Family 3.A.10 - The H+- Translocating Pyrophosphatase Family       

Family ID: 52654

Proteins of the H+-PPase family are found in the vacuolar (tonoplast) membranes of higher plants, algae, and protozoa, and in both bacteria and archaea. They are therefore ancient enzymes. The plant enzymes probably pump one H+ upon hydrolysis of pyrophosphate, thereby generating a proton motive force, postive and acidic in the tonoplast lumen. They establish a pmf of similar magnitude to that generated by the H+-translocating ATPases in the same vacuolar membrane. The bacterial and archaeal proteins may catalyze fully reversible reactions. The enzyme from R.rubrum contributes to the pmf when light intensity is insufficient to generate a pmf sufficient in magnitude to support rapid ATP synthesis.

Eukaryotic members of the H+-PPase family are large proteins of about 770 amino acyl residues with fifteen putative transmembrane a-helical spanners (TMSs). The N-termini are predicted to be in the vacuolar lumen while the C-termini are thought to be in the cytoplasm. These proteins exhibit a region that shows convincing sequence similarity to the regions surrounding the DCCD-sensitive glutamate in the C-terminal regions of the c-subunits of F-type ATPases (TC # 3.A.2). Several acidic residues in the Arabidopsis H+-PPase have been shown to be important for function. Some plants possess closely related H+-PPase isoforms. These enzymes have the enzyme commission number EC 3.6.1.1.

Full length homologues of the plant PPases have been sequenced from Rhodospirillum rubrum (660 aas), Thermatoga maritima (726aas), Streptomyces coelicolor (794aas), Pyrobaculum aerophilum (721aas), Plasmodium falciparum (717aas), and Acetabularia mediterranea (751aas). A membrane-bound, reversible, proton-translocating pyrophosphatase has been demonstrated biochemically in the benzoate degrading bacterium, Syntrophus gentianae, but the sequence of this protein is not available.