A ba3 oxygen reductase from the thermohalophilic bacterium Rhodothermus marinus

The aerobic respiratory chain of the thermohalophilic bacterium Rhodothermus marinus has been extensively studied. In this study the isolation and characterization of a third oxygen reductase expressed in this organism are described. This newly isolated enzyme is a typical member of the type B family of haem-copper oxygen reductases, showing 43% amino acid sequence identity and 63% similarity with the ba3 oxygen reductase from Thermus thermophilus. It constitutes two subunits with apparent molecular masses of 42 and 38 kDa. It contains a low-spin B-type haem and a high-spin A-type haem. A stoichiometry of 1B: 1A haem per protein was obtained by spectral integration of UV-visible spectra. Metal analysis showed the presence of two iron and three copper ions, which is in agreement with the existence of a CuA centre. Taking advantage of having two spectroscopically distinct haems, the redox behaviour of the ba3 oxygen reductase was analysed and discussed in the framework of a model with interacting centres. Both haems, B and A, present two transitions, have unusually low reduction potentials of -65 mV and an interaction potential of -52.5 mV.