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CO2 fixation is a thermodynamically and kinetically challenging process, but nature has its own way of transforming CO2 into diverse organic molecules. Of our particular interest is 2-oxoacid:ferredoxin oxidoreductase (OFOR) that catalyzes the anaerobic, reversible inter-conversion of 2-oxoacids and CO2, making use of a small electron-transfer protein, ferredoxin (Fd), as the redox partner. This dissertation characterizes OFORs and Fds from organisms that exhibit different metabolic patterns and investigates how the interplay of OFOR and Fd could impact the fate of CO2 metabolism, asking the question What controls the catalytic bias of OFOR for CO2 evolution versus fixation? The study of OFORs and Fds from Desulfovibrio africanus and Hydrogenobacter thermophilus through an electrocatalytic assay reveals that the reduction potential of Fd is possibly associated with the biological function of OFOR and that CO2 fixation requires a low-potential electron donor. The Fd from H. thermophilus (HtFd1) is used as a model to probe the factors that govern iron-sulfur cluster potential. The dependence of OFOR activity on Fd potential is systematically studied with HtFd1 and its molecular variants through the electrocatalytic assay and a coupled enzyme assay. The results suggest there is a Fd “potential optimum” for OFOR-catalyzed CO2 fixation. The study of a 2-oxoglutarate:ferredoxin oxidoreductase (OGOR) and three Fds from Magnetococcus marinus MC-1 further highlights other factors such as the intramolecular electron-transfer within Fd and the electrostatic and hydrophobic interactions at the protein-protein interface in determining OFOR-Fd interaction. The characterization of an OGOR from M. marinus MC-1 (MmOGOR) also provides kinetic, structural and spectroscopic details for a CO2-fixing OFOR that contains only one iron-sulfur cluster. Overall, this work furthers the scientific understanding of how nature achieves CO2 fixation through supplying reducing equivalents and with enzymes as efficient catalysts, and how intermolecular electron-transfer mediated by protein-protein interaction could regulate enzyme catalysis. / 2019-10-08T00:00:00Z