Bacterial Reaction Centers with New Photochemical Properties

Arizona State University, Scottsdale AZ

Investigators

Abstract

In this project, modifications of the photosynthetic reaction center from the purple bacterium, Rhodobacter sphaeroides, will be designed to incorporate original oxidation reactions involving the primary electron donor, a bacteriochlorophyll dimer. This strategy has already been shown to be successful in elevating the oxidizing capacity of the reaction center, enabling the oxidation of nearby tyrosine amino acid residues. A variety of techniques such as vibrational, magnetic resonance, and transient optical spectroscopy will be employed in the characterization of the modified reaction centers. Specific goals include: (1) To understand the role of proton transfer in the generation of amino acid radicals. Recently developed hydrogen abstraction models have proposed a critical role for protons in the function of tyrosyl radicals. The involvement of specific protons will be identified by performing measurements on mutants with alterations of possible proton-accepting amino acid residues. (2) To incorporate binding sites for manganese compounds that can be oxidized by the reaction center. Water oxidation in photosystem II takes place at a manganese cluster that remains a challenging aspect of photosynthesis to understand at the molecular level. The ability of the highly oxidizing reaction centers to oxidize manganese at increasingly more complex levels will be characterized. (3) To increase the yield of electron transfer in the highly oxidizing reaction centers by replacement of bacteriochlorophyll with chlorophyll in the dimer. The yield of the modified reaction centers is presently restricted because the bacteriochlorophyll dimer uses relatively low energy near infrared radiation for excitation. This limitation will be overcome by incorporation of chlorophyll, which absorbs higher energy visible radiation, into this site. In all photosynthetic systems, the primary conversion of light into chemical energy occurs in pigment-protein complexes. By coupling the primary process in these complexes to secondary events, the photosynthetic apparatus is capable of creating energy-rich compounds. Although the specific pathways vary among organisms, the general pattern of energy conversion is remarkably conserved, and an understanding of this process serves as the basis for current designs of artificial systems that mimic the photosynthesis. The expansion of the capabilities of the reaction center by the design of reactions involving amino acid radicals, metal complexes, and alternate pigments will not only provide a novel strategy for studying reactions that are crucial in photosynthesis and other biological processes, but also will lead to the development of a well-controlled system for oxidation of specific substrates.