ATP7A

The ATP7A gene is located on the long (q) arm of the X chromosome at band Xq21.1. The encoded ATP7A protein has 1,500 amino acids. At least 12 disease-causing mutations in this gene have been discovered. Mutations/additions/deletions of this gene often cause copper deficiency, which leads to progressive neurodegeneration and death in children. == Structure ==

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: • Eight transmembrane segments that form a channel and allow for Cu(I) to pass through the membrane; • An ATP-binding domain; • A large N-terminal cytosolic domain that contains six repeated Cu(I)-binding sites, each containing a GMTCXXC motif. Many motifs in the ATP7A structure are conserved: • The TGEA motif lies in the loop on the cytosolic side between transmembrane segments 4 and 5 and is involved in energy transfer. • 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. The six Cu(I)-binding sites at the N-terminal bind one Cu(I) each. This binding site is not specific for Cu(I) and can bind various transition metal ions. Cd(II), Au(III) and Hg(II) bind to the binding site more tightly than does Zn(II), whereas Mn(II) and Ni(II) have lower affinities relative to Zn(II). In the case of Cu(I), a possible cooperative-binding mechanism is observed. When the Cu(I) concentration is low, Cu(I) has a lower affinity for ATP7A compared to Zn(II); as the Cu(I) concentration increases, a dramatic increasing affinity of Cu(I) for the protein is observed. == Conformational change ==

The two cysteine (C) residues in each Cu(I)-binding site are coordinated to Cu(I) with a S-Cu(I)-S angle between 120 and 180° and a Cu-S distance of 2.16 Å. Experimental results from a homologous protein ATP7B suggests that reducing reagents are involved, and upon Cu(I) binding the disulfide bonding between the cysteine residues is broken as cysteine starts to bind to Cu(I), leading to a series of conformational changes at the N-terminal of the protein, and possibly activating the Cu(I)-transporting activity of other cytosolic loops. Of the six copper(I)-binding sites, two are considered enough for the function of Cu(I) 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(I) concentration detector. == Transport mechanism ==

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(I) 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(I) release. A subsequent 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(I) is tightly bound to the binding sites on the cytoplasmic side; in the E2 state, the affinity of ATP7A for Cu(I) decreases and Cu(I) is released on the extracellular side. == Function ==

ATP7A is important for regulating copper Cu(I) in mammals. The functions of ATP7A in some tissues of the human body are as follows: == Interactions ==

ATP7A has been shown to interact with ATOX1 and GLRX. Antioxidant 1 copper chaperone (ATOX1) is required to maintain Cu(I) copper homeostasis in the cell. It can bind and transport cytosolic Cu(I) to ATP7A in the trans-Golgi-network. Glutaredoxin-1 (GRX1) has is also essential for ATP7A function. It promotes Cu(I) 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 Cu(I)-binding motifs GMTCXXC. == Clinical significance ==

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 Cu(I). Other mutations insert additional DNA base pairs or use the wrong base pairs, which leads to ATP7A proteins that do not function properly. A comprehensive resource of clinically annotated genetic variants in ATP7A gene has been made available confirming to the American College of Medical Genetics and Genomics guidelines for interpretation of sequence variants. == Inhibition ==

A proton pump inhibitor, Omeprazole, has been shown to block ATP7A, in addition to its more established role of blocking ATP4A. == See also ==