Mitochondrial oxidative phosphorylation makes
possible ATP synthesis using the energy available from substrate
oxidation at the respiratory chain. These processes are coupled
through the proton electrochemical potential gradient generated
during the transfer of electrons from the substrate to oxygen.
The uncoupling proteins (UCPs) are mitochondrial inner membrane
proteins that are considered to be transporters functioning as
enzymatic uncouplers of oxidative phosphorylation. They are capable
of returning protons pumped by the respiratory chain to the mitochondrial
matrix. Uncoupling proteins currently comprise UCP1, UCP2, UCP3,
UCP4, and UCP5. UCP1 is a 32 kDa protein that is active as a proton
channelforming dimer. It can bind purine nucleotides and is capable
of being stimulated by fatty acids. Proton transport by UCP1 has
been shown to depend on CoQ (ubiquinone) as an obligatory cofactor.
UCP1 is exclusively expressed in BAT in rodents and in neonates
where it is regulated by norepinephrine and thyroid hormones. Stimulated
BAT is able to dissipate energy as heat via uncoupled mitochondrial
respiration. The liberated heat can serve several physiological
functions, e.g. for body heating during emergence from hibernation
or during cold exposure, for burning body fat and consequently
for body weight regulation.
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