With this award, the Chemistry of Life Processes Program in the Chemistry Division is funding Ekaterina Pletneva from Dartmouth College to investigate mechanisms of reactions that take place in proteins and involve transfer of both protons and electrons. Proton-coupled electron-transfer (PCET) reactions are central to a wide variety of biological processes and also underlie the action of many chemical systems for energy conversion in small-molecule catalysis, electrochemical devices, and solar cells. The specific assignment of sites with protons in proteins is challenging, especially when time dependent information is necessary. Using unique site modifications and protein reengineering, Dr. Pletneva and her students will develop and test new approaches for tracking protons in reactions and will explore the effects of the protein in controlling these processes. They will detect the presence of protons at specific protein sites. The project will provide opportunities for multidisciplinary training of students of all levels and for instilling appreciation for quantitative descriptions of biological phenomena. Dr. Pletneva will offer guidance to the volunteers from the Dartmouth Undergraduate Journal of Science (DUJS) in the design of outreach activities at schools in the Upper Valley. She, her students, and DUJS staff will also work together to discuss biological reactions and strategies for artificial photosynthesis with local citizens.
Propionate groups at the periphery of the heme cofactor are sites of proton transfer in PCET reactions of a variety of enzymes, electron carriers, and redox complexes. Despite their prominent role in biological redox mechanisms, heme propionates (HPs) have received little attention and their reactivity is not well understood. Using three systems of progressively increasing complexity (a heme peptide, a monoheme protein cytochrome (cyt) c2, and a tetraheme protein cyt c3), this project is establishing the factors set by the biological environment to control the interplay between proton transfer (PT) and electron transfer (ET) in HP-mediated PCET reactions. The effects of the biologically relevant solvent, protein scaffold, neighboring amino acids, and multiple redox centers on the PCET reaction landscape are being examined. With special emphasis on methods to monitor elusive PT steps, thorough characterization of the reaction mechanisms, the studies are addressing major challenges in the PCET field. Site-specific labeling techniques are enabling determination of redox-dependent pKa values of HPs by NMR and explicit observation of both ET and PT steps in photoinduced studies with electronic absorption and vibrational spectroscopic methods. Isothermal titration calorimetry is being used as a novel label-free approach to identify redox-dependent changes in HP protonation, examine a possibility of intraprotein proton relay, and evaluate reactions with buffers. The detailed studies of less understood HP-mediated reactions is expanding the mechanistic database of PCET reactions, which is important for the development of predictive tools. These studies are advancing the understanding of ubiquitous heme proteins and are yielding valuable tools and tactics for mechanistic studies of other redox centers, both biological and man-made.
