ADP/ATP translocase

ADP/ATP translocases

Cytoplasmic view of the binding pocket of ATP–ADP translocase 1, PDB: 1OKC​.)
Identifiers
Symbol Aden_trnslctor
Pfam PF00153
InterPro IPR002113
TCDB 2.A.29.1.2
OPM superfamily 21
OPM protein 2c3e

ADP/ATP translocases, also known as adenine nucleotide translocases (ANT) and ADP/ATP carrier proteins (AAC), are transporter proteins that enable the exchange of cytosolic adenosine diphosphate (ADP) and mitochondrial adenosine triphosphate (ATP) across the inner mitochondrial membrane. Free ADP is transported from the cytoplasm to the mitochondrial matrix, while ATP produced from oxidative phosphorylation is transported from the mitochondrial matrix to the cytoplasm, thus providing the cells with its main energy currency.[1]

ADP/ATP translocases are exclusive to eukaryotes and are thought to have evolved during eukaryogenesis.[2] Human cells express four ADP/ATP translocases: SLC25A4, SLC25A5, SLC25A6 and SLC25A31, which constitute more than 10% of the protein in the inner mitochondrial membrane.[3] These proteins are classified under the mitochondrial carrier superfamily.

Structure

A side view of the translocase spanning the inner mitochondrial membrane. The six α-helices are denoted by different colors. The binding pocket is currently open to the cytoplasmic side and will bind to ADP, transporting it into the matrix. (From PDB: 1OKC​)
The translocase (as a molecular surface, green) viewed from both sides of a lipid bilayer representing the inner mitocondrial membrane. Left panel (IM): view from the intermembrane space. The protein is in the open conformation towards this side. Right panel (M): view from the matrix. The protein is closed towards this side.

ADP/ATP translocase 1 is the major AAC in human cells and the archetypal protein of this family. It has a mass of approximately 30 kDa, consisting of 297 residues.[4] It forms six transmembrane α-helices that form a barrel that results in a deep cone-shaped depression accessible from the outside where the substrate binds. The binding pocket, conserved throughout most isoforms, mostly consists of basic residues that allow for strong binding to ATP or ADP and has a maximal diameter of 20 Å and a depth of 30 Å.[5] Indeed, arginine residues 96, 204, 252, 253, and 294, as well as lysine 38, have been shown to be essential for transporter activity.[6]

Translocase mechanism

Under normal conditions, ATP and ADP cannot cross the inner mitochondrial membrane due to their high negative charges, but ADP/ATP translocase, an antiporter, couples the transport of the two molecules. The depression in ADP/ATP translocase alternatively faces the matrix and the cytoplasmic sides of the membrane. ADP in the intermembrane space, coming from the cytoplasm, binds the translocase and induces its eversion, resulting in the release of ADP into the matrix. Binding of ATP from the matrix induces eversion and results in the release of ATP into the intermembrane space, subsequently diffusing to the cytoplasm, and concomitantly brings the translocase back to its original conformation.[1] ATP and ADP are the only natural nucleotides recognized by the translocase.[5]

The net process is denoted by:

ADP3−cytoplasm + ATP4−matrix → ADP3−matrix + ATP4−cytoplasm

ADP/ATP exchange is energetically expensive: about 25% of the energy yielded from electron transfer by aerobic respiration, or one hydrogen ion, is consumed to regenerate the membrane potential that is tapped by ADP/ATP translocase.[1]

Biological function

ADP/ATP translocase transports ATP synthesized from oxidative phosphorylation into the cytoplasm, where it can be used as the principal energy currency of the cell to power thermodynamically unfavorable reactions. After the consequent hydrolysis of ATP into ADP, ADP is transported back into the mitochondrial matrix, where it can be rephosphorylated to ATP. Because a human typically exchanges the equivalent of his/her own mass of ATP on a daily basis, ADP/ATP translocase is an important transporter protein with major metabolic implications.[1][5]

