Adenine nucleotide translocator (ANT), also known as the ADP/ATP translocase (ANT), ADP/ATP carrier protein (AAC) or mitochondrial ADP/ATP carrier, exchanges free
ATP with free ADP across the
inner mitochondrial membrane
The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space.
Structure
The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. ...
.
ANT is the most abundant protein in the
inner mitochondrial membrane
The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space.
Structure
The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. ...
and belongs to
mitochondrial carrier
Mitochondrial carriers are proteins from solute carrier family 25 which transfer molecules across the membranes of the mitochondria. Mitochondrial carriers are also classified in thTransporter Classification Database The Mitochondrial Carrier (M ...
family.
Free ADP is transported from the cytoplasm to the mitochondrial matrix, while ATP produced from
oxidative phosphorylation is transported from the
mitochondrial matrix
In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribo ...
to the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
, thus providing the cells with its main energy currency.
ADP/ATP translocases are exclusive to eukaryotes and are thought to have evolved during
eukaryogenesis
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
. Human cells express four ADP/ATP translocases:
SLC25A4,
SLC25A5,
SLC25A6
ADP/ATP translocase 3, also known as solute carrier family 25 member 6, is a protein that in humans is encoded by the ''SLC25A6'' gene.
Identical copies of this gene reside on the pseudoautosomal regions of the X and Y chromosomes.
See also
...
and
SLC25A31, which constitute more than 10% of the protein in the inner mitochondrial membrane. These proteins are classified under the
mitochondrial carrier
Mitochondrial carriers are proteins from solute carrier family 25 which transfer molecules across the membranes of the mitochondria. Mitochondrial carriers are also classified in thTransporter Classification Database The Mitochondrial Carrier (M ...
superfamily.
Types
In humans, there exist three
paraologous ANT
isoforms
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...
:
*
SLC25A4 – found primarily in heart and skeletal muscle
*
SLC25A5 – primarily expressed in
fibroblasts
*
SLC25A6
ADP/ATP translocase 3, also known as solute carrier family 25 member 6, is a protein that in humans is encoded by the ''SLC25A6'' gene.
Identical copies of this gene reside on the pseudoautosomal regions of the X and Y chromosomes.
See also
...
– primarily express in liver
Structure
![ATP-ADP Translocase Side View (w) drawn over](https://upload.wikimedia.org/wikipedia/commons/8/81/ATP-ADP_Translocase_Side_View_%28w%29_drawn_over.png)
ANT has long been thought to function as a homodimer, but this concept was challenged by the projection structure of the yeast Aac3p solved by electron crystallography, which showed that the protein was three-fold symmetric and monomeric, with the translocation pathway for the substrate through the centre.
The atomic structure of the bovine ANT confirmed this notion, and provided the first structural fold of a mitochondrial carrier.
Further work has demonstrated that ANT is a monomer in detergents
and functions as a monomer in mitochondrial membranes.
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.
It forms six
transmembrane
A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequent ...
α-helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
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
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...
, 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.
Indeed,
arginine residues 96, 204, 252, 253, and 294, as well as
lysine 38, have been shown to be essential for transporter activity.
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
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
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.
ANT transports the free, i.e. deprotonated, non-
Magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
, non-
Calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
bound forms of
ADP and
ATP, in a 1:1 ratio.
Transport is fully reversible, and its directionality is governed by the concentrations of its substrates (ADP and ATP inside and outside mitochondria), the chelators of the adenine nucleotides, and the mitochondrial membrane potential. The relationship of these parameters can be expressed by an equation solving for the 'reversal potential of the ANT" (Erev_ANT), a value of the mitochondrial membrane potential at which no net transport of adenine nucleotides takes place by the ANT.
The ANT and the
F0-F1 ATP synthase are not necessarily in directional synchrony.
Apart from exchange of ADP and ATP across the inner mitochondrial membrane, the ANT also exhibits an intrinsic uncoupling activity
ANT is an important modulatory
and possible structural component of the Mitochondrial Permeability Transition Pore, a channel involved in various pathologies whose function still remains elusive. Karch et al. propose a "multi-pore model" in which ANT is at least one of the molecular components of the pore.
