Abstract
The Fenna-Matthews-Olson (FMO) pigment-protein complex in green sulfur bacteria transfers excitation energy from the chlorosome antenna complex to the reaction center. In understanding energy transfer in the FMO protein, the individual contributions of the bacteriochlorophyll pigments to the FMO complex's absorption spectrum could provide detailed information with which molecular and energetic models can be constructed. The absorption properties of the pigments, however, are such that their spectra overlap significantly. To overcome this, we used site-directed mutagenesis to construct a series of mutant FMO complexes in the model green sulfur bacterium Chlorobaculum tepidum (formerly Chlorobium tepidum). Two cysteines at positions 49 and 353 in the C. tepidum FMO complex, which reside near hydrogen bonds between BChls 2 and 3, and their amino acid binding partner serine 73 and tyrosine 15, respectively, were changed to alanine residues. The resulting C49A, C353A, and C49A C353A double mutants were analyzed with a combination of optical absorption and circular dichroism (CD) spectroscopies. Our results revealed changes in the absorption properties of several underlying spectral components in the FMO complex, as well as the redox behavior of the complex in response to the reductant sodium dithionite. A high-resolution X-ray structure of the C49A C353A double mutant reveals that these spectral changes appear to be independent of any major structural rearrangements in the FMO mutants. Our findings provide important tests for theoretical calculations of the C. tepidum FMO absorption spectrum, and additionally highlight a possible role for cysteine residues in the redox activity of the pigment-protein complex.
Original language | English |
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Pages (from-to) | 1455-1463 |
Number of pages | 9 |
Journal | Biochimica et Biophysica Acta - Bioenergetics |
Volume | 1857 |
Issue number | 9 |
DOIs | |
State | Published - Sep 1 2016 |
Funding
This work was supported as part of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC 0001035 . X-ray data were collected at Southeast Regional Collaborative Access Team (SER-CAT) 22-ID (or 22-BM) beamline at the Advanced Photon Source, Argonne National Laboratory. Supporting institutions may be found at www.ser-cat.org/members.html . Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. We are also grateful for the assistance with data collation from Macromolecular X-ray Crystallography Laboratory, Department of Molecular and Structural Biochemistry, North Carolina State University.
Keywords
- Bacteriochlorophyll
- Energy transfer
- Exciton
- Fenna-Matthews-Olson
- Photosynthesis
- Spectroscopy