Molecular diffusion into horse spleen ferritin: a nitroxide radical spin probe study.
Abstract: Electron paramagnetic resonance spectroscopy and gel permeation chromatography were employed to study the molecular diffusion of a number of small nitroxide spin probes (approximately 7-9 A diameter) into the central cavity of the iron-storage protein ferritin. Charge and polarity of these radicals play a critical role in the diffusion process. The negatively charged radical 4-carboxy-2,2,6,6-tetramethylpiperidine-N-oxyl (4-carboxy-TEMPO) does not penetrate the cavity whereas the positively charged 4-amino-TEMPO and 3-(aminomethyl)-proxyl radical and polar 4-hydroxy-TEMPO radical do. Unlike the others, the apolar TEMPO radical does not enter the cavity but instead binds to ferritin, presumably at a hydrophobic region of the protein. The kinetic data indicate that diffusion is not purely passive, the driving force coming not only from the concentration gradient between the inside and outside of the protein but also from charge interactions between the diffusant and the protein. A model for diffusion is derived that describes the observed kinetics. First-order half-lives for diffusion into the protein of 21-26 min are observed, suggesting that reductant molecules with diameters considerably larger than approximately 9 A would probably enter the protein cavity too slowly to mobilize iron efficiently by direct interaction with the mineral core.
Publication Date: 1996-09-01 PubMed ID: 8874032PubMed Central: PMC1233625DOI: 10.1016/S0006-3495(96)79361-XGoogle Scholar: Lookup
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- Journal Article
- Research Support
- U.S. Gov't
- P.H.S.
Summary
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The research examined how small molecules, specifically nitroxide spin probes, diffuse into the core of ferritin, a protein that stores iron in the body. The factors affecting diffusion process, such as charge, polarity, and the nature of diffusant, were studied using sophisticated techniques. Key findings highlight that the diffusion is not wholly passive and the model for diffusion developed from the study could help understand certain biological processes.
Experimental Procedure and Materials
- For analyzing molecular diffusion, the researchers used a combination of Electron Paramagnetic Resonance Spectroscopy and Gel Permeation Chromatography.
- Nitroxide spin probes with specific attributes (7-9 A diameter) were chosen for the study.
- The prime protein under investigation was ferritin, an iron-storage protein.
- Various types of nitroxide spin probes were used, each having different charge and polarity.
Role of Charge and Polarity in Diffusion
- The research found that the charge and polarity of the molecules play a critical role in the diffusion into the ferritin protein.
- For instance, the negatively charged radical 4-carboxy-TEMPO didn’t penetrate the protein cavity.
- In contrast, probes with positive charge and polarity, like 4-amino-TEMPO and 3-(aminomethyl)-proxyl radical, and the polar 4-hydroxy-TEMPO radical, did diffuse into the protein structure.
Distinct Diffusion Behavior of ‘Apolar’ TEMPO Radical
- The study demonstrated that the apolar TEMPO radical behaves differently since it doesn’t penetrate the cavity but instead binds to ferritin protein, potentially on a hydrophobic region.
Finding on Kinetics of Diffusion
- The kinetic data revealed that the diffusion process is not purely passive.
- The driving force for diffusion doesn’t just come from the concentration gradient but also includes the influence of charge interactions between the diffusing molecule and the protein.
Model for Diffusion and Practical Implications
- The researchers derived a diffusion model interpreting the observed kinetics.
- First-order half-lives for diffusion into the protein were observed to be in the range of 21-26 minutes.
- Given these half-lives, it can be inferred that larger molecules, those with diameters significantly larger than 9 A, might not enter the protein cavity quickly enough to efficiently mobilize iron via direct interaction with the mineral core.
Cite This Article
APA
Yang X, Chasteen ND.
(1996).
Molecular diffusion into horse spleen ferritin: a nitroxide radical spin probe study.
Biophys J, 71(3), 1587-1595.
https://doi.org/10.1016/S0006-3495(96)79361-X Publication
Researcher Affiliations
- Department of Chemistry, University of New Hampshire, Durham 03824, USA.
MeSH Terms
- Animals
- Binding Sites
- Biophysical Phenomena
- Biophysics
- Cyclic N-Oxides
- Diffusion
- Electrochemistry
- Electron Spin Resonance Spectroscopy
- Ferritins / chemistry
- Free Radicals
- Horses
- Kinetics
- Nitrogen Oxides
- Spin Labels
- Spleen / chemistry
Grant Funding
- R37 GM20194 / NIGMS NIH HHS
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