[Karch J, Bround MJ, Khalil H, et al. Inhibition of mitochondrial permeability transition by deletion of the ANT family and CypD. Sci Adv. 2019;5(8):eaaw4597. Published 2019 Aug 28. doi:10.1126/sciadv.aaw4597]
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.
ATP and ADP are the only natural
nucleotides
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules with ...
recognized by the translocase.
The net process is denoted by:
:ADP
3−cytoplasm + ATP
4−matrix → ADP
3−matrix + ATP
4−cytoplasm
ADP/ATP exchange is energetically expensive: about 25% of the energy yielded from
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
by
aerobic respiration, or one
hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
, is consumed to regenerate the
membrane potential that is tapped by ADP/ATP translocase.
The translocator cycles between two states, called the cytoplasmic and matrix state, opening up to these compartments in an alternating way.
There are structures available that show the translocator locked in a cytoplasmic state by the inhibitor
carboxyatractyloside
Carboxyatractyloside (CATR) is a highly toxic diterpene glycoside that inhibits the ADP/ATP translocase. It is about 10 times more potent than its analog atractyloside. While atractyloside is effective in the inhibition of oxidative phosphorylat ...
,
or in the matrix state by the inhibitor
bongkrekic acid.
Alterations
Rare but severe diseases such as
mitochondrial myopathies
Mitochondrial myopathies are types of myopathies associated with mitochondrial disease. On biopsy, the muscle tissue of patients with these diseases usually demonstrate "ragged red" muscle fibers. These ragged-red fibers contain mild accumulation ...
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.
In particular,
autosomal dominant progressive external
ophthalmoplegia
Ophthalmoparesis refers to weakness (-paresis) or paralysis (-plegia) of one or more extraocular muscles which are responsible for eye movements. It is a physical finding in certain neurologic, ophthalmologic, and endocrine disease.
Internal ...
(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
Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
patterns but is characterized by large-scale
mitochondrial DNA (mtDNA) deletions. mtDNA contains few
introns
An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene ...
, 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,
particularly residues involved in the binding pocket which will compromise translocase efficacy.
MM is commonly associated with dysfunctional ADP/ATP translocase, but MM can be induced through many different mitochondrial abnormalities.
Inhibition
![Bongkrekic acid](https://upload.wikimedia.org/wikipedia/commons/4/40/Bongkrekic_acid.svg)
ADP/ATP translocase is very specifically inhibited by two families of compounds. The first family, which includes
atractyloside
Atractyloside (ATR) is a natural, toxic glycoside present in numerous plant species worldwide in the daisy family including '' Atractylis gummifera'' and '' Callilepis laureola'', and it's used for a variety of therapeutic, religious, and toxic ...
(ATR) and
carboxyatractyloside
Carboxyatractyloside (CATR) is a highly toxic diterpene glycoside that inhibits the ADP/ATP translocase. It is about 10 times more potent than its analog atractyloside. While atractyloside is effective in the inhibition of oxidative phosphorylat ...
(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.
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.
There are structures available that show the translocator locked in a cytoplasmic state by the inhibitor
carboxyatractyloside
Carboxyatractyloside (CATR) is a highly toxic diterpene glycoside that inhibits the ADP/ATP translocase. It is about 10 times more potent than its analog atractyloside. While atractyloside is effective in the inhibition of oxidative phosphorylat ...
,
or in the matrix state by the inhibitor
bongkrekic acid.
History
In 1955, Siekevitz and Potter demonstrated that
adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deri ...
nucleotides were distributed in cells in two pools located in the mitochondrial and cytosolic compartments.
Shortly thereafter, Pressman hypothesized that the two pools could exchange nucleotides.
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.
Soon after, an overwhelming amount of research was done in proving the existence and elucidating the link between ADP/ATP translocase and energy transport.
cDNA of ADP/ATP translocase was sequenced for bovine in 1982
and a yeast species ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...
'' in 1986
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.
See also
*
Mitochondrial carrier
Mitochondrial carriers are proteins from solute carrier family 25 which transfer molecules across the membranes of the mitochondria. Mitochondrial carriers are also classified in thTransporter Classification Database The Mitochondrial Carrier (M ...
*
Cellular respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
*
Oxidative phosphorylation
References
External links
*
*
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{{Membrane transport proteins
Solute carrier family
Cellular respiration