Structural Biochemistry, Membrane Proteins, P-Type ATPases

Calcium ATPase (Image: Andrei Lomize, CC BY-SA 3.0)

Sodium-Potassium ATPase (Image: Andrei Lomize, CC BY-SA 3.0)

P-type ATPases are a family of cation transport enzymes present in eukaryotes, archea and bacteria. These alpha helical bundle primary transportser (P-type ATPases) are known for catalysis of self phosphorylation of a key conserved aspartate residue within an ion pump. They generate essential ion gradients that are the basis for a variety of cellular functions like signaling, energy storage and secondary transport. Prominent examples of P-type ATPases are the sodium-potassium pump(Na+,K+-ATPase), the proton pump (H+-ATPase), the proton-potassium pump (H+,K+-ATPase) and the calcium pump (Ca2+-ATPase).

Discovered in 1957 by Nobel Laureate Jens Christian Skou, the first P-type ATPase was the Na+, K+ - ATPase. As of June 2007, 400 unique and confirmed members of the P-type ATPase family have been discovered.

These enzymes have a significant impact on cellular function. Indeed, more than one third of the ATP consumed by a resting animal is used by a single ion pump, the Na+,K+-ATPase. The calcium pump (located in the sarcoplasmic reticulum (SR) membrane) also plays a significant role because it is responsible for pumping calcium from the muscle cell to cause a muscle to relax.

P-type ATPases can be divided into five phylogenetic subfamilies:

• Type I — Transition/heavy metal ATPases. These ATPases are involved in transport of metals such as K+, Cu+, Ag+, Cu2+, Zn2+, Cd2+, Pb2+ and Co2+.
• Type II — Includes Ca2+-ases, such as SERCA1a. Also includes Na+/K+ and H+/K+ ATPases.

Type IIA and IIB transports Ca2+.

• Type IIC contains Na+/K+ and H+/K+ ATPases from animal cells.
• Type IID contains few fungal ATPases of unknown function.
• Type III — Includes plasma membrane H+-ATPases from plants and fungi and Mg2+-ATPases from three bacterial species
• Type IV — Includes ATPases have been shown to be involved in the transport of phospholipids.
• Type V — ATPases with unknown specificity. This large group are only found in eukaryotes and are believed to be involved in cation transport in the endoplasmic reticulum.

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