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Role of Redox-Active Relay Residues in Protein Electron Transfer

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Abstract

Protein electron transfer reactions are essential in biological and bioenergetic systems. They are capable of mobilizing charges over vast distances through the use of relay residues, which behave as intermediary sites for electron localization and hopping. The electron transfer rate is primarily determined by driving force, reorganization energies, and electronic coupling between the donor and acceptor species, as modeled by the Marcus equation. However, these parameters are not readily obtained by in vitro experiments and the reaction may be further complicated by proton transfer events. Thus, systems amenable to facile manipulation of these parameters are highly advantageous for in vitro studies. Tyrosine and tryptophan are among the most prevalent relay residues in electron transfer systems, but despite their similarities, one can not easily replace the functionality of the other. In our first protein model system, yeast cytochrome c peroxidase:cytochrome c, we substituted the conserved tryptophan electron hopping site with a tyrosine residue, allowing us to gain insight into the behavior of tyrosyl radicals in electron transfer and the effect of its local environment on activity. Our findings evince that inclusion of a basic side chain that coordinates to the phenolic proton is essential for augmenting electron transfer rates through the hopping site. Other relay amino acids also play significant roles in proteins, including methionine and cysteine residues. In our second protein model, we use a flavoprotein photosensor (VIVID) variant that is devoid of an active-site cysteine residue essential for generating the canonical light-oxygen-voltage domain flavin signaling state. We demonstrated that this protein variant is surprisingly capable of in vivo signaling and light-dependent conformational changes. From spectroscopic investigations, we identified the elusive electron donors responsible for the formation of this signaling state and show the importance of methionine residues in transferring electrons. Herein, I have disseminated our findings on these two projects, furthering our understanding of the intricacies of protein electron transfer and the necessary properties of relay residues for facilitating electron transfer reactions. Firstly, in Chapter 2 and Appendix A, I discuss our findings on intermolecular electron transfer through a tyrosyl hopping site in the cytochrome c peroxidase complex and recovery of activity through manipulation of the tyrosyl environment. Following this, I relate investigations of a photoactive model system comprised of internal electron donors. In Chapter 3, I evaluate the ability of the active-cysteine-less VIVID variant to induce a biologically relevant signaling response in blue light, and in Chapter 4, I describe continued investigations on the intrinsic electron transfer mechanism involved in photoactivation, with some burgeoning discoveries on an analogous photoactive system presented in Appendix B.

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Date Issued

2018-08-30

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Keywords

LOV domains; Marcus theory; protein electron transfer; Biophysics; crystallography; Physical chemistry; Biochemistry; Spectroscopy; cytochrome c peroxidase

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Crane, Brian

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Gruner, Sol Michael
Petersen, Poul B.

Degree Discipline

Chemistry and Chemical Biology

Degree Name

Ph. D., Chemistry and Chemical Biology

Degree Level

Doctor of Philosophy

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Government Document

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Attribution-NonCommercial-NoDerivatives 4.0 International

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dissertation or thesis

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