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Variable-temperature study of the heme-reorientation process in equine myoglobin
1Department of Chemistry, Portland State University, OR 97207-0751, USA.
Biochimica Et Biophysica Acta
|October 25, 1995
Summary
This study compares nuclear magnetic resonance and optical spectroscopy to investigate heme reorientation in myoglobin (Mb). Findings suggest heme reorientation in Mb is likely non-dissociative, with an activation energy close to dissociation/recombination mechanisms.
Area of Science:
- Biochemistry
- Biophysics
- Spectroscopy
Background:
- Myoglobin (Mb) heme-insertion isomers redistribute from a 50/50 to a 90/10 ratio.
- This redistribution was previously assumed to involve heme iron-protein bond rupture.
Purpose of the Study:
- Compare nuclear magnetic resonance (NMR) and optical spectroscopy for studying heme reorientation in myoglobin.
- Determine the mechanism and kinetics of heme reorientation within myoglobin.
Main Methods:
- Utilized nuclear magnetic resonance (NMR) spectroscopy.
- Employed optical spectroscopy for kinetic determinations.
- Performed variable-temperature analysis on horse myoglobin at pH 8.4.
Main Results:
- Optical spectroscopy kinetics for heme isomer redistribution in Mb are quantitatively consistent with NMR results.
- Activation energy parameters (delta H++, delta S++, delta G++) were determined using optical spectroscopy.
- The measured activation energy for heme reorientation is close to the estimated lower bound for a complete dissociation/recombination mechanism.
Conclusions:
- Heme reorientation in myoglobin (Mb) is likely a non-dissociative process.
- The activation energy is comparable to, though slightly lower than, that expected for complete heme dissociation and recombination.
- The measured entropy of activation may be influenced by significant solvent contributions.