Alternative NAD(P)H dehydrogenase and alternative oxidase: Proposed physiological roles in animals

Abstract The electron transport systems in mitochondria of many organisms contain alternativerespiratory enzymes distinct from those of the canonical respiratory systemdepicted in textbooks. Two of these enzymes, the alternative NADH dehydrogenaseand the alternative oxidase, were of interest to a limited circle of researchers until they were envisioned as gene therapy tools for mitochondrial diseasetreatment. Recently, these enzymes were discovered in several animals. Here, we analyse the functioning of alternative NADH dehydrogenasesand oxidases in different organisms. We propose that both enzymes ensure bioenergeticand metabolic flexibility during environmental transitions or other conditions which may compromise the operation of the canonical respiratory system.