Membrane Na+-pyrophosphatases Can Transport Protons at Low Sodium Concentrations

Abstract

Background: Prokaryotic membrane Na ϩ -PPases couple PP i hydrolysis with active Na ϩ transport.

On the principle of ion selectivity in Na + /H + -coupled membrane proteins: Experimental and theoretical studies of an ATP synthase rotor

“…However, it is likely that some of these systems are actually H + selective, like the Na + -dependent ATP synthases; given that a large excess of Na + over H + is a feature of many natural environments, there is no evolutionary pressure to create binding sites that are highly Na + selective. This intrinsic H + selectivity would explain, for example, the observation that Na + -dependent pyrophosphates are coupled to H + under low Na + concentrations (although still 1,000 times greater than that of H + ) (19). Moreover, whereas Na + coordination typically requires five to six ligands, proton binding requires only an ionizable side chain and a hydrogen bond acceptor, and therefore one or two hydrophobic substitutions in a Na + site may result in H + -coupling, as observed among ATP synthases.…”

“…Indeed, pmf-and smf-driven systems are often found within the same organism, and sometimes with the same or similar function; for example, malate uptake in Bacillus subtilis is mediated by the H + -coupled symporter CimH (14) and by the Na + -coupled MaeN (15). Moreover, specific membrane transporters and enzymes are sometimes coupled to both Na + and H + , either concurrently (using multiple binding sites), such as the multidrug efflux pump NorM of Vibrio cholera (16) and the Methanosarcina acetivorans ATP synthase (17), or alternately (using a single binding site), such as the Escherichia coli melibiose permease (18) and some membraneintegral pyrophosphatases (19).…”

“…Indeed, pmf-and smf-driven systems are often found within the same organism, and sometimes with the same or similar function; for example, malate uptake in Bacillus subtilis is mediated by the H + -coupled symporter CimH (14) and by the Na + -coupled MaeN (15). Moreover, specific membrane transporters and enzymes are sometimes coupled to both Na + and H + , either concurrently (using multiple binding sites), such as the multidrug efflux pump NorM of Vibrio cholera (16) and the Methanosarcina acetivorans ATP synthase (17), or alternately (using a single binding site), such as the Escherichia coli melibiose permease (18) and some membraneintegral pyrophosphatases (19).At the molecular level, the architecture of pmf-and smf-driven membrane proteins within the same family has consistently been found to be largely conserved (20-28). Thus, it appears as if the Na + or H + specificity of these systems is dictated by localized variations in their amino acid sequence, rather than by major structural or mechanistic adaptations.…”

Evolutionarily divergent, Na+-regulated H+-transporting membrane-bound pyrophosphatases

“…In contrast, K + (150 mM) enhanced H + transport activity. Divergent mPPases are therefore fully capable of H + transport and differ from Na + -PPases, which pump H + only at low (<5 mM) Na + concentrations [18].…”

“…The latter transition appears relatively unstable as the phylogenetic clade of Na + ,H + -PPases additionally includes enzymes that apparently have reverted to exclusive Na + pumping [17]. A rationale for the surprising frequency of Na + →H + pumping transition during the mPPase evolutionary history was recently provided by the finding that Na + -PPases are capable of low-efficiency H + transport at sub-physiological Na + concentrations [18]. The evolutionary pressure thus modified a pre-existing function rather than creating it de novo.…”