Spectroscopic characterization of nitrosylheme in nitric oxide complexes of ferric and ferrous cytochrome c′ from photosynthetic bacteria

Abstract

Reactions of ferric and ferrous cytochromes c′ from four photosynthetic bacteria ( Rhodobacter capsulatus ATCC 11166, Rhodopseudomonas palustris ATCC 17001, Rhodospirillum rubrum ATCC 11170, and Chromatium uinosum ATCC 17899) with nitric oxide have been investigated by electronic absorption and electron paramagnetic resonance spectroscopies. The heme iron(III) of these ferric cytochromes c′ has been recently reported to be in a quantum mechanically admixed ( S = sol5 2 , 3 2 ) state [Fujii, S., Yoshimura, T., Kamada, H., Yamaguchi, K., Suzuki, S., Shidara, S. and Takakuwa, S. (1995) Biochim. Biophys. Acta 1251, 161–169]. The affinity of ferric cytochromes c′ for NO among these bacterial species ( C. vinosum > Rps. palustris ∼ Rb. capsulatus ⪢ R. rubrum) was apparently related to the S = 3 2 content in the or der. In the reaction of ferrous cytochrome c′ with NO, six- and five-coordinated nitrosylhemes, which represent species with and without a ligand at the axial position trans to nitrosyl group, have been formed. The content of six-coordinated nitrosylheme in NO-ferrous cytochrome c′ has been determined to be Rb. capsulatus ∼ Rps. palustris > C. vinosum > R rubrum, suggesting that a stability of iron-to-histidine bond decreases with this order. The NO reactions of ferric and ferrous cytochromes c′ from photosynthetic bacteria have been compared with those of cytochromes c′ from denitrifying bacteria.