Pre-steady-state kinetics and solvent isotope effects support the "billiard-type" transport mechanism in Na+ -translocating pyrophosphatase.

Membrane-bound pyrophosphatase (mPPase) found in microbes and plants is a membrane H+ pump that transports the H+ ion generated in coupled pyrophosphate hydrolysis out of the cytoplasm. Certain bacterial and archaeal mPPases can in parallel transport Na+ via a hypothetical "billiard-type" mechanism, also involving the hydrolysis-generated proton. Here, we present the functional evidence supporting this coupling mechanism. Rapid-quench and pulse-chase measurements with [32 P]pyrophosphate indicated that the chemical step (pyrophosphate hydrolysis) is rate-limiting in mPPase catalysis and is preceded by a fast isomerization of the enzyme-substrate complex. Na+ , whose binding is a prerequisite for the hydrolysis step, is not required for substrate binding. Replacement of H2 O with D2 O decreased the rates of pyrophosphate hydrolysis by both Na+ - and H+ -transporting bacterial mPPases, the effect being more significant than with a non-transporting soluble pyrophosphatase. We also show that the Na+ -pumping mPPase of Thermotoga maritima resembles other dimeric mPPases in demonstrating negative kinetic cooperativity and the requirement for general acid catalysis. The findings point to a crucial role for the hydrolysis-generated proton both in H+ -pumping and Na+ -pumping by mPPases.

The tetrameric structure of nucleotide-regulated pyrophosphatase and its modulation by deletion mutagenesis and ligand binding.

A quarter of prokaryotic Family II inorganic pyrophosphatases (PPases) contain a regulatory insert comprised of two cystathionine ß-synthase (CBS) domains and one DRTGG domain in addition to the two catalytic domains that form canonical Family II PPases. The CBS domain-containing PPases (CBS-PPases) are allosterically activated or inhibited by adenine nucleotides that cooperatively bind to the CBS domains. Here we use chemical cross-linking and analytical ultracentrifugation to show that CBS-PPases from Desulfitobacterium hafniense and four other bacterial species are active as 200-250-kDa homotetramers, which seems unprecedented among the four PPase families. The tetrameric structure is stabilized by Co2+, the essential cofactor, pyrophosphate, the substrate, and adenine nucleotides, including diadenosine tetraphosphate. The deletion variants of dhPPase containing only catalytic or regulatory domains are dimeric. Co2+ depletion by incubation with EDTA converts CBS-PPase into inactive tetrameric and dimeric forms. Dissociation of tetrameric CBS-PPase and its catalytic part by dilution renders them inactive. The structure of CBS-PPase tetramer was modelled from the structures of dimeric catalytic and regulatory parts. These findings signify the role of the unique oligomeric structure of CBS-PPase in its multifaced regulation.

Residue Network Involved in the Allosteric Regulation of Cystathionine ß-Synthase Domain-Containing Pyrophosphatase by Adenine Nucleotides.

Inorganic pyrophosphatase containing regulatory cystathionine ß-synthase (CBS) domains (CBS-PPase) is inhibited by adenosine monophosphate (AMP) and adenosine diphosphate and activated by adenosine triphosphate (ATP) and diadenosine polyphosphates; mononucleotide binding to CBS domains and substrate binding to catalytic domains are characterized by positive cooperativity. This behavior implies three pathways for regulatory signal transduction - between regulatory and active sites, between two active sites, and between two regulatory sites. Bioinformatics analysis pinpointed six charged or polar amino acid residues of Desulfitobacterium hafniense CBS-PPase as potentially important for enzyme regulation. Twelve mutant enzyme forms were produced, and their kinetics of pyrophosphate hydrolysis was measured in wide concentration ranges of the substrate and various adenine nucleotides. The parameters derived from this analysis included catalytic activity, Michaelis constants for two active sites, AMP-, ATP-, and diadenosine tetraphosphate-binding constants for two regulatory sites, and the degree of activation/inhibition for each nucleotide. Replacements of arginine 295 and asparagine 312 by alanine converted ATP from an activator to an inhibitor and markedly affected practically all the above parameters, indicating involvement of these residues in all the three regulatory signaling pathways. Replacements of asparagine 312 and arginine 334 abolished or reversed kinetic cooperativity in the absence of nucleotides but conferred it in the presence of diadenosine tetraphosphate, without effects on nucleotide-binding parameters. Modeling and molecular dynamics simulations revealed destabilization of the subunit interface as a result of asparagine 312 and arginine 334 replacements by alanine, explaining abolishment of kinetic cooperativity. These findings identify residues 295, 312, and 334 as crucial for CBS-PPase regulation via CBS domains.

