Abstract
Membrane-bound pyrophosphatases (M-PPases) are homodimeric enzymes that couple the generation and utilization of membrane potentials to pyrophosphate (PP i ) hydrolysis and synthesis. Since the discovery of the link between PP i use and proton transport in purple, non-sulphur bacteria in the 1960s, M-PPases have been found in all three domains of life and have been shown to have a crucial role in stress tolerance and in plant maturation. The discovery of sodium-pumping and sodium/proton-pumping M-PPases showed that the pumping specificity of these enzymes is not limited to protons, further suggesting that M-PPases are evolutionarily very ancient. The recent structures of two M-PPases, the Vigna radiata H + -pumping M-PPase and Thermotoga maritima Na + -pumping M-PPase, provide the basis for understanding the functional data. They show that M-PPases have a novel fold and pumping mechanism, different to the other primary pumps. This review discusses the current structural understanding of M-PPases and of ion selection among various M-PPases. © 2014 Elsevier Ltd.