Redox properties of cytochrome c.
ABSTRACT The redox properties of cytochromes (cyt) c, a ubiquitous class of heme-containing electron transport proteins, have been extensively investigated over the last two decades. The reduction potential (E degrees') is central to the chemistry of cyt c for two main reasons. First, E degrees' influences both the thermodynamic and kinetic aspects of the electron exchange reaction with redox partners. Second, this thermodynamic parameter is remarkably sensitive to changes in the properties of the heme and the protein matrix, and hence can be profitably used for the investigation of the solution chemistry of cyt c. This research area owes much to the exploitation of voltammetric techniques for the determination of E degrees' for metalloproteins, which dates back to the late 1970s. Since then, much effort has been devoted to the comprehension of the molecular factors that control E degrees' in cyt c, which include first coordination sphere effects on the heme iron, the interactions of the heme group with the surrounding polypeptide chain and the solvent, and also include medium effects related to the nature and ionic composition of the solvent, pH, the presence of potential protein ligands, and the temperature. This article provides an overview of the most significant advances made in this field recently.
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ABSTRACT: Six-coordinated heme groups are involved in a large variety of electron transfer reactions because of their ability to exist in both the ferrous (Fe2+) and ferric (Fe3+) state without any large differences in structure. Our studies on hemes coordinated by two histidines (bis-His) and hemes coordinated by histidine and methionine (His-Met) will be reviewed. In both of these coordination environments, the heme core can exhibit ferric low spin (electron paramagnetic resonance EPR) signals with large gmax values (also called Type I, highly anisotropic low spin, or highly axial low spin, HALS species) as well as rhombic EPR (Type II) signals. In bis-His coordinated hemes rhombic and HALS envelopes are related to the orientation of the His groups with respect to each other such that (i) parallel His planes results in a rhombic signal and (ii) perpendicular His planes results in a HALS signal. Correlation between the structure of the heme and its ligands for heme with His-Met axial ligation and ligand-field parameters, as derived from a large series of cytochrome c variants, show, however, that for such a combination of axial ligands there is no clear-cut difference between the large gmax and the “small g-anisotropy” cases as a result of the relative Met-His arrangements. Nonetheless, a new linear correlation links the average shift 〈δ〉 of the heme methyl groups with the gmax values. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 1064–1082, 2009.This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at email@example.comBiopolymers 11/2009; 91(12):1064 - 1082. · 2.29 Impact Factor
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ABSTRACT: Molecular dynamics (MD) simulation combined with inelastic neutron scattering can provide information about the thermal dynamics of proteins, especially the low-frequency vibrational modes responsible for large movement of some parts of protein molecules. We performed several 30-ns MD simulations of cytochrome c (Cyt c) in a water box for temperatures ranging from 110 to 300 K and compared the results with those from experimental inelastic neutron scattering. The low-frequency vibrational modes were obtained via dynamic structure factors, S(Q, ω), obtained both from inelastic neutron scattering experiments and calculated from MD simulations for Cyt c in the same range of temperatures. The well known thermal transition in structural movements of Cyt c is clearly seen in MD simulations; it is, however, confined to unstructured fragments of loops Ω(1) and Ω(2); movement of structured loop Ω(3) and both helical ends of the protein is resistant to thermal disturbance. Calculated and experimental S(Q, ω) plots are in qualitative agreement for low temperatures whereas above 200 K a boson peak vanishes from the calculated plots. This may be a result of loss of crystal structure by the protein-water system compared with the protein crystal.Biophysics of Structure and Mechanism 12/2012; · 2.44 Impact Factor
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ABSTRACT: Background: Cytochrome c (cyt c) is well known for its role in mitochondrial electron transport. The redox state of cyt c has recently been linked to apoptosis. In the present study, we investigated and compared the kinetics of cyt c reduction by various thiol antioxidants. Methods: All kinetic experiments were performed by measuring spectrophotometrically the changes in absorbance at 550 nm for 2 min at 25 o C in a reaction buffer (50 mM Tris HCl, pH 7.4) containing cyt c (5 μM) and thiols (100 μM). Electrostatic effects on the rate of reduction of cyt c were determined by varying the ionic strength of the reaction buffer. The kinetic data were treated by applying a pseudo fi rst-order approximation. Results: The results show that, among all thiols studied, cysteine reduces cyt c at the highest rate. The second-order rate constant is on the order of 10 2 M -1 s -1 for cellular thiol reductants, GSH and cysteine. Most thiols of therapeutic importance exhibit extremely low reduction rates. Moreover, the rate of cyt c reduction was found to correlate negatively with the pK a of the SH group. This study further demonstrates that a complex electrostatic interaction may occur between cyt c and thiols. Conclusions: Given the high abundance of GSH in cells, our kinetic data suggest that GSH may out-compete other re-ductants for cyt c; and is therefore an important mediator of the cellular redox state of cyt c.