Cooperativity in catalysis by canonical family II pyrophosphatases.

Bacterial family II pyrophosphatases (PPases) are homodimeric enzymes, with the active site located between two catalytic domains. Some family II PPases additionally contain regulatory cystathionine ß-synthase (CBS) domains and exhibit positive kinetic cooperativity, which is lost upon CBS domain removal. We report here that CBS domain-deficient family II PPases of Bacillus subtilis and Streptococcus gordonii also exhibit positive kinetic cooperativity, manifested as an up to a five-fold difference in the Michaelis constants for two active sites. An Asn79Ser replacement in S. gordonii PPase preserved its dimeric structure but abolished cooperativity. The results of our study indicated that kinetic cooperativity is an inherent property of all family II PPase types, is not induced by CBS domains, and is sensitive to minor structural changes. These findings may have inferences for other CBS-proteins, which include important enzymes and membrane transporters associated with hereditary diseases.

An arginine residue involved in allosteric regulation of cystathionine ß-synthase (CBS) domain-containing pyrophosphatase.

Inorganic pyrophosphatase containing a pair of regulatory CBS domains (CBS-PPase1) is allosterically inhibited by AMP and ADP and activated by ATP and diadenosine polyphosphates. Mononucleotide binding to CBS domains and substrate binding to catalytic domains are characterized by positive co-operativity. Bioinformatics analysis pinpointed a conserved arginine residue at the interface of the regulatory and catalytic domains in bacterial CBS-PPases as potentially involved in enzyme regulation. The importance of this residue was assessed by site-directed mutagenesis using the CBS-PPase from Desulfitobacterium hafniense (dhPPase) as a model. The mutants R276A, R276K and R276E were constructed and purified, and the impact of the respective mutation on catalysis, nucleotide binding and regulation was analysed. Overall, the effects decreased in the following order R276A > R276E > R276K. The variants retained ≥50% catalytic efficiency but exhibited reduced kinetic co-operativity or even its inversion (R276A). Negative co-operativity was retained in the R276A variant in the presence of mononucleotides but was reversed by diadenosine tetraphosphate. Positive nucleotide-binding co-operativity was retained in all variants but the R276A and R276E variants exhibited a markedly reduced affinity to AMP and ADP and greater residual activity at their saturating concentrations. The R276A substitution abolished activation by ATP and diadenosine tetraphosphate, while preserving the ability to bind them. The results suggest that the H-bond formed by the Arg276 sidechain is essential for signal transduction between the regulatory and catalytic domains within one subunit and between the catalytic but not regulatory domains of different subunits.

Role of the potassium/lysine cationic center in catalysis and functional asymmetry in membrane-bound pyrophosphatases.

Membrane-bound pyrophosphatases (mPPases), which couple pyrophosphate hydrolysis to transmembrane transport of H+ and/or Na+ ions, are divided into K+,Na+-independent, Na+-regulated, and K+-dependent families. The first two families include H+-transporting mPPases (H+-PPases), whereas the last family comprises one Na+-transporting, two Na+- and H+-transporting subfamilies (Na+-PPases and Na+,H+-PPases, respectively), and three H+-transporting subfamilies. Earlier studies of the few available model mPPases suggested that K+ binds to a site located adjacent to the pyrophosphate-binding site, but is substituted by the ε-amino group of an evolutionarily acquired lysine residue in the K+-independent mPPases. Here, we performed a systematic analysis of the K+/Lys cationic center across all mPPase subfamilies. An Ala → Lys replacement in K+-dependent mPPases abolished the K+ dependence of hydrolysis and transport activities and decreased these activities close to the level (4-7%) observed for wild-type enzymes in the absence of monovalent cations. In contrast, a Lys → Ala replacement in K+,Na+-independent mPPases conferred partial K+ dependence on the enzyme by unmasking an otherwise conserved K+-binding site. Na+ could partially replace K+ as an activator of K+-dependent mPPases and the Lys → Ala variants of K+,Na+-independent mPPases. Finally, we found that all mPPases were inhibited by excess substrate, suggesting strong negative co-operativity of active site functioning in these homodimeric enzymes; moreover, the K+/Lys center was identified as part of the mechanism underlying this effect. These findings suggest that the mPPase homodimer possesses an asymmetry of active site performance that may be an ancient prototype of the rotational binding-change mechanism of F-type ATPases.

Inorganic pyrophosphatases of Family II-two decades after their discovery.