Alterations

Rare but severe diseases such as mitochondrial myopathies are associated with dysfunctional human ADP/ATP translocase. Mitochondrial myopathies (MM) refer to a group of clinically and biochemically heterogeneous disorders that share common features of major mitochondrial structural abnormalities in skeletal muscle. The major morphological hallmark of MM is ragged, red fibers containing peripheral and intermyofibrillar accumulations of abnormal mitochondria.[7][8] In particular, autosomal dominant progressive external ophthalmoplegia (adPEO) is a common disorder associated with dysfunctional ADP/ATP translocase and can induce paralysis of muscles responsible for eye movements. General symptoms are not limited to the eyes and can include exercise intolerance, muscle weakness, hearing deficit, and more. adPEO shows Mendelian inheritance patterns but is characterized by large-scale mitochondrial DNA (mtDNA) deletions. mtDNA contains few introns, or non-coding regions of DNA, which increases the likelihood of deleterious mutations. Thus, any modification of ADP/ATP translocase mtDNA can lead to a dysfunctional transporter,[9] particularly residues involved in the binding pocket which will compromise translocase efficacy.[6] MM is commonly associated with dysfunctional ADP/ATP translocase, but MM can be induced through many different mitochondrial abnormalities.

Inhibition

ADP/ATP translocase is very specifically inhibited by two families of compounds. The first family, which includes atractyloside (ATR) and carboxyatractyloside (CATR), binds to the ADP/ATP translocase from the cytoplasmic side, locking it in a cytoplasmic side open conformation. In contrast, the second family, which includes bongkrekic acid (BA) and isobongkrekic acid (isoBA), binds the translocase from the matrix, locking it in a matrix side open conformation.[10] The negatively charged groups of the inhibitors bind strongly to the positively charged residues deep within the binding pocket. The high affinity (Kd in the nanomolar range) makes each inhibitor a deadly poison by obstructing cellular respiration/energy transfer to the rest of the cell.[5]

History

In 1955, Siekevitz and Potter demonstrated that adenine nucleotides were distributed in cells in two pools located in the mitochondrial and cytosolic compartments.[11] Shortly thereafter, Pressman hypothesized that the two pools could exchange nucleotides.[12] However, the existence of an ADP/ATP transporter was not postulated until 1964 when Bruni et al. uncovered an inhibitory effect of atractyloside on the energy-transfer system (oxidative phosphorylation) and ADP binding sites of rat liver mitochondria.[13] Soon after, an overwhelming amount of research was done in proving the existence and elucidating the link between ADP/ATP translocase and energy transport.[14][15][16] cDNA of ADP/ATP translocase was sequenced for bovine in 1982[17] and a yeast species Saccharomyces cerevisiae in 1986[18] before finally Battini et al. sequenced a cDNA clone of the human transporter in 1989. The homology in the coding sequences between human and yeast ADP/ATP translocase was 47% while bovine and human sequences extended remarkable to 266 out of 297 residues, or 89.6%. In both cases, the most conserved residues lie in the ADP/ATP substrate binding pocket.[4]

