ATP7A, also known as Menkes’ protein (MNK), is a copper-transporting P-type ATPase which uses the energy arising from ATP hydrolysis to transport copper across cell membranes. The ATP7A protein is a transmembrane protein and is expressed in the intestine and all tissues except liver. In the intestine, ATP7A regulates copper absorption in the human body by transporting copper from the small intestine into the blood. In other tissues, ATP7A shuttles between the Golgi apparatus and the cell membrane to maintain proper copper concentration in the cell and provide certain enzymes with copper. The X-linked, inherited, lethal genetic disorder of the ATP7A gene causes Menkes disease, a copper deficiency resulting in early childhood death.[1]
The ATP7A gene is located on the long (q) arm of the X chromosome between at position 13.3. The encoded ATP7A protein has 1,500 amino acids.[2] Genetic disorder of this gene causes copper deficiency, which leads to progressive neurodegeneration and death in children.[3]
ATP7A is a transmembrane protein with the N- and C-termini both oriented towards the cytosol (see picture). It is highly homologous to protein ATP7B. ATP7A contains three major functional domains:
Many motifs in the ATP7A structure are conserved, such as the TGEA, CPC, DKTG, SEHPL, and GDGXND motifs. The TGEA motif lies in the loop on the cytosolic side between transmembrane segments 4 and 5 and is involved in energy transduction. The CPC motif located in transmembrane segment 6 is common for all heavy metal transporting ATPases. Between transmembrane segments 6 and 7 is a large cytoplasmic loop, where three motifs are located: DKTG, SEHPL, and GDGXND. The DKTG motif is essential for the proper function of the ATPase. The aspartic acid (D) residue is phosphorylated during the transport cycles. The SEHPL motif only exists in heavy metal transporting P-type ATPases. Without the histidine (H) residue ATP7A may not function properly. The GDGXND motif near transmembrane segment 7 is thought to contain mainly α-helices and serves as a structural support.[6]
The six copper-binding sites at the N-terminal bind one Cu per binding site. This binding site is not specific for Cu and can bind various transition metal ions, depending on the identity of the metal ion. Cd(II), Au(III) and Hg(II) binds to the binding site more tightly than does Zn(II), whereas Mn(II) and Ni(II) have lower affinities than relative to Zn(II). In the case of Cu, a possible cooperative-binding mechanism is observed. When the Cu concentration is low, Cu has a lower affinity for ATP7A compared to Zn(II); as the Cu concentration increases, a dramatic increasing affinity of Cu for the protein is observed.[6]
The two cysteine (C) residues in each copper-binding site are coordinated to Cu(I) with a S-Cu-S angle between 120 and 180° and a Cu-S distance of 2.16 Å. Experimental results from a homologous protein ATP7B suggests that upon Cu binding, the disulfide bonding between the cysteine residues is broken as cysteine starts to bind to Cu, leading to a series of conformational changes at the N-terminal of the protein, and possibly activating the Cu-transporting activity of other cytosolic loops.[6]
Of the six copper-binding sites, two are considered enough for the function of Cu transport. The reason why there are six binding sites remains not fully understood. However, some scientists have proposed that the other four sites may serve as a Cu concentration detector.[4]
ATP7A belongs to a transporter family called P-type ATPases, which catalyze auto-phosphorylation of a key conserved aspartic acid (D) residue within the enzyme. The first step is ATP binding to the ATP-binding domain and Cu binding to the transmembrane region. Then ATP7A is phosphorylated at the key aspartic acid (D) residue in the highly conserved DKTG motif, accompanied by Cu release. A subsequence dephosphorylation of the intermediate finishes the catalytic cycle. Within each cycle, ATP7A interconverts between at least two different conformations, E1 and E2. In the E1 state, Cu is tightly bound to the binding sites on the cytoplasmic side; in the E2 state, the affinity of ATP7A for Cu decreases and Cu is released on the extracellular side.[8]
ATP7A is important for regulating copper levels in mammals.[5] This protein is found in most tissues, but it is not expressed in the liver.[6] In the small intestine, the ATP7A protein helps control the absorption of copper from food. After cupper ions are absorbed into enterocytes, ATP7A is required to transfer them across the basolateral membrane into the circulation.[4]
