Mitochondrial ion transport pathways: Role in metabolic diseases (original) (raw)
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Journal of Biological Chemistry
The uncoupling protein is one of a family of mitochondrial transport proteins involved in energy metabolism. It dissipates oxidative energy to generate heat, either by catalyzing proton transport directly or by catalyzing fatty acid anion transport, thus enabling fatty acids to act as cycling protonophores. This transport process is tightly regulated by purine nucleotides. W e have expressed uncoupling protein in yeast and examined its proton transport activity after its reconstitution into proteoliposomes. A directed change of Arg276 to Leu or Gln completely abolished nucleotide inhibition of protonophoretic action of the reconstituted mutant uncoupling proteins without affecting the transport process. Arg276 is the first residue of functional importance to be identified in uncoupling protein. Mutation of the homologous residue in the yeast ADP/ATP translocator prevented the growth of yeast on a nonfermentable carbon source, presumably by interfering with nucleotide exchange (Nelson, D. R., Lawson, J. E., Klingenberg, M., and Douglas, M. G. (1993) J. Mol. BioZ. 230, 1159-1170).
The uncoupling protein generates heat by catalyzing electrophoretic proton transport across the inner membrane of brown adipose tissue mitochondria. It also transports C1and other monovalent anions, and both proton and anion transport are inhibited by purine nucleotides. Several long-standing hypotheses bear on specific aspects of C1-transport, H+ transport, and nucleotide gating mechanisms in uncoupling protein. We reevaluated these hypotheses in mitochondria and liposomes reconstituted with purified uncoupling protein; GDP inhibition is strictly noncompetitive with C1-and unaffected by either transmembrane electrical potential or fatty acids. The K,,, and V , , values for C1-are independent of pH, arguing against a common binding site for C1-and OHions. C1-transport was inhibited by fatty acids and stimulated by fatty acid removal, refuting the consensus hypothesis that there is no interaction between fatty acids and anion transport through uncoupling protein. These results support a mechanism in which the transport pathway for anions is identical with the fatty acid binding site and distinct from the nucleotide binding site.
Transport Function and Regulation of Mitochondrial Uncoupling Proteins 2 and 3
Journal of Biological Chemistry, 1999
Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.
The role of mitochondrial transport in energy metabolism
Mitochondrion, 2003
Since mitochondria are closed spaces in the cell, metabolite traffic across the mitochondrial membrane is needed to accomplish energy metabolism. The mitochondrial carriers play this function by uniport, symport and antiport processes.
The mechanism of proton transport mediated by mitochondrial uncoupling proteins
FEBS Letters, 1998
The effort to understand the mechanism of uncoupling by UCP has devolved into two models^the fatty acid protonophore model and the proton buffering model. Evidence for each hypothesis is summarized and evaluated. We also evaluate the obligatory requirement for fatty acids in UCP1mediated uncoupling and the question of fatty acid affinity for UCP1. The structural bases of UCP transport function and nucleotide inhibition are discussed in light of recent mutagenesis studies and in relationship to the sequences of newly discovered UCPs.
Mitochondrial cation transport: A progress report
Journal of Bioenergetics and Biomembranes, 1994
This report summarizes recent work in our laboratory aimed at understanding proteinmediated mitochondrial cation transport. We are studying three distinct cation cycles that contain porters catalyzing influx and efllux of cations between cytosol and mitochondrial matrix. Each of these cation cycles plays a major physiological role in the overall energy economy. The K + cycle maintains the integrity of the vesicular structure and includes the K+/H + antiporter, the KAte channel, and K + leak driven by the high membrane potential. The Ca 2+ cycle relays the signals calling for modulation of ATP production and includes the Ca 2+ channel, the Na+/Ca 2+ antiporter, and the Na+/H + antiporter. The H + cycle of brown adipose tissue mitochondria provides heat to hibernating and newborn mammals and consists of the uncoupling protein, which catalyzes regulated H + influx.
Mitochondrial proton leak in obesity-resistant and obesity-prone mice
AJP: Regulatory, Integrative and Comparative Physiology, 2007
We quantified uncoupling proteins (UCPs) in molar amounts and assessed proton conductance in mitochondria isolated from interscapular brown adipose tissue (IBAT) and hindlimb muscle [known from prior work to contain ectopic brown adipose tissue (BAT) interspersed between muscle fibers] of obesity-resistant 129S6/SvEvTac (129) and obesity-prone C57BL/6 (B6) mice under conditions of low (LF) and high-fat (HF) feeding. With usual feeding, IBAT mitochondrial UCP1 content and proton conductance were greater in 129 mice than B6. However, with HF feeding, UCP1 and proton conductance increased more in B6 mice. Moreover, with HF feeding GDP-inhibitable proton conductance, specific for UCP1, equaled that seen in the 129 strain. UCP1 expression was substantial in mitochondria from hindlimb muscle tissue (ectopic BAT) of 129 mice as opposed to B6 but did not increase with HF feeding in either strain. As expected, muscle UCP3 expression increased with HF feeding in both strains but did not diffe...