title: Interaction between the regulatory domain of cardiac troponin C and the acidosis-resistant cardiac troponin I A162H.
abstract: OBJECTIVE
  Ischaemic heart disease is the leading cause of mortality worldwide. Acidosis is the main mediator of ischaemia and shielding against it might be possible. In this study, we characterize the nature of interaction between the regulatory domain of cardiac troponin C and the A162H-substituted cardiac troponin I (cTnI) that confers protection against acidosis.
  
  
  RESULTS
  We used nuclear magnetic resonance spectroscopy to study the interaction of the Ca(2+)-saturated N-domain of cardiac troponin C with the switch region of cTnI containing the A162H substitution under normal and acidic conditions. Our results show that H162 increases the affinity of TnI for troponin C at pH 7 and this affinity is further enhanced at pH 6. To investigate the nature of the interactions responsible for such improvement, we determined the acid dissociation constants of the glutamate residues in troponin C. The results show that E15 and E19 exhibit deviations in their acid dissociation constant (pK(a)) profiles and reflect a common high pK(a) value of 6.8, indicating electrostatic interactions with H162. Residue H171 in wild-type cTnI does not play a similar role.
  
  
  CONCLUSIONS
  This work provides evidence for the mechanism by which cTnI A162H improves myocardial performance during acidosis. The electrostatic interaction between residues E15 and E19 in troponin C and H162 in TnI at low pH is responsible for stabilizing the conformation of troponin C that leads to contraction, thus partially ablating the decreased Ca(2+)-sensitivity caused by acidosis.
authors.affiliation: Department of Biochemistry, Faculty of Medicine and Dentistry, School of Translational Medicine,  University of Alberta, 4-19B Medical Sciences Building, Edmonton, Alberta,  Canada T6G 2H7.
authors.forename: Sandra E
authors.initials: SE
authors.lastname: Pineda-Sanabria
authors.forename: Ian M
authors.initials: IM
authors.lastname: Robertson
authors.forename: Monica X
authors.initials: MX
authors.lastname: Li
authors.forename: Brian D
authors.initials: BD
authors.lastname: Sykes
mesh_terms: D000138:Acidosis / Q000503:physiopathology / Q000517:prevention & control*; D000595:Amino Acid Sequence; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D017202:Myocardial Ischemia / Q000503:physiopathology; D009206:Myocardium / Q000378:metabolism*; D054730:Protein Interaction Domains and Motifs / Q000502:physiology*; D019209:Troponin C / Q000378:metabolism*; D019210:Troponin I / Q000378:metabolism*
chemical_list: D019209:Troponin C; D019210:Troponin I