Mapping of Electron Tunneling Pathways in Proteins

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Project Summary Drug metabolism, programmed cell death, DNA biosynthesis and repair, respiration, and photosynthesis all occur via electron-transfer (ET) reaction mechanisms. As such, the malfunction of ET pathways is an underlying cause of numerous human diseases. Since ET is a process common to all forms of life, a molecular-level understanding of the ET pathways in pathogenic organisms may be exploited for therapeutic advantage as well. The long-term objective of our research is to understand, at the molecular level, how biological structure and dynamics control the rates of critical ET reactions. Theoretical progress from our laboratory over two decades has found that protein structure and dynamics determine ET reaction mechanisms and rates, and we have established useful methods to predict these rates. We have also come to understand that structured water can play an essential role in mediating electron flow. In the last grant period, we found that very long- range multi-step electron hopping establishes ET pathways in some anaerobic bacteria. The specific aims of this proposal are: (1) to understand the mechanism that couples long-distance electron transfer, in the proximity of a water filled protein cleft, to substrate catalysis in monooxygenases, and (2) to understand a new mechanism for micron to centimeter distance charge flow in bacterial appendages. The former reactions are essential for the processing of hormones and neurotransmitters in higher eukaryotes and the latter reactions are essential for metabolism in some anaerobic bacteria. In the case of monooxygenase function, we will use theoretical methods to understand the origins of the very effective "coupling" of the ET reaction to catalysis, namely the close synchronization of electron delivery over long distances to the chemistry of covalent bond formation. In the case of long distance ET along bacterial charge transfer conduits, also known as bacterial nanowires, we will employ a new theoretical framework to describe "flickering resonant" transport, a mixed incoherent-coherent regime of ET, and will assess viable mechanisms in these fascinating micrometer-to-centimeter distance regime ET reactions. Reaching our aim of establishing molecular- level predictive theories of biological ET could lead to new strategies to address the malfunction of electron transfer pathways, to understand the origins of oxidative stress, and to disrupt the essential redox chains of pathogenic organisms. The research described here will be carried out using a combination of approaches from statistical mechanics and quantum mechanics. The hallmark of our research has been the close collaboration of our theoretical research program with cutting-edge experimental groups, and this essential strategy will continue into the next research cycle.