ReviewUCP1 - A sophisticated energy valve

- Mechanism and regulation of H+ transport by the energy dissipating uncoupling protein 1 in mitochondria from brown adipose tissue, destined to generate heat in mammals.

This review focuses on the biochemical work of UCP1 starting from the early observation by Ricquier and Kader in 1976. We entered this field in 1980 with the isolation of native UCP1 and then reported the amino acid sequence structure discovering a strong homology to the ADP/ATP carrier. With the isolated native UCP1 we studied structural and functional features, in particular the complex characteristics of nucleotide binding. A strong pH dependence of binding and herein the differences between diphopho- and triphopho-nucleotides were observed, resulting in the identification of residues which control binding site access by their H+ dissociation. Newly synthesized fluorescent nucleotide derivatives provided tools to determine a two state nucleotide binding in line with loose and tight UCP1 conformations and H+ transport inhibition. The slow transition between these states were a notable feature. The reconstitution of isolated UCP1 in vesicles demonstrated that UCP1 protein is in fact the uncoupling factor and not only a nucleotide controlled regulator. The H+ transport was shown to be electrophoretic with a linear relation to the membrane potential. The dependence of H+ transport on fatty acids (FA) was characterized and is elaborated here with a view of the experimental conditions of other research groups which had different views of the role of FA in H+ transport. Furthermore, to explain the contrast of the FA - nucleotide competition between mitochondria and reconstituted system, indirect paths for FA to relieve the inhibition in mitochondria are here proposed, such as a FA induced upward pH shift and a FA induced increase of cardiolipin level around UCP1 since cardiolipin has been found by us to relieve nucleotide binding on isolated UCP1. Recently reported patch clamp results on mitoplasts led to a reformulation of the H+ transport mechanism of FA in UCP1 in which bound FA shuttles with the carboxyl group between the two membrane sides along the translocation channel outward as FA− and inward as FA−H+. We propose here a modified version, where FA forms an immobile prosthetic group surrounded by the inner and outer gate of the H+ translocation channel. By alternating opening of the gates FA takes up H+ from the cytosol side and releases H+ to the matrix.