Characterization of a quinole-oxidase activity in crude extracts of Thermoplasma acidophilum and isolation of an 18-kDa cytochrome.

A quinol-oxidase activity was detected in crude extracts of the thermoacidophilic archaebacterium Thermoplasma acidophilum. The activity was optimal at pH 5.4 and 50 degrees C. The Km for ubiquinol-10 was 18 microM. The enzyme was inhibited by 2n-heptyl-4-hydroxyquinoline N-oxide with a Ki of 150 nM. Ubiquinols with different side-chain lengths were oxidized at similar rates, whereas menaquinols were converted at 6-12-fold higher rates compared to ubiquinols. Membranes from T. acidophilum contain cytochromes of b, d and a1 types, as shown by optical spectroscopy. CO difference spectroscopy suggests the existence of a cytochrome o. A b-type cytochrome with an apparent molecular mass of 18 kDa was purified in the presence of high detergent concentrations. It readily forms dimers on SDS/PAGE. This cytochrome also contains Cu, as shown by atomic-absorption spectroscopy. Redox titration suggests that the 18-kDa cytochrome may contain 2 heme groups; b558 with a midpoint potential of 75 mV and b562/553 with a midpoint potential of -150 mV.