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 Haloba cterium salinarum were investigated by spectroscopic and polarographic tech niques. 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 succinate-cytochrome-c oxidore ductase as well as an NADH and succinate oxidase activity. These activities could be blocked by the following inhibitors: 7-jodocarboxylic acid, givin g evidence for the presence of a type II NADH dehydrogenase, antimycin A, a nd myxothiazol, indicating the presence of a complex III analog, and the ty pical succinate dehydrogenase (SDH) and terminal oxidase inhibitors. Comple x I inhibitors like rotenone and annonine were inactive, clearly excluding the presence of a coupled NADH dehydrogenase. In addition, no [Fe-S] resona nces in the region of the NADH dehydrogenase (NDH) clusters could be observ ed after NADH addition. One of the terminal oxidases could be shown to act as a cytochrome-c oxidase with a K-m value of 37 mu M and an activation ene rgy of 23.7 kJ/mol. The relative molecular mass of the endogenous c-type cy tochrome could be determined as 14.1 kD. The complex III analog could be en riched after detergent extraction with Triton X-100 and hydroxylapatite (HT P) chromatography. The partially purified complex contained a Rieske iron-s ulfur cluster, b- and c-type cytochromes, and was catalytically active in t he decylubiquinone-cytochrome-c oxidoreductase assay.