Abstract
Biological macromolecules involved in electron transfer reactions display chains of closely packed redox cofactors when long distances must be bridged. This is a consequence of the need to maintain a rate of transfer compatible with metabolic activity in the framework of the exponential decay of electron tunneling with distance. In this work intermolecular electron transfer was studied in kinetic experiments performed with the small tetraheme cytochrome from Shewanella oneidensis MR-1 and from Shewanella frigidimarina NCIMB400 using non-physiological redox partners. This choice allowed the effect of specific recognition and docking to be eliminated from the measured rates. The results were analyzed with a kinetic model that uses the extensive thermodynamic characterization of these proteins reported in the literature to discriminate the kinetic contribution of each heme to the overall rate of electron transfer. This analysis shows that, in this redox chain that spans 23 A, the kinetic properties of the individual hemes establish a functional specificity for each redox center. This functional specificity combined with the thermodynamic properties of these soluble proteins ensures directional electron flow within the cytochrome even outside of the context of a functioning respiratory chain.
Highlights
The anaerobic respiratory flexibility found in these bacteria is associated with the presence of numerous multiheme cytochromes [4]
Functional aspects of the redox activity of these proteins remain unexplored: do the intrinsic redox properties of the hemes lead to functional specificity as proposed on the basis of the thermodynamic characterization, or will this specificity arise only upon interaction with physiological partners? To answer this question, kinetic studies of reduction and oxidation of SoSTC and SfSTC were performed using a non-physiological electron donor and acceptor to avoid physiological bias that may result from specific recognition and docking of partners
Kinetic and thermodynamic information are essential to understand the molecular details of the electron transfer mechanisms in redox proteins
Summary
In this work intermolecular electron transfer was studied in kinetic experiments performed with the small tetraheme cytochrome from Shewanella oneidensis MR-1 and from Shewanella frigidimarina NCIMB400 using non-physiological redox partners This choice allowed the effect of specific recognition and docking to be eliminated from the measured rates. SoSTC was determined for the reduced and oxidized states by x-ray crystallography [7], and recently the solution structure was solved for the reduced state of SfSTC [8] These studies showed that, despite the diversity found in the amino acid sequence of these cytochromes (64% identical), the architecture of their heme core is highly conserved with the hemes organized in a chain spanning 23 Å for the most distant heme irons [7, 8]. Functional aspects of the redox activity of these proteins remain unexplored: do the intrinsic redox properties of the hemes lead to functional specificity as proposed on the basis of the thermodynamic characterization, or will this specificity arise only upon interaction with physiological partners? To answer this question, kinetic studies of reduction and oxidation of SoSTC and SfSTC were performed using a non-physiological electron donor and acceptor to avoid physiological bias that may result from specific recognition and docking of partners
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