H+‐PPases: a tightly membrane‐bound family

Abstract

The earliest known H⁺‐PPase (proton‐pumping inorganic pyrophosphatase), the integrally membrane‐bound H⁺‐PPi synthase (proton‐pumping inorganic pyrophosphate synthase) from Rhodospirillum rubrum, is still the only alternative to H⁺‐ATP synthase in biological electron transport phosphorylation. Cloning of several higher plant vacuolar H⁺‐PPase genes has led to the recognition that the corresponding proteins form a family of extremely similar proton‐pumping enzymes. The bacterial H⁺‐PPi synthase and two algal vacuolar H⁺‐PPases are homologous with this family, as deduced from their cloned genes. The prokaryotic and algal homologues differ more than the H⁺‐PPases from higher plants, facilitating recognition of functionally significant entities. Primary structures of H⁺‐PPases are reviewed and compared with H⁺‐ATPases and soluble PPases.