Mechanism of uncoupling protein action

Two competing models of uncoupling protein (T-TCP) transport mechanism agre e that fatty acids (FAs) are obligatory for uncoupling, but they disagree a bout which ion is transported. In Klingenberg's model, UCPs conduct protons . In Garlid's model, UCPs conduct anions, like all members of this gene fam ily. In the latter model, UCP transports the anionic FA head group from one side of the membrane to the other, and the cycle is completed by rapid fli p-flop of protonated FAs across the bilayer. The head groups of the FA anal ogues, long-chain alkylsulphonates, are translocated by UCP, but they canno t induce uncoupling, because these strong acids cannot be protonated for th e flip-flop part of the cycle. We have overcome this limitation by ion-pair transport of undecanesulphonate with propranolol, which causes the sulphon ate to deliver protons across the membrane as if it were an FA. Full GDP-se nsitive uncoupling is seen in the presence of propranolol and undecanesulph onate. This result confirms that the mechanism of UCP uncoupling requires t ransport of the anionic FA head group by UCP and that the proton transport occurs via the bilayer and not via UCP.