Uncoupling proteins in mitochondria of plants and some microorganisms.

Uncoupling proteins, members of the mitochondrial carrier family, are present in mitochondrial inner membrane and mediate free fatty acid-activated, purine-nucleotide-inhibited H+ re-uptake. Since 1995, it has been shown that the uncoupling protein is present in many higher plants and some microorganisms like non-photosynthetic amoeboid protozoon, Acanthamoeba castellanii and non-fermentative yeast Candida parapsilosis. In mitochondria of these organisms, uncoupling protein activity is revealed not only by stimulation of state 4 respiration by free fatty acids accompanied by decrease in membrane potential (these effects being partially released by ATP and GTP) but mainly by lowering ADP/O ratio during state 3 respiration. Plant and microorganism uncoupling proteins are able to divert very efficiently energy from oxidative phosphorylation, competing for ΔμH+ with ATP synthase. Functional connection and physiological role of uncoupling protein and alternative oxidase, two main energy-dissipating systems in plant-type mitochondria, are discussed.