Electron Transfer and the Roles of Flavins during Iron(III) Reduction by Shewanella Oneidensis

Open Access
Author:
Puls, Brendan William
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 25, 2013
Committee Members:
  • James David Kubicki, Thesis Advisor
Keywords:
  • electron transfer
  • flavins
  • Shewanella oneidensis
Abstract:
The purpose of our investigation is to explore the terminal electron-transfer reaction of Shewanella oneidensis, a dissimilatory metal-reducing bacterium (DMRB), during the anaerobic respiration of Fe3+-oxides. Our study proceeds on two scales: the molecular and the electronic. Our molecular-level work focuses on the binding of flavin mononucleotide (FMN) and riboflavin (RBF), the two flavins secreted by Shewanella, which have been shown to be vital for Fe3+-oxide reduction by Canstein et al. (2008), to Fe3+-oxides and aqueous Fe3+ through the use of flow-adsorption calorimetry, infrared spectroscopy, and Gibbs free-energy and vibrational-frequency calculations. Our electronic-level work focuses on the electron transfer from cytochrome STC, a periplasmic electron-transfer protein of Shewanella, to aqueous Fe3+-EDTA, a proxy system for Fe3+-oxide respiration with well-defined parameters, through the use of Gibbs free-energy and electron-transfer calculations and the results of stopped-flow kinetics experiments previously completed by Qian et al. (2011). The main strategy of our investigation is to model our system computationally, study our system experimentally, and compare the calculated model results to the measured experimental results. By doing this, we can benchmark our computational methods and reveal molecular- and electronic-level details, such as binding structures and electron-transfer rates, that cannot be obtained by experiment alone. Our findings from the first phase of our study are consistent with a functional difference between FMN and RBF as electron shuttles and a possible role of FMN and RBF as chelators during Fe3+ reduction. Our findings from the second phase of our study are consistent with electron transfer from cytochrome to electron shuttle being the rate-limiting step in the overall Fe3+-reduction reaction and reveal important differences between molecular- and electronic-level behavior based on differences between thermodynamic and quantum mechanical processes. Altogether, this investigation is a step towards a comprehensive picture of electron transfer during respiratory microbial Fe3+-oxide reduction.