The Redding group works on the assembly, function, and degradation of integral membrane proteins involved in energy transduction. As a model system, we are using Photosystem 1 (PS1), a multi-subunit membrane protein complex that uses the energy of absorbed photons to promote transmembrane electron transfer. The core of PS1 is a heterodimer of two homologous, integral membrane polypeptides, which form a framework holding the cofactors involved in electron transport.
1) Structure/function studies: One of the most interesting questions in biochemistry is "How does the protein environment affect the properties of bound molecules?" The phylloquinone cofactor embedded in PS1 is an excellent example. It is much more reducing when bound in PS1 than when isolated in organic solvent. Thus, the protein is able to "tune" the properties of the quinone so that it functions as a good intermediate in electron transfer. We are using site-directed mutagenesis to change amino acid residues that interact with the phylloquinone, and thus change its properties. Characterization of the mutants involves use of advanced techniques, such as electron paramagnetic resonance and kinetic spectroscopy.
2) Engineering electron transfer: The symmetric structure of PS1 includes two possible pathways of electron transfer. By changing amino acids around one quinone or the other, we have shown that both pathways can be used. We are altering the two quinone sites to see how the differences between them translate into different electron transfer rates. We hope to alter the sites enough to allow binding of alternate target molecules, which may lead to light-powered biomolecular devices capable of reductively destroying environmental pollutants, etc. We are also trying to see if we can control which pathway the electrons take by modify the environment near the electron donors.
3) Degradation of membrane proteins: Biological systems target aberrant membrane complexes for destruction. Although some human diseases are caused by this process, the systems that recognize and degrade aberrant membrane proteins remain largely unknown. In order to identify these in the chloroplast of green plants, a two-pronged attack is being used: a genetic approach to screen for mutants defective in degradation of PS1, and a biochemical approach to characterize and purify the proteins involved in the degradation process.
4) Electron transfer processes in Heliobacteria: Our newest project involves engineering of the most primitive photosynthetic organism currently known. They use a homodimeric reaction center that is superficially similar to PS1 in several ways. The genome of Heliobacterium modesticaldum, the only thermophilic organism in this group, was recently determined in a collaboration between TGen and ASU (http://genomes.tgen.org/). We have recently developed a transformation system for this organism, and are using it to delete key proteins involved in photosynthetic electron transfer and biosynthesis of cofactors. Long-term goals include: gaining insight into the evolution of asymmetric photosynthetic reaction centers, assessing alternative roles for quinones in this group of organisms, and optimizing their production of hydrogen.
