Redox Components of Cytochrome bc-type Enzymes in Acidophilic Prokaryotes

Abstract

The redox components of the cytochrome bc1 complex from the acidophilic chemolithotrophic organism Thiobacillus ferrooxidans were investigated by potentiometric and spectroscopic techniques. Optical redox titrations demonstrated the presence of two b-type hemes with differing redox midpoint potentials at pH 7.4 (-169 and + 20 mV for bL and bH, respectively). At pH 3.5, by contrast, both hemes appeared to titrate at about +20 mV. Antimycin A, 2-heptyl-4-hydroxyquinoline N-oxide, and stigmatellin induced distinguishable shifts of the b hemes' alpha-bands, providing evidence for the binding of antimycin A and 2-heptyl-4-hydroxyquinoline N-oxide near heme bH (located on the cytosolic side of the membrane) and of stigmatellin near heme bL (located on the periplasmic side of the membrane). The inhibitors stigmatellin, 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole, and 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone affected the EPR spectrum of the Rieske iron-sulfur center in a way that differs from what has been observed for cytochrome bc1 or b6f complexes. The results obtained demonstrate that the T. ferrooxidans complex, although showing most of the features characteristic for bc1 complexes, contains unique properties that are most probably related to the chemolithotrophicity and/or acidophilicity of its parent organism. A speculative model for reverse electron transfer through the T. ferrooxidans complex is proposed.