In other organs and tissues, the ATP7A protein has a dual role and shuttles between two locations within the cell. The protein normally resides in a cell structure called the Golgi apparatus, which modifies and transports newly produced enzymes and other proteins. Here, the ATP7A protein supplies copper to certain enzymes (e.g. peptidyl-α-monooxygenase, tyrosinase, and lysyl oxidase[4]) that are critical for the structure and function of brain, bone, skin, hair, connective tissue, and the nervous system. If copper levels in the cell environment are elevated, however, the ATP7A protein moves to the cell membrane and eliminates excess copper from the cell.[5][3]
The functions of ATP7A in some tissues of the human body are as follows:[5]
Tissue | Location | Function |
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Kidney | Expressed in epithelial cells of the proximal and distal renal tubules | Removes excess copper to maintain copper level in the renal |
Parenchyma | In the cytotrophoblast, syncytiotrophoblast and foetal vascular endothelial cells | Delivers copper to placental cuproenzymes and transports copper into the foetal circulation |
Central nervous system | Various locations | Distributes copper in the various compartments of the central nervous system |
ATP7A has been shown to interact with ATOX1 and GLRX. Antioxidant 1 copper chaperone (ATOX1) is required to maintain proper copper homeostasis in the cell. It can bind and transport cytosolic copper to ATP7A in the trans-Golgi-network. Glutaredoxin-1 (GRX1) has is also essential for ATP7A function. It promotes copper binding for subsequent transport by catalyzing the reduction of disulfide bridges. It may also catalyze de-glutathionylation reaction of the C (cysteine) residues within the six copper-binding motifs GMTCXXC.[5]
Menkes disease is caused by mutations in the ATP7A gene. Researchers have identified different ATP7A mutations that cause Menkes disease and occipital horn syndrome (OHS), the milder form of Menkes disease. Many of these mutations delete part of the gene and are predicted to produce a shortened ATP7A protein that is unable to transport copper. Other mutations insert additional DNA building blocks (base pairs) or use the wrong building blocks, which leads to ATP7A proteins that do not function properly.[2]
The altered proteins that result from ATP7A mutations impair the absorption of copper from food, fail to supply copper to certain enzymes, or get stuck in the cell membrane, unable to shuttle back and forth from the Golgi. As a result of the disrupted activity of the ATP7A protein, copper is poorly distributed to cells in the body. Copper accumulates in some tissues, such as the small intestine and kidneys, while the brain and other tissues have unusually low levels. The decreased supply of copper can reduce the activity of numerous copper-containing enzymes that are necessary for the structure and function of bone, skin, hair, blood vessels, and the nervous system.[5][3]
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PDB gallery
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1aw0: FOURTH METAL-BINDING DOMAIN OF THE MENKES COPPER-TRANSPORTING ATPASE, NMR, 20 STRUCTURES
1kvi: Solution Structure of the Reduced Form of the First Heavy Metal Binding Motif of the Menkes Protein
1kvj: Solution Structure of the Cu(I) bound form of the first heavy metal binding motif of the Menkes protein
1q8l: Second Metal Binding Domain of the Menkes ATPase
1s6o: Solution structure and backbone dynamics of the apo-form of the second metal-binding domain of the Menkes protein ATP7A
1s6u: Solution structure and backbone dynamics of the Cu(I) form of the second metal-binding domain of the Menkes protein ATP7A
1y3j: Solution structure of the copper(I) form of the fifth domain of Menkes protein
1y3k: Solution structure of the apo form of the fifth domain of Menkes protein
1yjr: Solution structure of the apo form of the sixth soluble domain A69P mutant of Menkes protein
1yjt: Solution structure of the Cu(I) form of the sixth soluble domain A69P mutant of Menkes protein
1yju: Solution structure of the apo form of the sixth soluble domain of Menkes protein
1yjv: Solution structure of the Cu(I) form of the sixth soluble domain of Menkes protein
2aw0: FOURTH METAL-BINDING DOMAIN OF THE MENKES COPPER-TRANSPORTING ATPASE, NMR, 20 STRUCTURES
2g9o: Solution structure of the apo form of the third metal-binding domain of ATP7A protein (Menkes Disease protein)
2ga7: Solution structure of the copper(I) form of the third metal-binding domain of ATP7A protein (menkes disease protein)
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3.6.1 |
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3.6.2 |
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3.6.3-4: ATPase |
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3.6.5: GTPase |
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F-, V-, and A-type ATPase (3.A.2) |
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P-type ATPase (3.A.3) |
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see also ATPase disorders |