Abstract
Myocardial ischemia is characterized by reduced blood flow to cardiomyocytes, which can lead to acidosis. Acidosis decreases the calcium sensitivity and contractile efficiency of cardiac muscle. By contrast, skeletal and neonatal muscles are much less sensitive to changes in pH. The pH sensitivity of cardiac muscle can be reduced by replacing cardiac troponin I with its skeletal or neonatal counterparts. The isoform-specific response of troponin I is dictated by a single histidine, which is replaced by an alanine in cardiac troponin I. The decreased pH sensitivity may stem from the protonation of this histidine at low pH, which would promote the formation of electrostatic interactions with negatively charged residues on troponin C. In this study, we measured acid dissociation constants of glutamate residues on troponin C and of histidine on skeletal troponin I (His-130). The results indicate that Glu-19 comes in close contact with an ionizable group that has a pK(a) of ∼6.7 when it is in complex with skeletal troponin I but not when it is bound to cardiac troponin I. The pK(a) of Glu-19 is decreased when troponin C is bound to skeletal troponin I and the pK(a) of His-130 is shifted upward. These results strongly suggest that these residues form an electrostatic interaction. Furthermore, we found that skeletal troponin I bound to troponin C tighter at pH 6.1 than at pH 7.5. The data presented here provide insights into the molecular mechanism for the pH sensitivity of different muscle types.
Highlights
PH sensitivity differences between skeletal and cardiac muscle originate from distinct troponin I isoforms
The results indicate that Glu-19 comes in close contact with an ionizable group that has a pKa of ϳ6.7 when it is in complex with skeletal troponin I but not when it is bound to cardiac troponin I
Two-dimensional 1H,15N HSQC and two-dimensional 1H,13C HCBCGCO NMR spectra were acquired over a range of pH values for four different complexes of Ca2ϩ with the N-terminal domain of cTnC (cNTnC) as follows: cNTnC(apo), cNTnCCa2ϩ,4 cNTnC-cardiac troponin I (cTnI)[147–163], and cNTnC-skeletal isoform of TnI (sTnI)[115–131]
Summary
This alanine of cTnI (Ala-162) was replaced by a histidine, a dramatic reduction in pH sensitivity was observed. Because the closest distances between the imidazole protons of His-130 and the side chain protons of Glu-20 are 5.4 and 5.6 Å (H1 and H2) in the skeletal x-ray structure [16], NOEs between these residues would be too weak to observe If these groups make an electrostatic interaction, knowledge of their acid dissociation constants could provide clues as to whether the same interaction occurs when sTnI is bound to cNTnC. The pH-dependent differences in the affinity of sTnI[115–131] for cNTnC revealed that the ionization state of His-130 fine-tunes the affinity of sTnI[115–131] for cNTnC These results provide structural insights into the mechanism by which a single histidine on troponin I can reverse the decrease in myofilament Ca2ϩ sensitivity during acidosis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
