Protein Dynamics in Electron Transfer

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This project integrates molecular dynamic simulation and experimental measurement to develop a dynamic model for photosynthetic electron transfer in reaction centers of the purple nonsulfur bacterium, Rhodobacter sphaeroides. Current simulations on this system have suggested that the involvement of slow, collective protein modes in mediating these reactions is key, and as a result, the amount of reorganization energy available during a reaction is profoundly dependant on the time scale of the reaction, particularly as one moves from picoseconds to nanoseconds. Here, a theoretical group and an experimental group have joined forces to explore how proteins dynamics is involved to balance two competing electron transfer reactions that are critical to solar energy conversion in photosynthesis. The first is a productive electron transfer reaction that takes place on the 200 ps timescale from a bacteriopheophytin intermediate to a quinone. The second is the competing unproductive recombination reaction in which the electron returns back to the original electron donor on the 10-20 ns time scale. Simulations and measurements will be performed as a function of the driving force and temperature for these two reactions and compared directly, the goal being to understand how Nature has optimized protein structure and dynamics to promote productive photosynthetic function. A series of surface charge mutants and different pH's will be investigated, testing the concept that that the charge distribution at the protein/water interface controls the shape of the initial and final state potential surfaces, as suggested by simulation. In addition, internal mutations, thought to alter the displacement between potential surfaces, will also be investigated. The ultimate objective is to provide the capability to quantitatively predict biochemical reaction dynamics and associated function from protein structure in this complex biological system. The development of a quantitative, structure-based model of the relationship between structure, dynamics and function in the photosynthetic reaction center would be broadly valuable. The reaction center in this respect represents something of a "lab on a molecule" with high resolution structural data and the inherent optical probes for monitoring electron transfer in considerable detail. The project will support the development of a complete, participatory learning experience for the students in the Mesa High School Biotechnology Academy, based on genetic manipulation and spectroscopic probing of photosynthetic bacteria. This will involve both teacher training in the ASU laboratories during one summer and then PIs working directly with the teachers and students in the academy to perform the activity during the subsequent years. The PIs are very heavily involved in undergraduate mentoring having had 25 undergraduates in the lab during the last 5 years.