See also

References

  1. 1 2 3 4 Stryer L, Berg JM, Tymoczko JL (2007). Biochemistry. San Francisco: W.H. Freeman. pp. 529–530. ISBN 0-7167-8724-5.
  2. ↑ Radzvilavicius, Arunas L.; Blackstone, Neil W. (14 October 2015). "Conflict and cooperation in eukaryogenesis: implications for the timing of endosymbiosis and the evolution of sex". Journal of The Royal Society Interface. 12 (111): 20150584. doi:10.1098/rsif.2015.0584.
  3. ↑ Brandolin G, Dupont Y, Vignais PV (April 1985). "Substrate-induced modifications of the intrinsic fluorescence of the isolated adenine nucleotide carrier protein: demonstration of distinct conformational states". Biochemistry. 24 (8): 1991–1997. doi:10.1021/bi00329a029. PMID 2990548.
  4. 1 2 Battini R, Ferrari S, Kaczmarek L, Calabretta B, Chen S-T & Baserga R (March 1987). "Molecular cloning of a cDNA for a human ADP/ATP carrier which is growth-regulated". J. Biol. Chem. 262 (2): 4355–4359. PMID 3031073.
  5. 1 2 3 4 Pebay-Peyroula E; Dahout C; Kahn R; Trezeguet V; Lauquin GJ-M; Brandolin G (November 2003). "Structure of mitochondrial ADP/ATP carrier complex with carboxyatractyloside". Nature. 426 (1): 39–44. doi:10.1038/nature02056. PMID 14603310.
  6. 1 2 Nelson DR, Lawson JE, Klingenberg M, Douglas MG (April 1993). "Site-directed mutagenesis of the yeast mitochondrial ADP/ATP translocator. Six arginines and one lysine are essential". J. Mol. Biol. 230 (4): 1159–1170. doi:10.1006/jmbi.1993.1233. PMID 8487299.
  7. ↑ Harding AE, Petty RKH & Morgan-Hughes JA (Aug 1988). "Mitochondrial myopathy: a genetic study of 71 cases". Journal of Medical Genetics. 25 (8): 528–35. doi:10.1136/jmg.25.8.528. PMC 1080029Freely accessible. PMID 3050098.
  8. ↑ Rose, MR (Jan 1998). "Mitochondrial myopathies: genetic mechanisms.". Archives of Neurology. 55 (1): 17–24. doi:10.1001/archneur.55.1.17. PMID 9443707.
  9. ↑ Kaukonen J, Juselius JK, Tiranti V, Kyttälä A, Zeviani M, Comi GP, Keränen S, Peltonen L, Suomalainen A (4 August 2000). "Role of Adenine Nucleotide Translocator 1 in mtDNA Maintenance". Science. 289 (5480): 782–785. doi:10.1126/science.289.5480.782. PMID 10926541.
  10. ↑ Kunji ER, Harding M (2003-09-26). "Projection structure of the atractyloside-inhibited mitochondrial ADP/ATP carrier of Saccharomyces cerevisiae.". The Journal of Biological Chemistry. 278 (39): 36985–8. doi:10.1074/jbc.C300304200. PMID 12893834.
  11. ↑ Siekevitz P, Potter VR (July 1955). "Biochemical structure of mitochondria. II. Radioactive labeling of intra-mitochondrial nucleotides during oxidative phosphorylation". J. Biol. Chem. 215 (1): 237–255. PMID 14392158.
  12. ↑ Pressman BC (June 1958). "Intramitochondrial nucleotides. I. Some factors affecting net interconversions of adenine nucleotides". J. Biol. Chem. 232 (2): 967–978. PMID 13549480.
  13. ↑ Bruni A, Luciani S, Contessa AR (March 1964). "Inhibition by atractyloside of the binding of adenine-nucleotides to rat-liver mitochondria". Nature. 201 (1): 129–1220. doi:10.1038/2011219a0. PMID 14151375.
  14. ↑ Duee ED, Vignais PV (August 1965). "Exchange between extra- and intramitochondrial adenine nucleotides". Biochim. Biophys. Acta. 107 (1): 184–188. doi:10.1016/0304-4165(65)90419-8. PMID 5857365.
  15. ↑ Pfaff E, Klingenberg M, Heldt HW (June 1965). "Unspecific permeation and specific exchange of adenine nucleotides in liver mitochondria". Biochim. Biophys. Acta. 104 (1): 312–315. doi:10.1016/0304-4165(65)90258-8. PMID 5840415.
  16. ↑ Saks, V; Lipina N; Smirnov V; Chazov E (1 March 1976). "Studies of energy transport in heart cells The functional coupling between mitochondrial creatine phosphokinase and ADP/ATP translocase: Kinetic evidence". Archives of Biochemistry and Biophysics. 173 (1): 34–41. doi:10.1016/0003-9861(76)90231-9. PMID 1259440.
  17. ↑ Aquila H, Misra D, Eulitz M, Klingenberg M (1 January 1982). "Complete aminoacid sequence of the ADP/ATP carrier from beef heart mitochondria". Hoppe-Seyler´s Zeitschrift für physiologische Chemie. 363 (1): 345–350. doi:10.1515/bchm2.1982.363.1.345. PMID 7076130.
  18. ↑ Adrian GS, McCammon MT, Montgomery DL, Douglas MG (Feb 1986). "Sequences required for delivery and localization of the ADP/ATP translocator to the mitochondrial inner membrane.". Molecular and Cellular Biology. 6 (2): 626–34. PMC 367554Freely accessible. PMID 3023860.
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