Executive Summary : | The computational modeling of electron transfer processes in natural photosynthetic systems is one of the fundamental research endeavors. Heliobacteria, similar to the other phototrophic systems, make use of the photosynthetic reaction center (RC) to convert electrical energy into useful chemical energy. The RC of heliobacteria (HbRC) is of particular interest as several recent experimental and structural studies showed that it may be a key factor in understanding the approx. 3 billion years old evolution of photosynthetic systems into type I and type II RCs. In HbRC, a type I RC, the incoming light is absorbed by antenna pigments and the resulting excitation energy is transferred to the primary electron acceptor. Further, electron transfer takes place via a series of cofactors to a terminal acceptor, an iron-sulfur complex (Fx). Recent experimental studies showed the presence of a menaquinone molecule in the vicinity of the primary electron acceptor and speculated it to act as an alternative electron acceptor molecule, indicating that HbRC behaves like a type II RC. Therefore, HbRC may represent a functional intermediate between type I and type II RCs and an atomic-level understanding of the associated electron transfer processes will be crucial in that regard. Our goal is to map the light-driven electron transfer (ET) pathway in HbRC, in particular the role of quinone. It is here that insights from computational chemistry will be essential, as the structure does not reveal an obvious mechanism beyond providing a predicted location of the menaquinone. Computational approaches are necessary, in addition to the experiments, to learn how the electron transfer process takes place as experiment alone cannot resolve all the relevant details. First, we will study the electron transfer from the primary electron acceptor, a bacteriochlorophyll molecule, to the terminal electron acceptor, Fx. Here, classical molecular dynamics (MD) followed by ab-initio quantum mechanical (QM) will be performed to map the role of the protein environment in the electron transfer and compute the associated energetics and kinetics. Next, we will investigate the much-debated reduction pathway of menaquinone. Two pathways will be considered here differing in the sequence of electron and proton transfer. The structural arrangement of the involved cofactors and protein residues will be evaluated using hybrid quantum mechanical / molecular mechanical (QM/MM) method. These calculations will allow us to identify the rate-limited steps as well as possible amino-acid mutations to enhance the rate of ET. Completion of the objectives in the proposed work will begin to illuminate the electron and proton transfer processes in HbRC, which will shed light on how nature has optimized the photosynthetic systems to use solar energy with incredible efficiency and may help constitute the prototype of a nature-inspired photosynthesis center. |