The purified SoxABCD quinol oxidase complex of Sulfolobus acidocaldarius contains a novel haem

Summary A respiratory quinol oxidase complex that is encoded by the soxABCD operon has been purified from the thermoacidophilic archaeon Sulfolobus acidocaldarius. The enzyme was solubilized with dodecyl maltoside and purified in the presence of this detergent and ethylene glycol. The complex is hydro‐dynamically homogeneous and contains at least five different polypeptides. In addition to the major subunits SoxA, SoxB and SoxC, it has two small polypeptides. One of these is the translation product of a short open reading frame (now called the soxD gene) at the end of the operon. The optical and electron paramagnetic resonance spectra of the SoxABCD compiex have been characterized. It probably contains four A‐type haems which are bound to SoxB and SoxC. The structure of these haems is not identical to haem A. The novel haem Aa has a 2‐hydroxyethyl geranylgeranyl in position 2 of the porphyrin ring whereas haem A has the related farnesyl‐containing side‐chain.