ATPase

200px|thumb|[[Adenosine triphosphate]]

200px|thumb|[[Adenosine diphosphate]]

200px|thumb|[[Adenosine monophosphate]]

ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or the inverse reaction. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. This process is widely used in all known forms of life.

Some such enzymes are integral membrane proteins (anchored within biological membranes), and move solutes across the membrane, typically against their concentration gradient. These are called transmembrane ATPases.

Functions

thumb|250px|[[NaKATPase|Na⁺/K⁺ATPase ]]

Transmembrane ATPases import metabolites necessary for cell metabolism and export toxins, wastes, and solutes that can hinder cellular processes. An important example is the sodium-potassium pump (Na⁺/K⁺ATPase) that maintains the cell membrane potential. Another example is the hydrogen potassium ATPase (H⁺/K⁺ATPase or gastric proton pump) that acidifies the contents of the stomach. ATPase is genetically conserved in animals; therefore, cardenolides which are toxic steroids produced by plants that act on ATPases, make general and effective animal toxins that act dose dependently.

Besides exchangers, other categories of transmembrane ATPase include co-transporters and pumps (however, some exchangers are also pumps). Some of these, like the Na⁺/K⁺ATPase, cause a net flow of charge, but others do not. These are called electrogenic transporters and electroneutral transporters, respectively. Genetic variants in ATPases result in a wide spectrum of human diseases, from prenatal to later onset disease.

Structure

The Walker motifs are a telltale protein sequence motif for nucleotide binding and hydrolysis. Beyond this broad function, the Walker motifs can be found in almost all natural ATPases, with the notable exception of tyrosine kinases. The Walker motifs commonly form a Beta sheet-turn-Alpha helix that is self-organized as a Nest (protein structural motif). This is thought to be because modern ATPases evolved from small NTP-binding peptides that had to be self-organized.

Protein design has been able to replicate the ATPase function (weakly) without using natural ATPase sequences or structures. Importantly, while all natural ATPases have some beta-sheet structure, the designed "Alternative ATPase" lacks beta sheet structure, demonstrating that this life-essential function is possible with sequences and structures not found in nature.

Mechanism

ATPase (also called F_oF₁-ATP Synthase) is a charge-transferring complex that catalyzes ATP to perform ATP synthesis by moving ions through the membrane. The number of peripheral stalks is dependent on the type of ATPase: F-ATPases have one, A-ATPases have two, and V-ATPases have three. The F₁ catalytic domain is located on the N-side (negative-side) of the membrane and is involved in the synthesis and degradation of ATP and is involved in oxidative phosphorylation. The F_o transmembrane domain is involved in the movement of ions across the membrane.

The bacterial F_oF₁-ATPase consists of the soluble F₁ domain and the transmembrane F_o domain, which is composed of several subunits with varying stoichiometry. There are two subunits, γ, and ε, that form the central stalk and they are linked to F_o. F_o contains a c-subunit oligomer in the shape of a ring (c-ring). The α subunit is close to the subunit b₂ and makes up the stalk that connects the transmembrane subunits to the α3β3 and δ subunits. F-ATP synthases are identical in appearance and function except for the mitochondrial F_oF₁-ATP synthase, which contains 7-9 additional subunits.

• F-ATPases (F1FO-ATPases) in mitochondria, chloroplasts and bacterial plasma membranes are the prime producers of ATP, using the proton gradient generated by oxidative phosphorylation (mitochondria) or photosynthesis (chloroplasts).
• F-ATPases lacking a delta/OSCP subunit move sodium ions instead. They are proposed to be called N-ATPases, since they seem to form a distinct group that is further apart from usual F-ATPases than A-ATPases are from V-ATPases.
• V-ATPases (V1VO-ATPases) are primarily found in eukaryotic vacuoles, catalysing ATP hydrolysis to transport solutes and lower pH in organelles like proton pump of lysosome.
• A-ATPases (A1AO-ATPases) are found in Archaea and some extremophilic bacteria. They are arranged like V-ATPases, but function like F-ATPases mainly as ATP synthases.
• Many homologs that are not necessarily rotaty exist. See .
• P-ATPases (E1E2-ATPases) are found in bacteria, fungi and in eukaryotic plasma membranes and organelles, and function to transport a variety of different ions across membranes.
• E-ATPases are cell-surface enzymes that hydrolyze a range of NTPs, including extracellular ATP. Examples include ecto-ATPases, CD39s, and ecto-ATP/Dases, all of which are members of a "GDA1 CD39" superfamily.
• AAA proteins are a family of ring-shaped P-loop NTPases.

P-ATPase

P-ATPases (sometime known as E1-E2 ATPases) are found in bacteria and also in eukaryotic plasma membranes and organelles. Its name is due to short time attachment of inorganic phosphate at the aspartate residues at the time of activation. Function of P-ATPase is to transport a variety of different compounds, like ions and phospholipids, across a membrane using ATP hydrolysis for energy. There are many different classes of P-ATPases, which transports a specific type of ion. P-ATPases may be composed of one or two polypeptides, and can usually take two main conformations, E1 and E2.

Human genes

• Na⁺/K⁺ transporting: ATP1A1, ATP1A2, ATP1A3, ATP1A4, ATP1B1, ATP1B2, ATP1B3, ATP1B4
• Ca²⁺ transporting: ATP2A1, ATP2A2, ATP2A3, ATP2B1, ATP2B2, ATP2B3, ATP2B4, ATP2C1, ATP2C2
• H⁺/K⁺ exchanging: ATP4A
• H⁺ transporting, mitochondrial: ATP5F1A, ATP5F1B, ATP5F1C, ATP5C2, ATP5F1D, ATP5F1E, ATP5F1, ATP5MC1, ATP5MC2, ATP5MC3, ATP5PD, ATP5ME, ATP5PF, ATP5MF, ATP5MG, ATP5L2, ATP5PO, ATP5S, MT-ATP6, MT-ATP8
• H⁺ transporting, lysosomal: ATP6AP1, ATP6AP2, ATP6V1A, ATP6V1B1, ATP6V1B2, ATP6V1C1, ATP6V1C2, ATP6V1D, ATP6V1E1, ATP6V1E2, ATP6V1F, ATP6V1G1, ATP6V1G2, ATP6V1G3, ATP6V1H, ATP6V0A1, ATP6V0A2, ATP6V0A4, ATP6V0B, ATP6V0C, ATP6V0D1, ATP6V0D2, ATP6V0E
• Cu²⁺ transporting: ATP7A, ATP7B
• Class I, type 8: ATP8A1, ATP8B1, ATP8B2, ATP8B3, ATP8B4
• Class II, type 9: ATP9A, ATP9B
• Class V, type 10: ATP10A, ATP10B, ATP10D
• Class VI, type 11: ATP11A, ATP11B, ATP11C
• H⁺/K⁺ transporting, nongastric: ATP12A
• type 13: ATP13A1, ATP13A2, ATP13A3, ATP13A4, ATP13A5

See also

• ATP synthase
• ATP synthase alpha/beta subunits
• AAA proteins
• P-ATPase

References

External links

• "ATP synthase - a splendid molecular machine"
• Electron microscopy structures of ATPases from the EM Data Bank(EMDB)