3.A.10 The H⁺-translocating Pyrophosphatase (H⁺-PPase) Family 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, positive 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, thus being able to synthesize pyrophosphate when the pmf is sufficient. 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 α-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 members of the H⁺-PPase family have been sequenced from numerous bacteria, archaea and eukaryotes. These H⁺ pumping enzymes, which are probably homodimeric, have been reported to fall into two phylogenetic subfamilies (Belogurov et al., 2002). One subfamily invariably contains a conserved cysteine (Cys²²²) and includes all known K⁺-independent H⁺-PPases while the other has another conserved cysteine (Cys⁵⁷³) but lacks Cys²²² and includes all known K⁺-dependent H⁺-PPases (Belogurov et al., 2000). All H⁺-PPases require Mg²⁺, and those from plant vacuoles, acidocalcisomes of protozoa and fermentative bacteria require mM K⁺. Those from respiratory and photosynthetic bacteria as well as archaea do not require K⁺ or are less sensitive to it. However, exceptions may exist (Belogurov et al., 2000). It is not sure whether K⁺ is transported. The archaeon, Methansarcina mazei Gö1, encodes within its genome two H⁺-translocating pyrophosphatases, Mvp1 and Mvp2. Mvp1 resembles bacterial PPases while Mvp2 resembles plant PPases (Bäumer et al., 2002). Mvp2 was shown to translocate 1 H⁺ per pyrophosphate hydrolyzed. The generalized transport reaction catalyzed by H⁺-PPases is: pyrophosphate (P₂) + H₂O + H⁺ (cytoplasm) → inorganic phosphate (2 P_i) + H⁺ (vacuolar lumen).
References:
Baltscheffsky, M. and H. Baltscheffsky. (1993). Inorganic pyrophosphate and inorganic pyrophosphatases. In: L. Ernster ( ed.), Molecular Mechanisms in Bioenergetics. Amsterdam, The Netherlands: Elsevier Science Publishers B.V., pp. 331-348.
Baltscheffsky, M., A. Schultz, and H. Baltscheffsky. (1999). H⁺-PPases: a tightly membrane-bound family. FEBS Lett. 457: 527-533.
Belogurov, G.A., M.V. Turkina, A. Penttinen, S. Huopalahti, A.A. Baykov, and R. Lahti. (2002). H⁺-pyrophosphatase of Rhodospirillum rubrum. High yield expression in Escherichia coli and identification of the Cys residues responsible for inactivation by mersalyl. J. Biol. Chem. 277: 22209-22214.
Bäumer, S., S. Lentes, G. Gottschalk, and U. Deppenmeier. (2002). Identification and analysis of proton-translocating pyrophosphatases in the methanogenic archaeon Methansarcina masei. Archaea 1: 1-7.
Drozdowicz, Y.M., Y.-P. Lu, V. Patel, S. Fitz-Gibbon, J.H. Miller, and P.A. Rea. (1999). A thermostable vacuolar-type membrane pyrophosphatase from the archaeon Pyrobaculum aerophilum: implication for the origins of pyrophosphate-energized pumps. FEBS Lett. 460: 505-512.
Rea, P.A. and R.J. Poole. (1993). Vacuolar H⁺-translocating pyrophosphatase. Ann. Rev. Plant Physiol. Plant Mol. Biol. 44: 157-180.
Ruiz, F.A., N. Marchesini, M. Seufferheld, Govindjee, and R. Docampo. (2001). The polyphosphate bodies of Chlamydomonas reinhardtii possess a proton-pumping pyrophosphatase and are similar to acidocalcisomes. J. Biol. Chem. 276: 46196-46203.
Schöcke, L. and B. Schink. (1998). Membrane-bound proton-translocating pyrophosphatase of Synthrophus gentianae,a syntrophically benzoate-degrading fermenting bacterium. Eur. J. Biochem. 256: 589-594.
Zhen, R.-G., E.J. Kim, and P.A. Rea. (1997a). Acidic residues necessary for pyrophosphate-energized pumping and inhibition of the vacuolar H⁺-pyrophosphatase by N,N'-dicyclohexylcarbodiimide. J. Biol. Chem. 272: 22340-22348.
Zhen, R.-G., E.J. Kim, and P.A. Rea. (1997b). The molecular and biochemical basis of pyrophosphate-energized proton translocation at the vacuolar membrane. Adv. Bot. Res. 25: 297-337.
Examples:
TC#	Name	Organismal Type	Example
3.A.10.1.1	H⁺-translocating vacuolar pyrophosphatase	Plant vacuoles	V-PPase of Arabidopsis thaliana

3.A.10.1.2	H⁺-translocating acidocalcisome pyrophosphatase	Algae	V-PPase of Chlamydomonas reinhardtii

3.A.10.2.1	H⁺-translocating pyrophosphate synthase	Bacteria	H⁺-PPase of Rhodospirillum rubrum

3.A.10.3.1	H+-translocating pyrophosphatase	Archaea	H⁺-PPase of Pyrobaculum aerophilum