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Variable-temperature study of the heme-reorientation process in equine myoglobin.
Abstract: The redistribution of the initially-formed myoglobin heme-insertion isomers from the initially formed 50/50 mixture to the equilibrium ratio of 90/10 has long been assumed to occur by one of two mechanisms, both of which require the rupture of the heme iron-protein bond (La Mar, G.N., Toi, H. and Krishnamoorthi, K. (1984) J. Am. Chem. Soc. 106, 6395-6401). In this study we compared the use of nuclear magnetic resonance and optical spectroscopic techniques as methods for studying the reorientation of heme within myoglobin. We found that kinetics determinations of the heme insertion isomer redistribution process in Mb by optical spectroscopy are quantitatively compatible with the results obtained by nuclear magnetic resonance spectroscopy. A variable-temperature analysis for horse myoglobin using the optical method at pH 8.4 +/- 0.1 yielded the following activation energy parameters: delta H++ = 31 kcal/mol, delta S++ = 34 cal/mol per K, and delta G++21 degrees C = 21 kcal/mol. The value of delta G++ expected for complete dissociation of the heme from myoglobin can be estimated, from its dissociation constant and insertion rate, to be on the order of 23-27 kcal/mol under the same conditions as our determination. Therefore, although the mechanism for heme reorientation in Mb is likely non-dissociative, it has an activation energy which is not far from the lower bound expected for a complete-dissociation/recombination mechanism. Our measured entropy of activation is not especially large, perhaps owing to a large contribution by the solvent.
Publication Date: 1995-10-25 PubMed ID: 7578236DOI: 10.1016/0167-4838(95)00115-bGoogle Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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The study investigates how the structure of proteins in horse myoglobin changes over time and under different conditions. It reveals that the process does not require complete dissociation of its components, contrary to previous beliefs.
Research Objective and Background
The researchers aimed to understand a specific process in the protein myoglobin: the redistribution of initially-formed myoglobin heme-insertion isomers. Equine myoglobin (the myoglobin found in horses) was particularly explored here. For a long time, scientists believed that this redistribution process could only happen through two mechanisms, both requiring the break of the bond between heme iron and protein. The study aimed to investigate this commonly held view further.
Methods Used
- The researchers employed two different techniques—nuclear magnetic resonance (NMR) and optical spectroscopic techniques—to study the restructuring of heme within myoglobin.
- The research compared the results produced by these two methods regarding the redistribution process of heme insertion isomers in equine myoglobin.
Findings
- Both the spectroscopy and NMR techniques provided compatible results, indicating that they can reliably be used to study the heme reorientation process.
- The study showed that the activation energy parameters resulting from a variable-temperature analysis (using the optical method) at pH 8.4 +/- 0.1 were 31 kcal/mol for ΔH++, 34 cal/mol per K for ΔS++, and 21 kcal/mol for ΔG++ at 21 degrees C.
- The expected value of ΔG++ for complete dissociation of the heme from myoglobin was calculated to be between 23 and 27 kcal/mol, based on its dissociation constant and insertion rate.
Implications
By comparing the activation energy parameters and the estimated value for dissociation, the researchers found that they are close. This means the heme reorientation mechanism might occur without requiring total dissociation. Furthermore, the entropy of activation was measured to be not very high, which can be attributed to a massive contribution by the solvent. This new revelation could change the current understanding of myoglobin behavior and impact further scientific research in protein study.
Cite This Article
APA
Yee S, Peyton DH.
(1995).
Variable-temperature study of the heme-reorientation process in equine myoglobin.
Biochim Biophys Acta, 1252(2), 295-299.
https://doi.org/10.1016/0167-4838(95)00115-b Publication
Researcher Affiliations
- Department of Chemistry, Portland State University, OR 97207-0751, USA.
MeSH Terms
- Animals
- Heme / chemistry
- Horses
- Magnetic Resonance Spectroscopy
- Mathematics
- Molecular Structure
- Myoglobin / chemistry
- Spectrophotometry
- Temperature
Citations
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