Sulfolobus acidocaldarius terminal oxidase. A kinetic investigation and its structural interpretation

The thermoacidophilic archaebacterium Sulfolobus acidocaldarius possesses a very unusual terminal oxidase. We report original kinetic experiments on membranes of this microorganism carried out by stopped flow, using time- resolved optical spectroscopy combined with singular value decomposition analysis. The reduced-oxidized kinetic difference spectrum of the Sulfolobus membranes is characterized by three significant peaks in the visible region at 605, 586, and 560 nm. The 605-nm peak and part of the 586-nm peak (cytochrome aa₃-type quinol oxidase) are reduced synchronously by both ascorbate plus N,N,N',N'-tetramethyl-p-phenylen-diamine (TMPD) and dithionite, and they are very rapidly oxidized by molecular oxygen. A second pool of cytochromes seems to contribute to the 586-nm peak which is not reduced by ascorbate plus TMPD and reacts very slowly with dithionite. The b- type cytochromes (560 nm peak) are reduced by both reductants and are essentially 'non-autoxidizable' at room temperature. Only one CO binding site with spectral features, kinetic properties, and ligand affinity not very dissimilar from those of mammalian cytochrome oxidase can be detected in the ascorbate-reduced membranes. On the contrary, a second CO binding site having unusual properties for aa₃ terminal oxidases can be detected in the dithionite-reduced membranes.