Figure 9.

Possible scenario of the evolution of the mitochondrion in Kinetoplastida. A facultative anaerobic, 'pluripotent' ancestral eukaryote was formed by endosymbiosis of a facultative anaerobic α-proteobacterium by an archaebacterial host. It has been proposed [118] that different capacities of the original symbiont were lost in different eukaryotic lineages, leading to different types of organelles (aerobic mitochondria, anaerobic mitochondria, hydrogenosomes). In Kinetoplastida, the original hydrogenase, anaerobic terminal oxidases and the capacity to synthesize quinones of low redox potential were lost. Indications for the retainment of the PFO have been reported, but its presence still requires unequivocal confirmation. The kinetoplastid mitochondrion acquired an AOX and its metabolism became partially fermentative (see text). Abbreviations: AOX, alternative oxidase; ASCT, acetate:succinate CoA transferase; PDH, pyruvate dehydrogenase complex; PFO, pyruvate:ferredoxin oxidoreductase; terminal oxidase O, O₂-dependent terminal oxidase; terminal oxidases A, terminal oxidases using 'anaerobic' electron acceptors.