Some thermodynamic and kinetic properties of the primary photochemical reactants in a complex from a green photosynthetic bacterium.
We have examined the bacteriochlorophyll reaction-center complex of Chlorobium limicola f. thiosulfatophilum, strain Tassajara. Our results indicate that the midpoint potential of the primary electron donor bacteriochlorophyll of the reaction center is +250 mV at pH 6.8, while that of cytochrome c-553 is +165 mV. There are two cytochrome c-553 hemes per reaction center, and the light-induced oxidation of each is biphasic (t1/2 of less than 5 mus and approximately 50 mus). We belive that this indicates a two state equilibrium with each cytochrome heme being either close to, or a little removed from, the reaction-center bacteriochlorophyll. We have also titrated the primary electron acceptor of the reaction center. Its equilibrium midpoint potential at pH 6.8 is below -450 mV. This is very much lower than the previous estimate for green bacteria, and also substantially lower than values obtained for purple bacteria. Such a low-potential primary acceptor would be thermodynamically capable of direct reduction of NAD+ via ferredoxin in a manner analagous to photosystem I in chloroplasts and blue-green algae.
D001419:Bacteria / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000378:metabolism*; D002734:Chlorophyll / Q000031:analogs & derivatives*; D003574:Cytochrome c Group / Q000378:metabolism; D003624:Darkness; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction; D010777:Photochemistry; D010788:Photosynthesis; D013816:Thermodynamics
D001429:Bacteriochlorophylls; D003574:Cytochrome c Group; D002734:Chlorophyll