Crystal structure of the bacteriochlorophyll a protein from Chlorobium tepidum

The bacteriochlorophyll (BChl) a protein from Chlorobium tepidum, which participates in energy transfer in green photosynthetic bacteria, has been crystallized using the sitting drop method of vapor diffusion. X-ray diffraction data collected from these crystals indicate that the crystals belong to the cubic space group P4₁32 with cell dimensions of a = b = c = 169.5 Å. A native X-ray diffraction data set has been collected to a resolution of 2.2 Å. The initial solution was determined by using the molecular replacement method using the structure of the previously solved BChl a protein from Prosthecochloris aestunarii. A unique rotation and translation solution was obtained fur two monomers in the asymmetric unit giving a pseudo-body centered packing. After rebuilding and refinement the model yields an X factor of 19.0%, a free R-factor of 28.3%, and good geometry with root-mean-square deviations of 0.013 Å and 2.1°for the bond lengths and angles, respectively. The structure of the BChl. a protein from C. tepidum consists of three identical subunits related by a 3-fold axis of crystallographic symmetry. In each subunit the polypeptide backbone forms large β-sheets and encloses a central core of seven BChl a molecules. The distances between neighboring bacteriochlorin systems within a subunit range between 4 Å to 11 Å and that between two bacteriochlorins from different subunits is more than 20 Å. The overall structure is comparable with that of P. aestuarii but significant differences are observed for the individual bacteriochlorophyll structures. The surface of the trimer has a hydrophobic region that is modeled as the complex being a peripheral membrane protein partially embedded in the membrane. A general model is presented for the membrane organization with two of the bacteriochlorophyll structures in the membrane and transferring energy to the reaction center complex. In this model these two bacteriochlorophyll structures serve a similar role to the cofactors of integral membrane light-harvesting complexes although the protein structure surrounding the cofactors is significantly different for the BChl a protein compared with the integral membrane complexes.