In the course of evolution, only two or three enzymatic mechanisms  have been invented that can catalyze ATP-dependent active transport across membranes.  P-type ATPases are one of those inventions, and ion-transporting members of this family are well known for their role in nerve and muscle function, and well characterized at atomic resolution.  Surprisingly, the largest subfamily of these transporters are specific for an odd class of ions – the phospholipids that make up the membrane itself.  Active transport of these large amphipathic ions poses unique questions about the mechanism of these enzymes.