Evidence for a cysteine-mediated mechanism of excitation energy regulation in the FMO antenna complex

Robert E. Blankenshipa,b,c, Gregory S. Orfa,b,c, Rafael G. Saerb,c, Dariusz M. Niedzwiedzkic, Hao Zhanga,c, Chelsea L. McIntoshb, Jason W. Schultza, Liviu M. Miricaa
Departments of aChemistry and bBiology, Washington University in St. Louis, St. Louis, MO 63130; cPhotosynthetic Antenna Research Center (PARC), Washington University in St. Louis, St. Louis, MO 63130

Light-harvesting antenna complexes not only aid in the capture of solar energy for photosynthesis, but regulate the quantity of transferred energy as well. Light-harvesting regulation is important for protecting reaction center complexes from over-excitation, generation of reactive oxygen species, and metabolic overload. Usually, this regulation is controlled by association of light-harvesting antennas with accessory quenchers such as carotenoids. One antenna complex, the FMO antenna protein from green sulfur bacteria, completely lacks carotenoids and other known accessory quenchers. Despite this, the FMO protein is able to effectively quench energy transfer in aerobic conditions, indicating a new type of regulatory mechanism.

Through de novo sequencing mass spectrometry, chemical modification, and mutagenesis, we have pinpointed the source of the quenching action to cysteine residues (Cys49 and Cys353) situated near two low-energy bacteriochlorophylls in the FMO protein from Chlorobaculum tepidum. Removal of these cysteines (particularly the completely conserved Cys353) through N-ethylmaleimide modification or mutagenesis to alanine abolishes the aerobic quenching effect. Electrochemical analysis and electron paramagnetic resonance spectra suggest that the cysteine thiols are converted to thiyl radicals in aerobic conditions, which are then capable of quenching bacteriochlorophyll excited states through electron transfer photochemistry. This simple, novel mechanism has implications for the design of bio-inspired light-harvesting antennas and re-design of natural photosynthetic systems.