RESOLUTION OF THE AEROBIC RESPIRATORY SYSTEM OF THE THERMOACIDOPHILICARCHAEON, SULFOLOBUS SP, STRAIN-7 .2. CHARACTERIZATION OF THE ARCHAEAL TERMINAL OXIDASE SUBCOMPLEXES AND IMPLICATION FOR THE INTRAMOLECULARELECTRON-TRANSFER

The terminal segment of the aerobic respiratory chain of the thermoaci dophilic archaeon Sulfolobus sp. strain 7 is an unusual caldariellaqui nol 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 a nd characterization of two different forms of the archaeal a-type cyto chromes, namely, a three-subunit cytochrome a(583)-aa(3) subcomplex an d a single-subunit cytochrome aa(3) derived from the cytochrome subcom plex, in order to facilitate fur ther studies on the terminal oxidase segment of Sulfolobus. The optical and EPR spectroscopic analyses sugg est the presence of two different low-spin heme centers and one high-s pin heme center in the purified cytochrome a(583)-aa(3) subcomplex, an d one low-spin and one high-spin hemes in cytochrome aa(3), respective ly. The Rieske-type FeS center detected in the purified cytochrome sup ercomplex was absent in two forms of the a-type cytochrome oxidase, in dicating its association with cytochrome b(562). The crystal field par ameters of the low-spin heme a(583) center indicate that its axial lig ands may be similar to those of cytochromes c, rather than conventiona l bis-histidine ligation, In spite of the absence of any c-type cytoch rome, a ferrocytochrome c oxidase activity was detected in the archaea l purified cytochrome a(583)-aa(3) subcomplex with no quinol oxidase a ctivity, but not in the purified cytochrome oxidase supercomplex, whic h has been tentatively interpreted as a representative of electron tra nsfer from the Rieske FeS center to cytochrome a(583) in vivo. Thus, o ur results indicate the following scheme for the intramolecular electr on transfer of the terminal oxidase supercomplex from Sulfolobus sp. s train 7: [caldariellaquinol -->] b(562) --> Rieske FeS center --> a(58 3) aa(3) --> molecular oxygen.