin complex. It has two forms, one in skeletal muscle and one in cardiac muscle, distinguished by the isoforms of the gene encoding them.
Troponin T
Troponin T (TnT) binds the troponin complex to tropomyosin, anchoring the complex to the thin filament of the sarcomere. It also has different isoforms for cardiac and skeletal muscle, encoded by different genes.
Troponin I
Troponin I (TnI) is the inhibitory component of the troponin complex. It binds to actin in a calcium-dependent manner, preventing contraction in the absence of calcium. Like TnC and TnT, TnI has different isoforms for cardiac and skeletal muscle, encoded by different genes.
Function
In relaxed muscle, tropomyosin blocks the myosin-binding sites on actin, preventing the myosin cross bridges from binding to the actin filaments. The binding of calcium to troponin C causes conformational changes in the troponin complex, displacing tropomyosin from the myosin-binding sites on actin, allowing the myosin cross bridges to bind to the actin filaments and produce a muscle contraction.
Clinical significance
Troponin is used as a diagnostic marker for various heart disorders. The most common use of troponin testing is for the diagnosis of myocardial infarction (heart attack). Troponin levels in the blood are normally very low, but if the heart is damaged, as in a heart attack, troponin is released into the blood.