Conformation of the critical pH sensitive region of troponin depends upon a single residue in troponin I.

•The conformation of sTnI bound to cNTnC is the same as in skeletal troponin.•H130 of sTnI forms a pH-dependent salt-bridge with E19 from cNTnC.•A162H cTnI adopts the same conformation as sTnI when bound to cNTnC.•cNTnC can be in different positions to respond to phosphorylation on cTnI.•Structure of cTnI preserves the position of the mobile domain relative to actin.