SPECTROSCOPIC CHARACTERIZATION OF FLAVOCYTOCHROME C-552 FROM THE PHOTOSYNTHETIC PURPLE SULFUR BACTERIUM CHROMATIUM-VINOSUM

The identities of the axial ligands to the two hemes of the flavocytoc hrome c-552 isolated from the photosynthetic purple sulfur bacterium C hromatium vinosum have been investigated by visible/near-infrared abso rption and magnetic circular dichroism (MCD) spectroscopies, with para llel electron paramagnetic resonance (EPR) studies. One of the hemes h as histidine and methionine as axial ligands and has a local environme nt that is relatively insensitive to the composition of the bulk mediu m. The second heme, the local environment of which is sensitive to cha nges in the composition of the bulk medium, exists as a mixture of two forms, only one of which has histidine/methionine axial ligation. On the basis of its EPR characteristics, the other form most likely has h istidine/lysine axial ligation. In aqueous solution near neutral pH, m ore than half of the second heme is present as the histidine/lysine fo rm, while in 50:50 water/ethylene glycol the histidine/methionine form is the dominant one.