Fig. (5).

Different selective targets for different life strategies. The electron transport chain can be envisioned as a motorway, where electrons travel from NADH to molecular oxygen. Thus, complex I represents the motorway entrance while complex IV corresponds to the motorway exit. In this metaphor, uncoupling proteins (UCPs) assume the role of emergency exits. Whenever a traffic jam may be a problem, the dissipation of the proton-motive force through UCPs will speed up the flow of electrons and their proper exit through complex IV. Long-lived organisms require an efficient energy metabolism. This requirement, beside a low metabolic rate, imposes a rather tight coupling between substrate oxidation and ADP phosphorylation, which in turn urges for low proton conductances through UCPs. Therefore, in this life-history context, controlling the electron entrance through complex I seems to have been an important target of natural selection. In contrast, short-lived animals demand a high metabolic rate, even if that is at the expense of low efficiencies. In this scenario, reducing the entrance of electron to the ETC does not seem to be an affordable option. Thus, UCPs gain relevance as anti-oxidant devices.