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Abstract

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 P4(3)32 and the structure has been refined to a resolution 2.2 A with a R factor of 19.7%. The electron density maps show that the structure is composed of two beta 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 A 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.

Robustness, efficiency, and optimality in the Fenna-Matthews-Olson photosynthetic pigment-protein complex

“…2 and 3 demonstrate that FMO is strongly robust to disruption of the Bchl network, but what about the role of environmental perturbations? Several GSB species are known to be thermophilic; for example, C. tepidum is found in hot springs at temperatures of up to 52 • C. 39 However, the crystal structures of the FMO complex derived from thermophilic and non-thermophilic GSB are very similar, 15,36,43 suggesting that the FMO complex is sufficiently robust to changes in the thermal fluctuations of the protein environment such that no special adaptations are necessary. Here, we investigate this environmental resilience in the light of the inherent heterogeneity of the FMO network observed above.…”

“…15,36,39,43 In particular, we generate 2 × 10 3 alternative eight-site electronic Hamiltonians by adding Gaussian random noise to all elements of the FMO Hamiltonian (see the supplementary material 46 ). We note the possibility that some Hamiltonians generated in this manner may be physically unrealistic; however, in general, this approach allows us to generate Hamiltonians which are representative of FMO complexes with modified interchromophore distances and relative molecular orientations, as well as electronic changes at the Bchl sites themselves.…”

“…The relative positions and orientations of Bchl sites 1 to 7 are remarkably well-conserved across GSB species, 36,43 despite differences in the amino acid sequence in the surrounding FMO protein environment; 15 furthermore, the dominant spectral characteristics of FMO complexes from different species are similarly well-conserved, falling into one of two classes which only differ due to different orientations of Bchl 8. 15 Given the conserved nature of the FMO complex in GSB, it is tempting to ask whether this particular PPC motif is in any way optimised for its EET functionality.…”

“…Specifically, we adopt a network-based view of photosynthetic EET systems and focus on investigating quantum transport dynamics in a prototypical PPC, the FMO complex. 9,[33][34][35][36][37][38] In green sulfur bacteria (GSB) such as C. tepidum, 39 FMO trimers facilitate EET from the chlorosome to the membranebound RC; in this process, each FMO monomer acts as an independent "molecular wiring circuit" connecting the light-harvesting antenna complex (energy source) to the RC (energy sink) ( Fig. 1(a)).…”

61 The Structural Chemistry of Isolated Chlorophylls

“…The orientations of the chlorin rings were confirmed as had been reported earlier, 84 except for one that had to be flipped 180°. The structure of the FMO protein from Chlorobaculum tepidum at 2.2A resolution that was reported in 2003 (PDB ID: 1M50) 85 has recently been re-refined (PDB ID: 3ENI) resulting in an improved R-value of 0.166 (obs.). Each monomer subunit of the trimer envelopes seven Bchls with two sheets, formed by a series of β strands, in what has been termed a "taco shell" arrangement.…”