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A set of metabolite carriers operates the traffic of numerous molecules consumed or produced in mitochondrial matrix and/or cytosolic compartments. As their existence had been predicted by the chemiosmotic theory, the first challenge, in the late sixties, was to prove their presence in the inner mitochondrial membrane and to describe the various transports carried out. The second challenge was to understand their mechanisms by the kinetic approach in intact mitochondria (seventies). The third challenge (late seventies-eighties) was to isolate and to reconstitute the carriers in liposomes in order to characterize the proteins and to establish the concept of a structural and a functional family as well as some structure-function relationship with the help of primary sequences. Genetics, molecular biology and genomic sequencing bring the fourth challenge (nineties): a raising number of putative carriers becomes known only by their primary sequences but their functions have to be discovered. The actual challenge of the future is the elucidation of the ternary structure of carrier proteins that together with site-directed mutagenesis and kinetic mechanism will permit to advance in the understanding of molecular mechanisms of transport processes.
Mitochondria of the amoeba Acanthamoeba castellanii possess a free fatty acid-ac­tivated uncoupling protein (AcUCP) that mediates proton re-uptake driven by the mi­tochondrial proton electrochemical gradient. We show that AcUCP activity diverts energy from ATP synthesis during state 3 mitochondrial respiration in a fatty acid-dependent way. The efficiency of AcUCP in mitochondrial uncoupling increases when the state 3 respiratory rate decreases as the AcUCP contribution is constant at a given linoleic acid concentration while the ATP synthase contribution decreases with respiratory rate. Respiration sustained by this energy-dissipating process remains constant at a given linoleic acid concentration until more than 60% inhibition of state 3 respiration by n-butyl malonate is achieved. The present study supports the validity of the ADP/O method to determine the actual contributions of AcUCP (activated with various linoleic acid concentrations) and ATP synthase in state 3 respiration of A. castellanii mitochondria fully depleted of free fatty acid-activated and describes how the two contributions vary when the rate of succinate dehydrogenase is decreased by succinate uptake limitation.
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