Antenna Complexes in Photosynthetic Systems
The enhancement of the absorption cross section of photosystems is only one function of photosynthetic antenna complexes. They additionally have regulatory functions, such as the dissipation of excess energy in order to avoid the generation of singlet oxygen at high light conditions (see Hader, 1999 for an overview). The structural arrangement of the pigments together with the fine tuning of the Chl molecules by the environment cause a directed energy transfer from the antenna to the photochemically active reaction center (RC). The absorbed light energy is able to bridge distances of 30 nm to the RC with up to 99% quantum efficiency (Hader, 1999). In contrast to the common architecture of the RC, the antenna structures are found to vary in different photosynthetic organisms. The exact pigment compositions of the antennae are not completely determined for a certain species but depend on growth conditions, light intensity and light quality during growth. The latter effect is known as chromatic adaption. Therefore the following subsections present a short outline of some selected antenna systems of anoxygenic bacteria, oxygenic bacteria and higher plants.
It is interesting to consider why the structure of the RC is similar for most photosynthetic organisms while the structures of the antenna complexes vary. There might be several reasons for this discrepancy. It seems most probable that the variations in light spectra and intensity that the antenna complexes are exposed to are the main reason for the difference between structures found in the antennae. The antenna is the protein structure that mainly interacts with sunlight. The RC itself is more accurately understood as the acceptor of excited states of Chl molecules. While all plants that use Chl as light-harvesting pigment in close proximity to the RC are facing the same task to drive water splitting with the free energy of the excited Chl, the task of light-harvesting in different environments is a less clearly defined task and one that is carried out under varying conditions. It seems reasonable that an evolutionary development of the optimized structures forms different structures under different conditions while the same conditions lead to the same structure.
Anoxygenic photosynthetic bacteria contain bacteriochlorophyll (BChl) in the antenna complexes and reaction centers. BChl a is found in e.g. the light-harvesting antennae (LH1, LH2) of purple bacteria as presented in chap. 2.4.2 and the Fenna-Matthews-Olson (FMO) complex of green sulphur bacteria (chap. 2.4.3). In contrast to anoxygenic bacteria the oxygenic photosynthetic organisms contain two separated photosystems (PS I and PS II). Both PS have own reaction centers and antenna complexes. The major membrane-extrinsic, light-harvesting antenna of cyanobacteria is phycocyanobilin, which contains protein structures that are mostly associated with PS II but also found to undergo so-called “state transitions” between PS II and PS I. The PBPs organize in different forms: as huge phycobilisomes in most cyanobacteria (chap. 2.4.4), or as minor rod shaped PBP antennae in the cyanobacterium A.marina (chap. 2.4.5). Cyanobacteria contain several additional PCB complexes (recently also named “chlorophyll binding proteins” CBP) surrounding the reaction center and core proteins (see chap. 2.3 and Bibby et al., 2003; Chen et al., 2005). Higher plants contain the trimeric major light-harvesting complex II (LHCII), which is located inside the thylakoid membrane (see Figure 18 and chap. 2.4.1) and present in ratios of four or even more trimers per PS II core dimer (Hader, 1999; Lambrev et al., 2011). The LHCII is one of the most prevalent proteins found on earth.