CorpusHuman Anatomy
MuscularMolecule

Troponin complex

Troponinum

Also known as: troponin C, troponin I, troponin T, TnC, TnI, TnT, cardiac troponin

A three-subunit, Ca²⁺-sensing regulatory complex on the thin filament of striated muscle. When it binds Ca²⁺ it shifts tropomyosin and allows cross-bridges to form.

Sarcomere & Sliding Filaments

Overview

The troponin complex sits about every 38.5 nm along the thin filament, one per tropomyosin molecule. Troponin C (TnC) binds Ca²⁺. Fast skeletal TnC has two regulatory low-affinity sites; cardiac TnC has one. Troponin I (TnI) binds actin and inhibits actomyosin interaction at rest. Troponin T (TnT) anchors the complex to tropomyosin.

When cytosolic Ca²⁺ rises, Ca²⁺ binds TnC, and TnC pulls TnI away from actin. Tropomyosin is then free to roll into the actin groove, exposing the myosin-binding sites ("steric blocking" model). When Ca²⁺ is pumped back into the SR, the process reverses. Smooth muscle has no troponin; it uses calmodulin and myosin light-chain kinase instead.

Parts & subdivisions

  1. Troponin C
    Binds Ca²⁺ and starts the switch.
  2. Troponin I
    The inhibitory subunit, which holds tropomyosin over the binding sites. The cardiac isoform (cTnI) is a highly specific marker of myocardial injury.
  3. Troponin T
    Binds tropomyosin and links the complex together. The cardiac isoform (cTnT) is also a marker of myocardial injury.

Function

  • Couples a rise in Ca²⁺ to activation of the thin filament
  • Inhibits contraction at rest

Clinical significance

Cardiac troponins in myocardial infarction

Cardiac TnI and TnT have isoforms distinct from skeletal ones, so high-sensitivity assays detect myocardial necrosis specifically. Levels rise within ≈3–4 h of infarction, peak at ≈24–48 h and stay raised for ≈7–10 days (cTnT up to ≈14 days).

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.