Structural relaxation and nonexponential kinetics of CO-binding to horse myoglobin. Multiple flash photolysis experiments.
Abstract: The geminate recombination kinetics of CO-myoglobin strongly deviates from single exponential behavior in contrast to what is expected for unimolecular reactions (1). At low temperatures, this result was attributed to slowly exchanging conformational states which differ substantially in barrier height for ligand binding. Above 160 K the kinetics apparently slow down with temperature increase. Agmon and Hopfield (2) explain this result in terms of structural relaxation perpendicular to the reaction coordinate, which enhances the activation energy. In their model, structural relaxation homogenizes the kinetic response. Recently, Steinbach et al. (3) proposed a relaxation model which conserves the kinetic inhomogeneity. Below we test these conjectures by single and multiple excitation experiments. This method allows for discrimination between parallel (inhomogeneous) and sequential (homogeneous) kinetic schemes. The kinetic anomaly above 160 K is shown to result from a homogeneous, structurally relaxed intermediate. However a second anomaly is found above 210 K concerning the inhomogeneous phase which may indicate either a shift in activation energy or entropy.
Publication Date: 1993-06-01 PubMed ID: 8369410PubMed Central: PMC1262517DOI: 10.1016/S0006-3495(93)81554-6Google Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The researchers studied the uncommon, nonexponential recombination kinetics of CO-myoglobin, particularly at low temperatures. They discovered this might relate to slowly exchanging conformational states, differing in barrier height for ligand binding. The research tests theories on temperature effect, structural relaxation, and kinetic schemes, showing the existence of a structurally relaxed intermediate and a second, higher temperature anomaly affecting the inhomogeneous phase.
Understanding the Research Subject – CO-Binding to Horse Myoglobin
- Horse myoglobin is a single-chain globular protein of 153 amino acids, the main function of which is to store oxygen in muscle cells, ensuring a steady supply during physical activity. The researchers are particularly interested in how carbon monoxide (CO) binds to it and its ensuing influence on myoglobin kinetics. This kinetic behavior of CO binding to myoglobin ensures an optimal ratio of oxygen in the respiratory cycle.
Findings at Low Temperatures
- The study found that the geminate recombination kinetics of CO-myoglobin is nonexponential, deviating from what is generally expected for unimolecular reactions.
- The study attributes these unusual kinetics to slowly exchanging conformational states. Each of these states seems to have different barrier heights, i.e., energy required for ligand binding.
Description of Different Models
- Agmon and Hopfield offered a model where increased temperature causes structural relaxation, thereby increasing activation energy needed for reactions. This relaxation makes the kinetic behaviour of CO-myoglobin more uniform.
- Contrarily, Steinbach et al. present a model where structural relaxation instead conserves kinetic inhomogeneity — maintaining variation in kinetic behaviour.
Testing the Models and Findings
- The researchers have tested these models by conducting single and multiple excitation experiments which enable differentiation between parallel (inhomogeneous) and sequential (homogeneous) kinetic schemes.
- The research results corroborate the existence of a homogeneous, structurally relaxed intermediate acting upon the anomaly noticed above 160 K (Kelvin). This supports and extends the model proposed by Agmon and Hopfield.
- Interestingly, the research also reveals a second anomaly when the temperature is above 210 K. This anomaly affects the inhomogeneous phase and could denote a shift in either activation energy or entropy, a concept which requires further investigation.
Cite This Article
APA
Post F, Doster W, Karvounis G, Settles M.
(1993).
Structural relaxation and nonexponential kinetics of CO-binding to horse myoglobin. Multiple flash photolysis experiments.
Biophys J, 64(6), 1833-1842.
https://doi.org/10.1016/S0006-3495(93)81554-6 Publication
Researcher Affiliations
- Technische Universität München, Physik-Department E13, Garching, Germany.
MeSH Terms
- Animals
- Horses
- Kinetics
- Mathematics
- Myoglobin / chemistry
- Myoglobin / metabolism
- Photolysis
- Protein Binding
- Thermodynamics
- Whales
References
This article includes 13 references
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Citations
This article has been cited 13 times.- Wang J, Huang W, Lu H, Wang E. Downhill kinetics of biomolecular interface binding: globally connected scenario.. Biophys J 2004 Oct;87(4):2187-94.
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