Human troponin I is presented in cardiac muscle tissue by a single isoform with molecular weight 23876 Da and it consists of 209 amino acid residues. 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. Description: Human heart tissue.