Resolution of the aerobic respiratory system of the thermoacidophilic archaeon, Sulfolobus sp, strain 7 .2. Characterization of the archaeal terminal oxidase subcomplexes and implication for the intramolecular electron transfer

The terminal segment of the aerobic respiratory chain of the thermoacidophilic archaeon Sulfolobus sp. strain 7 is an unusual caldariellaquinol oxidase supercomplex, which contains at least one b-type and three spectroscopically distinguishable a-type cytochromes, one copper, and a Rieske-type FeS center. In this paper, we report the purification and characterization of two different forms of the archaeal a-type cytochromes, namely, a three-subunit cytochrome a(583)-aa(3) subcomplex and a single-subunit cytochrome aa(3) derived from the cytochrome subcomplex, in order to facilitate fur ther studies on the terminal oxidase segment of Sulfolobus. The optical and EPR spectroscopic analyses suggest the presence of two different low-spin heme centers and one high-spin heme center in the purified cytochrome a(583)-aa(3) subcomplex, and one low-spin and one high-spin hemes in cytochrome aa(3), respectively. The Rieske-type FeS center detected in the purified cytochrome supercomplex was absent in two forms of the a-type cytochrome oxidase, indicating its association with cytochrome b(562). The crystal field parameters of the low-spin heme a(583) center indicate that its axial ligands may be similar to those of cytochromes c, rather than conventional bis-histidine ligation, In spite of the absence of any c-type cytochrome, a ferrocytochrome c oxidase activity was detected in the archaeal purified cytochrome a(583)-aa(3) subcomplex with no quinol oxidase activity, but not in the purified cytochrome oxidase supercomplex, which has been tentatively interpreted as a representative of electron transfer from the Rieske FeS center to cytochrome a(583) in vivo. Thus, our results indicate the following scheme for the intramolecular electron transfer of the terminal oxidase supercomplex from Sulfolobus sp. strain 7: [caldariellaquinol -->] b(562) --> Rieske FeS center --> a(583) aa(3) --> molecular oxygen.