A single external enzyme confers alternative NADH : ubiquinone oxidoreductase activity in Yarrowia lipolytica

NADH:ubiquinone oxidoreductases catalyse the first step within the diverse pathways of mitochondrial NADH oxidation. In addition to the energy-conserv ing form commonly called complex I, fungi and plants contain much simpler a lternative NADH:ubiquinone oxido-reductases that catalyze the same reaction but do not translocate protons accross the inner mitochondrial membrane. L ittle is known about the distribution and function of these enzymes. We hav e identified YLNDH2 as the only gene encoding an alternative NADH:ubiquinon e oxidoreductase (NDH2) in the obligate aerobic yeast Yarrowia lipolytica, Cells carrying a deletion of YLNDH2 were fully viable; full inhibition by p iericidin A indicated that complex I activity was the sole NADH:ubiquinone oxidoreductase activity left in the deletion strains. Studies with intact m itochondria revealed that NDH2 in Y. lipolytica is oriented towards the ext ernal face of the mitochondrial inner membrane. This is in contrast to the situation seen in Saccharomyces cerevisiae, Neurospora crassa and in green plants, where internal alternative NADH:ubiquinone oxidoreductases have bee n reported. Phylogenetic analysis of known NADH:ubiquinone oxidoreductases suggests that during evolution conversion of an ancestral external alternat ive NADH:ubiquinone oxidoreductase to an internal enzyme may have paved the way for the loss of complex I in fermenting yeasts like S. cerevisiae.