Inorganic pyrophosphatases (PPases) convert pyrophosphate (PPi ) to phosphate and are present in all cell types. Soluble PPases belong to three nonhomologous families, of which Family II is found in approximately a quarter of prokaryotic organisms, often pathogenic ones. Each subunit of dimeric canonical Family II PPases is formed by two domains connected by a flexible linker, with the active site located between the domains. These enzymes require both magnesium and a transition metal ion (manganese or cobalt) for maximal activity and are the most active (kcat ≈ 104 s-1 ) among all PPase types. Catalysis by Family II PPases requires four metal ions per substrate molecule, three of which form a unique trimetal center that coordinates the nucleophilic water and converts it to a reactive hydroxide ion. A quarter of Family II PPases contain an autoinhibitory regulatory insert formed by two cystathionine ß-synthase (CBS) domains and one DRTGG domain. Adenine nucleotide binding either activates or inhibits the CBS domain-containing PPases, thereby tuning their activity and, hence, PPi levels, in response to changes in cell energy status (ATP/ADP ratio).

Two independent evolutionary routes to Na+/H+ cotransport function in membrane pyrophosphatases.

Membrane-bound pyrophosphatases (mPPases) hydrolyze pyrophosphate (PPi) to transport H(+), Na(+) or both and help organisms to cope with stress conditions, such as high salinity or limiting nutrients. Recent elucidation of mPPase structure and identification of subfamilies that have fully or partially switched from Na(+) to H(+) pumping have established mPPases as versatile models for studying the principles governing the mechanism, specificity and evolution of cation transporters. In the present study, we constructed an accurate phylogenetic map of the interface of Na(+)-transporting PPases (Na(+)-PPases) and Na(+)- and H(+)-transporting PPases (Na(+),H(+)-PPases), which guided our experimental exploration of the variations in PPi hydrolysis and ion transport activities during evolution. Surprisingly, we identified two mPPase lineages that independently acquired physiologically significant Na(+) and H(+) cotransport function. Na(+),H(+)-PPases of the first lineage transport H(+) over an extended [Na(+)] range, but progressively lose H(+) transport efficiency at high [Na(+)]. In contrast, H(+)-transport by Na(+),H(+)-PPases of the second lineage is not inhibited by up to 100 mM Na(+) With the identification of Na(+),H(+)-PPase subtypes, the mPPases protein superfamily appears as a continuum, ranging from monospecific Na(+) transporters to transporters with tunable levels of Na(+) and H(+) cotransport and further to monospecific H(+) transporters. Our results lend credence to the concept that Na(+) and H(+) are transported by similar mechanisms, allowing the relative efficiencies of Na(+) and H(+) transport to be modulated by minor changes in protein structure during the course of adaptation to a changing environment.

An asparagine residue mediates intramolecular communication in nucleotide-regulated pyrophosphatase.

Many prokaryotic soluble PPases (pyrophosphatases) contain a pair of regulatory adenine nucleotide-binding CBS (cystathionine ß-synthase) domains that act as 'internal inhibitors' whose effect is modulated by nucleotide binding. Although such regulatory domains are found in important enzymes and transporters, the underlying regulatory mechanism has only begun to come into focus. We reported previously that CBS domains bind nucleotides co-operatively and induce positive kinetic co-operativity (non-Michaelian behaviour) in CBS-PPases (CBS domain-containing PPases). In the present study, we demonstrate that a homodimeric ehPPase (Ethanoligenens harbinense PPase) containing an inherent mutation in an otherwise conserved asparagine residue in a loop near the active site exhibits non-co-operative hydrolysis kinetics. A similar N312S substitution in 'co-operative' dhPPase (Desulfitobacterium hafniense PPase) abolished kinetic co-operativity while causing only minor effects on nucleotide-binding affinity and co-operativity. However, the substitution reversed the effect of diadenosine tetraphosphate, abolishing kinetic co-operativity in wild-type dhPPase, but restoring it in the variant dhPPase. A reverse serine-to-asparagine replacement restored kinetic co-operativity in ehPPase. Molecular dynamics simulations revealed that the asparagine substitution resulted in a change in the hydrogen-bonding pattern around the asparagine residue and the subunit interface, allowing greater flexibility at the subunit interface without a marked effect on the overall structure. These findings identify this asparagine residue as lying at the 'crossroads' of information paths connecting catalytic and regulatory domains within a subunit and catalytic sites between subunits.

Cystathionine ß-Synthase (CBS) Domain-containing Pyrophosphatase as a Target for Diadenosine Polyphosphates in Bacteria.

