EN
The eukaryotic respiratory complexes carry out conserved reactions compared to their prokaryotic equivalents and contain conserved catalytic subunits. Despite these resemblances, eukaryotic complexes are more complex, containing additional subunits. Various lines of evidence suggest that these additional subunits protect against proteolysis, modulate catalytic activity and/or play a role in assembly. We suggest, however, that none of the present-day functions was the original driving force for their acquisition by the complex during evolution of the mitochondrial respiratory chain and that the incorporation was largely accidental, the small accessory subunits possibly being derived from imported presequences.