The electrochemical characterization of the redox properties of FeS clusters in metalloenzymes: investigating the AdoMet radical enzyme superfamily and carbon dioxide reducing enzymes
Walker, Lindsey M.
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FeS clusters play a multitude of roles in metalloenzymes from transporting electrons, binding substrates, and acting as a sulfur source along with many others. Importantly, each FeS cluster has its own redox properties which help impart the role it plays in enzyme function. Therefore, by characterizing FeS cluster containing enzymes through the lens of their reduction potentials, more can be understood about the function of the clusters. This thesis addresses the question of how the redox properties of FeS clusters provide clues to enzymatic role. The radical S-adenosylmethionine enzyme (ARE) superfamily is extremely diverse in function, structure, and cofactor content. The SPASM/Twitch domain containing subclass of AREs houses either one of two FeS clusters in addition to the canonical AdoMet binding [4Fe-4S] cluster. This work electrochemically characterizes the SPASM AREs Tte1186, anSMEcpe, MftC, and PqqE along with the Twitch domain enzymes MoaA and BtrN to identify what role the auxiliary cluster(s) are playing within the enzyme. Working with a broad scope of AREs in this subclass shows that the role the auxiliary clusters play within the SPASM/Twitch domain is as diverse as the chemistry they each perform with roles ranging from simply transferring electrons in Tte1186, MftC, and anSMEcpe, tobinding substrate and providing enzymatic efficiency in MoaA, to being both anelectron acceptor for substrate formation as well as an electron donor for AdoMet cleavage in PqqE. This work explores how redox properties can be used to propose FeS cluster function over AREs with SPASM/Twitch domains. Additionally, FeS clusters act as redox wires in the CO2 fixation enzymes pyruvate:ferredoxin oxidoreductase from Chlorobium tepidum and formate dehydrogenase from Ralstonia eutropha. Characterization of both of these enzymes in terms of their redox properties adds another layer of fundamental knowledge to how they transfer electrons and proposes possibilities for how manipulating reduction potentials can change catalytic bias to favor CO2 reduction. Through electrochemical characterization of FeS clusters in both AREs and CO2 fixing enzymes, we identify how these cofactors play an essential role in enzyme function.