Abstract
Electron transfer between redox active proteins and mineral oxides is important in a variety of natural as well as technological processes, including electron transfer from dissimilatory metal-reducing bacteria to minerals. One of the pathways that could trigger electron transfer between proteins and minerals is redox-linked conformation change. We present electrochemical evidence that mitochondrial cytochrome c (Mcc) undergoes significant conformation change upon interaction with hematite and indium-tin oxide (ITO) surfaces. The apparent adsorption-induced conformation change causes the protein to become more reducing, which makes it able to transfer electrons to the hematite conduction band. Although Mcc is not a protein thought to be involved in interaction with mineral surfaces, it shares (or can be conformed so as to share) some characteristics with multiheme outer-membrane cytochromes thought to be involved in the transfer of electrons from dissimilatory iron-reducing bacteria to ferric minerals during respiration. We present evidence that a 10.1 kDa monohoeme cytochrome isolated and purified from Acidiphilium cryptum, with properties similar to those of Mcc, also undergoes conformation change as a result of interaction with hematite surfaces.
Highlights
Globular proteins such as mitochondrial cytochrome c (Mcc) are polypeptide chains folded such that hydrophobic amino acid residues are positioned in the interior of the molecule and the hydrophilic residues coat the exterior
The initial Cyclic voltammetry (CV) work is in agreement with earlier work using oxide electrodes in suggesting that Mcc retains native conformation when interacting with oxides, subsequent repeat experiments sometimes showed no redox peaks, introducing a disquieting lack of consistency from experiment to experiment
A mixture of reduced and oxidized Mcc, when injected into an electrochemical cell containing a hematite electrode held at À200 mV vs. Ag/AgCl results in anodic current on the first injection, but not on a subsequent injection
Summary
Globular proteins such as mitochondrial cytochrome c (Mcc) are polypeptide chains folded such that hydrophobic amino acid residues are positioned in the interior of the molecule and the hydrophilic residues coat the exterior. The function of redox proteins is often to act as energy transducers in which conformational change is transformed into electrochemical energy An example of this is found in cytochrome cd nitrite reductase (Williams et al, 1997), in which ligand-switching by the d-heme of the protein is mechanically linked to ligand-switching by the c-heme so as to control the redox state of the c-heme and the timing of electron transfer from the c-heme to the d-heme during the catalytic cycle. Far from necessarily representing ‘‘denaturation’’, conformation change can be key to protein operation; the physiological role of Mcc is to shuttle electrons from the mitochondrial bc complex to cytochrome c oxidase (e.g., Lange and Hunte, 2002), and there is evidence that interaction between Mcc and its physiological partners triggers conformational change that results in changed redox properties (Witt et al, 1998; Cruciat et al, 2000; Dick et al, 2000). It is reasonable to expect that interaction of such a protein with a mineral particle may trigger conformation change with redox consequences, and the phenomenon of redox-linked conformation change is the subject of interest here
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