The Fenna-Matthews-Olson (FMO) Complex of Green Sulfur Bacteria
The FMO complex of green sulphur bacteria is another fascinating complex; here the fine-tuning of pigments by surrounding protein plays a more important role for the EET than in B800 and B850. The different site energies of the excitonically coupled BChl in the FMO lead to a stable pathway from the outer antenna complex to the RC. The FMO complex as shown in Figure 27 is described in more detail in e.g. (Adolphs and Renger, 2006; Muh et al, 2007; Wen et al., 2011).
The local protein surrounding of the BChl molecules in the FMO complex (see Figure 27C) modulates their energetic states. In Figure 27C only seven BChl molecules are shown while findings gathered from mass spectroscopic analysis unraveled the existence of an eight BChl (Tronrud et al., 2009; Wen et al., 2011). Fine-tuning the electronic states of the BChls leads to a localization of the lowest energetic states near the chlorophylls number 3 and 4, which are in close contact to the RC (see Figure 27C). Excited electronic states populated at pigments 1 and 6 are rapidly transferred within 5 ps to the pigments number 3 and 4 (Adolphs and Renger, 2006). The pathway of this EET is determined by pigment- protein coupling rather than pigment-pigment coupling. The directed
Figure 27. Schematic view of the FMO complex found in green sulfur bacteria (A). The organisation of the trimeric protein structure (green, blue) containing eight BChl molecules (red) per subunit is shown in panel (B). In (Muh et al., 2007) it was assumed that the FMO complex contains only seven BChl per subunit. New findings disclosed the existence of an eighth BChl [Tronrud et al., 2009, Wen et al., 2011]. The detailed structural arrangement of the chlorophylls is presented in panel (C). This figure was published in (Muh et al., 2007). Image reproduced with permission.
EET that is guided along a certain pathway is necessarily correlated with the existence of a pigment array within an asymmetric protein structure.