Characterization of electron-transfer proteins: archaeal disulfide bonds and bacterial multi-heme cytochromes c

Abstract

Electron-transfer proteins that are responsible for redox homeostasis and long. range electron transfer are vital to intracellular and extracellular processes. In this thesis, several examples of electron-transfer proteins are studied in order to determine the emergent properties of multi-electron transfer chemistry. Thioredoxin (Trx) is a small redox-active protein that functions via its disulfide bond. These disulfides, characterized by a CXXC motif, are found to have a range of redox potentials that are often linked to function. Chapter 2 uses a set of archaeal thioredoxins from Thermoplasma acidophilum and Archaeoglobus fulgidus to test the current hypotheses using protein film voltammetry and solution-based experiments that examine folding energies. Thioredoxin reductase (TrxR) functions to provide reducing equivalents to Trx to keep it active in the cell. The TrxR from Thermoplasma acidophilum has been noted to be unusual in that it does not use NADPH as a reductant, as found in most TrxRs. The reaction between T. acidophilum Trx and TrxR is explored in Chapter 3 and a bioinfonnatic analysis of TaTrxR is included in Chapter 4 to better understand its relationship in the TrxR protein family, as well as attempt to identity its native reductant. In Chapter 5, the periplasmic decaheme cytochrome DmsE from Shewanella oneidensis is biochemically characterized. This protein is part of the dimethyl sulfoxide reduction pathway and is compared with MtrA, the well-studied decaheme protein from the dissimilatory metal reduction pathway in Shewanella. Additionally, a Cytoscape analysis of the MtrA/DmsE and OmcA protein families is presented. Finally, Chapter 6 explores the electrochemical properties of two multi-heme proteins from Nitrosomonas europaea: cytochrome c554 and hydroxylamine oxidoreductase (HAO). Cytochrome c554, a tetraheme cytochrome, has been shown to have cooperativity between two of its heme groups and gating has been. observed in protein film voltammetry (PFV) experiments. This gating is further explored in this Chapter. The enzymatic hydroxylamine reduction by HAO, a reverse reaction, is also presented.