ATP7A

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:

1. Eight transmembrane segments that form a channel and allow for Cu(I) to pass through the membrane;
2. An ATP-binding domain;
3. 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. This protein is found in most tissues, but it is not expressed in the liver. In the small intestine, the ATP7A protein helps control the absorption of Cu(I) from food. After Cu(I) ions are absorbed into enterocytes, ATP7A is required to transfer them across the basolateral membrane into the circulation.

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, ATP7A supplies Cu(I) to certain enzymes (e.g. peptidyl-α-monooxygenase, tyrosinase, and lysyl oxidase) that are critical for the structures and functions of brain, bone, skin, hair, connective tissue, and the nervous system. If Cu(I) levels in the cell environment are elevated, however, ATP7A moves to the cell membrane and eliminates excess Cu(I) from the cell.

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.

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. Copper is also critical for the propagation of prion proteins, and mice with mutations in Atp7a have a delayed onset of prion disease. 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.