Protein dynamics play a key but poorly understood role in enzymatic mechanisms. The development of new methods to identify protein structural changes that facilitate chemical reactions will have a transformative effect on our understanding of biological catalysis. Photosynthetic reaction centers provide an example of biological reactions in which protein dynamics play an important role and in which the reactions are light-inducible. Therefore, the photosynthetic reactions serve as a model system in which kinetic mechanism can be investigated using laser flashes to synchronize and start reactions. To identify protein structural changes important in catalysis, novel, time-resolved spectroscopic studies of the oxygen-evolving complex (OEC) in photosystem II (PSII) will be conducted. PSII consists both of integral, membrane-spanning subunits and of extrinsic subunits. The OEC contains a tetranuclear manganese (Mn) cluster and accumulates the four photon-derived oxidizing equivalents necessary for oxygen production from water. The sequentially oxidized forms of the catalytic site are called the Sn states, where n refers to the number of oxidizing equivalents stored. Chloride plays an important but not completely understood role in the S state cycle. Despite decades of study, many aspects of the water oxidation mechanism remain to be elucidated. The intellectual merit of these activities is that new fundamental understandings of the role of protein dynamics in catalysis will result. The photosynthetic oxygen-evolving reactions are responsible for the maintenance of aerobic life on earth and thus are of intrinsic importance in biological chemistry. These reactions also serve as a model of other enzymatic reactions involving molecular oxygen.
BROADER IMPACTS: The broader impact of these activities results from the advancement of teaching, training, and learning, as well as from broadening the scientific participation of underrepresented groups. The principal investigator has had the privilege of training individuals from underrepresented groups, including both graduate and undergraduate students. Several of the principal investigator's former students and postdoctoral associates have gone on to teach at undergraduate or Ph.D.-granting institutions. The principal investigator has collaborations with individuals who teach at non-Ph.D. granting institutions, and these individuals are co-authors with the principal investigator on publications, which have been broadly disseminated to the scientific community. In addition, the principal investigator is program director for a new Georgia Tech Molecular Biophysics training program, which will enhance the infrastructure for research and training at Georgia Tech. The principal investigator teaches at both the undergraduate and graduate levels at Georgia Tech, and she incorporates research and literature-based exercises in these courses. Further, the principal investigator has been involved in outreach to the community through science fair activities. Plans for the next funding cycle include broadening graduate and undergraduate student involvement in the Molecular Biophysics research program by outreach to historically black institutions in the Southeast and Atlanta area. Also, the principal investigator will recruit summer students through the NSF REU program in the summer and through Georgia Tech research opportunities during the academic year.
Solar power has the capacity to meet the world's increasing needs for energy. In plants and algae, natural photosynthesis provides a model of the efficient conversion of light into transportable forms of chemical energy. Water is used as a source of electrons; the released byproduct, oxygen, is necessary to sustain life on earth. The processs is initiated by photoexcitation of chlorophyll; this photoexcitation causes a charge separation across a membrane. Transmembrane electron transfer is mediated stepwise by a chain of electron acceptors and leads to the conservation of solar energy. The water-oxidizing reaction is catalyzed in the photosynthetic reaction center protein, called photosystem II, by a metal containing, manganese-calcium, cluster and a redox-active tyrosine residue of the protein. The oxygen-evolving cycle involves a coordinated process, in which negatively charged electrons and positively charged protons are removed from water at the metal center. A critical feature of natural photosynthesis is the use of sunlight to oxidize water. Artificial systems, which use water as an electron source, would provide affordable and sustainable fuel production. To understand photosynthetic water oxidation chemistry, it is necessary to decipher the precise coordination of light-driven proton and electron transfer reactions. Precise coordination is necessary to promote the reaction and to avoid potentially damaging or off-pathway intermediates. This NSF funded research project gave new insight into the chemical steps necessary to carry out the water oxidizing reaction. In particular, information concerning the roles of internal water, of the essential calcium, and of the redox-active tyrosine was acquired. Although light is necessary to drive the production of oxygen, prolonged illumination of photosystem II damages the reaction center. This process, called photoinhibition, consists of a complex damage and repair cycle. In this NSF funded project, new insights into the chemical modifications and structural changes, which occur during photoinhibition, were obtained. The intellectual merit of this project is a new understanding of photosynthetic water oxidation, which will inform the design of bio-inspired solar energy convertors. These new design strategies will help to address the pressing global need for inexpensive and renewable sources of energy. The broader impact of these activities results from the advancement of teaching, training, and learning and from broadening the participation of underrepresented groups in science.
