The Analysis of Fenna-Matthews-Olson Protein Spectral Components Through Site-directed Mutagenesis

Rafael G. Saera, Gregory S. Orfb, Dariusz M. Niedwiedzkia, Hao Zhanga, Robert E. Blankenshipa
aDepartments of Chemistry, Biology and the Photosynthetic Antenna Research Center (PARC), Washington University in St. Louis, St. Louis, Missouri 63130, USA; bSchool of Molecular Sciences, Arizona State University, 1151 S Forest Ave, Tempe, Arizona 85281, USA

he Fenna-Matthews-Olson (FMO) pigment-protein complex has long been a biological model system for energy transfer, and more recently, for studies in quantum coherences. This protein behaves as molecular wire in green sulfur bacterial light harvesting, acting as a bridge between the large chlorosome antenna complexes, and a Fe-S-type (photosystem I-like) reaction center. FMO proteins contain eight molecules of bacteriochlorophyll a surrounded by a water-soluble protein matrix. Knowledge on the details of energy transfer in FMO has been advanced through a combination of new spectroscopic methods and advances in computational modelling. However, assigning the contributions of each pigment to the absorption spectrum remains a challenging, but important task. To this end, our laboratory developed a system for generating site-directed mutations in the FMO protein, allowing us to perturb regions of the protein in the vicinity of individual pigments. As a result, the optical contributions of each pigment can be analyzed through simple optical absorption shifts. Our results demonstrate that many optical components of the FMO absorption spectrum can be changed via the perturbation of a single pigment. Furthermore, the mutant samples are compatible with all optical techniques used to study the native protein, enabling a more detailed analysis of biological energy transfer in this model system.