Troponin I is a part of the troponin complex. It binds to actin in thin myofilaments to hold the actin-tropomyosin complex in place. Because of it, myosin cannot bind actin in relaxed muscle. When calcium binds to the Troponin C it causes conformational changes which lead to dislocation of troponin I and finally tropomyosin leaves the binding site for myosin on actin leading to contraction of muscle. The letter I is given due to its inhibitory character.
The tissue specific subtypes are:
Slow-twitch skeletal muscle isoform troponin I, TNNI1 (1q31.3, 191042)
Fast-twitch skeletal muscle isoform troponin I, TNNI2 (11p15.5, 191043)
Human troponin I, often denoted as cTnI, is presented in cardiac muscle tissue by a single isoform with molecular weight 23876 Da and it consists of 209 amino acid residues. The theoretical pI of cTnI is 9.87. cTnI molecule contains two serines in the 22 and 23 positions. Both amino acid residues can be phosphorylated in vivo by protein kinase A, so four forms of protein – one dephospho, two monophospho and one bisphospho – can coexist in the cell. Phosphorylation of cTnI changes the conformation of the protein and modifies its interaction with other troponins as well as the interaction with anti-TnI antibodies. According to the latest findings significant part of cTnI released into the patient’s blood stream is phosphorylated. For more than 15 years cTnI has been known as a reliable marker of cardiac muscle tissue injury. It is considered to be more sensitive and significantly more specific in diagnosis of the myocardial infarction than the “golden marker” of last decades – Creatin Kinase, as well as myoglobin and lactate dehydrogenaseisoenzymes.