The structure of the FMO protein from Chlorobium tepidum at 2.2 Å resolution

The bacteriochlorophyll protein, or FMO protein, from Chlorobium tepidum, which serves as a light-harvesting complex and directs light energy from the chlorosomes attached to the cell membrane to the reaction center has been crystallized in a new space group. The crystals belong to the cubic space group P4₃32 and the structure has been refined to a resolution 2.2 Å with a R factor of 19.7%. The electron density maps show that the structure is composed of two β sheets that surround seven bacteriochlorophylls as previously reported (Li et al. (1997) J Mol Biol 271: 456-471). The availability of the new data allows a more accurate refinement of the pigment-protein complex including identification of bound solvent molecules. Several structural differences probably contribute to the observed spectroscopic differences between the FMO proteins from Cb. tepidum and Prosthecochloris aestuarii, including differences in the planarity of corresponding tetrapyrroles. A citrate molecule is found on the surface of each protein subunit of the trimer from Cb. tepidum. However, the citrate molecule is over 15 Å from any bacteriochlorophyll. The presence of the citrate probably does not contribute to the function of the protein although it does contribute to the crystallization as it interacts with a crystallographically related trimer. Among the 236 water molecules found in the protein are four that appear to play a special role in the properties of bacteriochlorophyll 2, as this tetrapyrrole is coordinated by one of these water molecules and the waters form a hydrogen-bonded network that leads to the surface of the protein.