Photosynthetic reaction centre
The reaction center is the central component of all organisms that carry out photosynthesis. In any case it is a protein complex consisting of several subunits. In it, the absorbed light energy leads to a charge separation. Thus resulting electrons, which have a much higher redox potential, are then derived from the reaction center and used by many intermediate steps for the production of hydrocarbon compounds. The reaction center has hardly changed in the course of evolution, whereas, have adopted different kinds of shapes, the light -harvesting complexes that surround it. In more modern organisms, ie plants, algae and cyanobacteria, the reaction center is included in a larger complex, the photosystems. In bacteria can be isolated, the reaction center. The following section describes the function of the reaction center is exemplified in the reaction center of the purple bacterium Rhodopseudomonas viridis.
Operation
The reaction center of purple bacteria is anchored in the plasma membrane complex of three protein chains and various pigments and is of an annular light -harvesting complex ( LH1 ) surrounded. The three subunits are designated by the letters L, M and H, the latter projecting from the membrane. The nearly symmetrically designed L- and M- units are located in the membrane.
Both the L-and M- units each have two bacteriochlorophylls, also a phaeophytin and a ubiquinone. The ubiquinone molecule of the M unit can detach from the complex. The bacteriochlorophyll a molecule of L- and M- unit are close together and form the so-called special pair, which hosts the charge separation.
One of the two molecules of the bacteriochlorophyll special pair is excited usually by indirectly receiving the energy from the antenna pigments and transferred to the phaeophytin molecule of the L moiety. After 200.10 -12 seconds the tightly bound ubiquinone molecule accepts the electron. 200.10 -6 seconds, the other electron of the ubiquinone molecule of the M unit is transferred, that, after it has obtained in the same manner, a second electron to leave the reaction site, and transports the electrons to the cytochrome bc1 complex.
The missing electron of the special pair is filled by a cytochrome c, which binds to the reaction center, again.
The purpose of the H unit that has no pigments, it is unclear to this day (2006).
For the study of the reaction center of the purple bacterium Rhodopsuedomonas viridis, which operates anoxygenic photosynthesis, Hartmut Michel, Johann Deisenhofer and Robert Huber received the Nobel Prize for Chemistry.