CYTOCHROMES IN THIOBACILLUS-TEPIDARIUS AND THE RESPIRATORY-CHAIN INVOLVED IN THE OXIDATION OF THIOSULFATE AND TETRATHIONATE

Thiobacillus tepidarius was shown to contain cytochrome(s) c with abso rption maxima at 421, 522 and 552 nm in room temperature reduced minus oxidized difference spectra, present at 1.1-1.2 nmol per mg dry wt an d present in both membrane and soluble fractions of the cell. The memb rane-bound cytochrome c (1.75 nmol per mg membrane protein) had a midp oint potential (Em, pH 7.0) of 337 mV, while the soluble fractions app eared to contain cytochrome(s) c with Em (pH 7.0) values of about 270 and 360 mV. The organism also contained three distinct membrane-bound b-type cytochromes (totalling 0.33 nmol per mg membrane protein), each with absorption maxima in reduced minus oxidized difference spectra a t about 428, 532 and 561 nm. The Em (pH 7.0) values for the three cyto chromes b were 8 mV (47.8% of total), 182 mV (13.7%) and 322 mV (38.5% ). No a- or d-type cytochromes were detectable spectrophotometrically in the intact organism or its membrane and soluble fractions. Evidence is presented for both CO-binding and CO-unreactive cytochromes b or o , and CO-binding cytochrome(s) c. From redox effects observed with CO it is proposed that a cytochrome c donates electrons to a cytochrome b , and that a high potential cytochrome b or o may be acting as the ter minal oxidase in substrate oxidation. This may be the '445 nm pigment' , a photodissociable CO-binding membrane haemoprotein. Substrate oxida tion was relatively insensitive to CO-inhibition, but strongly inhibit ed by cyanide and azide. Thiosulphate oxidation couples directly to cy tochrome c reduction, but tetrathionate oxidation is linked (probably via ubiquinone Q-8) to reduction of a cytochrome b of lower potential than the cytochrome c. The nature of possible electron transport pathw ays in Thiobacillus tepidarius is discussed. One speculative sequence is: e- --> b8 --> b182 --> C270 --> C337 --> b322/c360 --> O2