Studies of the electron transport chain of the euryarcheon Halobacterium salinarum: indications for a type II NADH dehydrogenase and a complex III analog.

The components involved in the respiratory system of the euryarcheon Halobacterium salinarum were investigated by spectroscopic and polarographic techniques. Previous results about the cytochrome composition could be verified. However, under low oxygen tension, the expression of a d-type cytochrome was detected. Membranes exerted an NADH- and succinatecytochrome-c oxidoreductase as well as an NADH and succinate oxidase activity. These activities could be blocked by the following inhibitors: 7-jodocarboxylic acid, giving evidence for the presence of a type II NADH dehydrogenase, antimycin A, and myxothiazol, indicating the presence of a complex III analog, and the typical succinate dehydrogenase (SDH) and terminal oxidase inhibitors. Complex I inhibitors like rotenone and annonine were inactive, clearly excluding the presence of a coupled NADH dehydrogenase. In addition, no [Fe-S] resonances in the region of the NADH dehydrogenase (NDH) clusters could be observed after NADH addition. One of the terminal oxidases could be shown to act as a cytochrome-c oxidase with a Km value of 37 microM and an activation energy of 23.7 kJ/mol. The relative molecular mass of the endogenous c-type cytochrome could be determined as 14.1 kD. The complex III analog could be enriched after detergent extraction with Triton X-100 and hydroxylapatite (HTP) chromatography. The partially purified complex contained a Rieske iron-sulfur cluster, b- and c-type cytochromes, and was catalytically active in the decylubiquinone-cytochrome-c oxidoreductase assay.