CHARACTERIZATION OF A SOLUBLE CYTOCHROME C(4) ISOLATED FROM THIOBACILLUS-FERROOXIDANS

A soluble c-type cytochrome was purified to homogeneity from Thiobacil lus ferrooxidans. This cytochrome is characterised by an alpha-peak wa velength of 552 nm, a molecular mass of 21 193 Da (as determined by ma ss spectroscopy), and a pi value of 9. N-terminal sequencing yielded t he polypeptide sequence up to the 50th residue. The iron content of 1. 9 Fe/molecule and the heme/molecule ratio of 2.15 identified this cyto chrome as a diheme protein. Optical redox titrations at pH 3.0 reveale d the presence of two distinguishable redox species with E(m) = 385 mV +/- 20 mV and E(m) = 480 mV +/- 20 mV. EPR spectra recorded on this h eme protein showed the presence of two distinct spectral species with g(z) = 3.1 and g(z) = 3.35. The g(z) = 3.35 heme corresponds to the hi gher potential redox species. In line with the differences in E,values , the two heme species were oxidised by O-2 with significantly differi ng half-times. All the above mentioned properties demonstrate that thi s heme protein belongs to the c(4) family of diheme cytochromes. The c haracteristics and functional role of the studied heme protein are dis cussed with reference to other c-type cytochromes described in Thiobac illi. Its properties are furthermore compared to other members of the cytochrome c(4) family.