Two C_4 type cytochromes in Thiobacillus ferrooxidans: structural comparison and functional role

摘要

The bioenergetic metabolism of the ore-leaching acidophilic proteobacterium Thiobacillus ferrooxidans involves an electron transfer chain coupling oxidation of iron (Ⅱ) at the external cell wall surface to reduction of the terminal electron acceptor, molecular oxygen. Most of the metalloproteins involved in the periplasmic respiratory chain are acid-stable proteins exhibiting very high redox potentials. We have isolated and characterized two dihemic C_4-type cytochromes, cytochrome C_4 [Mr 21000] and cytochrome C_4 [Mr 26000]. These cytochromes present 34.5% amino acid identity, differ by molecular mass, EPR spectra, and redox potential. The physiological properties of both periplasmic cytochromes present in the same bacterium are compared. Both cytochromes interact with a blue copper protein: the rusticyanin. Cytochrome C_4 [Mr 26000], is found to be reduced by Fe~(2+) whereas no reduction of cytochrome C_4 [Mr 21000] could be observed at the same Fe~(2+) concentrations. We suggest different functional roles for these two cytochromes: cytochrome C_4 [Mr 26000] would be the primary cellular oxidant of ferrous iron. The electron are then transferred to rusticyanin, subsequently passed on to cytochrome C_4 [Mr 21000] and eventually to cytochrome oxidase. Thiobacillus ferrooxidans is the only organism reported so far that harbours two different representatives of this family of diheme cytochromes. This raises the question of the structural and functional characteristics favoring the presence of C_4-type diheme over the use of standard monoheme cytochromes. The diheme structure could be involved in the stabilization of the structure at extremely acid pH. The elucidation of the three-dimensional structure of cytochrome C_4 [Mr 21000] actually in progress may help to answer this question.