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Electrochemistry of Unfolded Cytochrome c in Neutral and Acidic Urea Solutions.
- Source :
-
Journal of the American Chemical Society . 5/25/2005, Vol. 127 Issue 20, p7638-7646. 9p. - Publication Year :
- 2005
-
Abstract
- The present investigation reports the first experimental measurements of the reorganization energy of unfolded metalloprotein in urea solution. Horse heart cytochrome c (cyt c) has been found to undergo reversible one-electron transfer reactions at pH 2 in the presence of 9 M urea. In contrast, the protein is electrochemically inactive at pH 2 under low-ionic strength conditions in the absence of urea. Urea is shown to induce ligation changes at the heme iron and lead to practically complete loss of the a-helical content of the protein. Despite being unfolded, the electron-transfer (El) kinetics of cyt c on a 2-mercaptoethanol- modified Ag(111) electrode remain unusually fast and diffusion controlled. Acid titration of ferric cyt c in 9 M urea down to pH 2 is accompanied by protonation of one of the axial ligands, water binding to the heme iron (pka = 5.2), and a sudden protein collapse (pH < 4). The formal redox potential of the urea-unfolded six-coordinate His18-Fe(II)-H2O/five-coordinate His18-Fe(ll) couple at pH 2 is estimated to be -0.083 V vs NHE, about 130 mV more positive than seen for bis-His-ligated urea-denatured cyt c at pH 7. The unusually fast ET kinetics are assigned to low reorganization energy of acid/urea-unfolded cyt c at pH 2 (0.41 ± 0.01 eV), which is actually lower than that of the native cyt c at pH 7 (0.6 ± 0.02 eV), but closer to that of native bis-His-ligated cyt b5 (0.44 ± 0.02 eV). The roles of electronic coupling and heme-flattening on the rate of heterogeneous ET reactions are discussed. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 00027863
- Volume :
- 127
- Issue :
- 20
- Database :
- Academic Search Index
- Journal :
- Journal of the American Chemical Society
- Publication Type :
- Academic Journal
- Accession number :
- 17180962
- Full Text :
- https://doi.org/10.1021/ja050321g