Among numerous proteins containing pairs of regulatory cystathionine ß-synthase (CBS) domains, family II pyrophosphatases (CBS-PPases) are unique in that they generally contain an additional DRTGG domain between the CBS domains. Adenine nucleotides bind to the CBS domains in CBS-PPases in a positively cooperative manner, resulting in enzyme inhibition (AMP or ADP) or activation (ATP). Here we show that linear P(1),P(n)-diadenosine 5'-polyphosphates (ApnAs, where n is the number of phosphate residues) bind with nanomolar affinity to DRTGG domain-containing CBS-PPases of Desulfitobacterium hafniense, Clostridium novyi, and Clostridium perfringens and increase their activity up to 30-, 5-, and 7-fold, respectively. Ap4A, Ap5A, and Ap6A bound noncooperatively and with similarly high affinities to CBS-PPases, whereas Ap3A bound in a positively cooperative manner and with lower affinity, like mononucleotides. All ApnAs abolished kinetic cooperativity (non-Michaelian behavior) of CBS-PPases. The enthalpy change and binding stoichiometry, as determined by isothermal calorimetry, were ~10 kcal/mol nucleotide and 1 mol/mol enzyme dimer for Ap4A and Ap5A but 5.5 kcal/mol and 2 mol/mol for Ap3A, AMP, ADP, and ATP, suggesting different binding modes for the two nucleotide groups. In contrast, Eggerthella lenta and Moorella thermoacetica CBS-PPases, which contain no DRTGG domain, were not affected by ApnAs and showed no enthalpy change, indicating the importance of the DTRGG domain for ApnA binding. These findings suggest that ApnAs can control CBS-PPase activity and hence affect pyrophosphate level and biosynthetic activity in bacteria.

Cytochrome cbb3 of Thioalkalivibrio is a Na+-pumping cytochrome oxidase.

Cytochrome c oxidases (Coxs) are the basic energy transducers in the respiratory chain of the majority of aerobic organisms. Coxs studied to date are redox-driven proton-pumping enzymes belonging to one of three subfamilies: A-, B-, and C-type oxidases. The C-type oxidases (cbb3 cytochromes), which are widespread among pathogenic bacteria, are the least understood. In particular, the proton-pumping machinery of these Coxs has not yet been elucidated despite the availability of X-ray structure information. Here, we report the discovery of the first (to our knowledge) sodium-pumping Cox (Scox), a cbb3 cytochrome from the extremely alkaliphilic bacterium Thioalkalivibrio versutus. This finding offers clues to the previously unknown structure of the ion-pumping channel in the C-type Coxs and provides insight into the functional properties of this enzyme.

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

Membrane-bound pyrophosphatase (mPPases) of various types consume pyrophosphate (PPi) to drive active H+ or Na+ transport across membranes. H+-transporting PPases are divided into phylogenetically distinct K+-independent and K+-dependent subfamilies. In the present study, we describe a group of 46 bacterial proteins and one archaeal protein that are only distantly related to known mPPases (23%-34% sequence identity). Despite this evolutionary divergence, these proteins contain the full set of 12 polar residues that interact with PPi, the nucleophilic water and five cofactor Mg2+ ions found in 'canonical' mPPases. They also contain a specific lysine residue that confers K+ independence on canonical mPPases. Two of the proteins (from Chlorobium limicola and Cellulomonas fimi) were expressed in Escherichia coli and shown to catalyse Mg2+-dependent PPi hydrolysis coupled with electrogenic H+, but not Na+ transport, in inverted membrane vesicles. Unique features of the new H+-PPases include their inhibition by Na+ and inhibition or activation, depending on PPi concentration, by K+ ions. Kinetic analyses of PPi hydrolysis over wide ranges of cofactor (Mg2+) and substrate (Mg2-PPi) concentrations indicated that the alkali cations displace Mg2+ from the enzyme, thereby arresting substrate conversion. These data define the new proteins as a novel subfamily of H+-transporting mPPases that partly retained the Na+ and K+ regulation patterns of their precursor Na+-transporting mPPases.

Cystathionine ß-synthase (CBS) domains confer multiple forms of Mg2+-dependent cooperativity to family II pyrophosphatases.

Regulated family II pyrophosphatases (CBS-PPases) contain a nucleotide-binding insert comprising a pair of cystathionine ß-synthase (CBS) domains, termed a Bateman module. By binding with high affinity to the CBS domains, AMP and ADP usually inhibit the enzyme, whereas ATP activates it. Here, we demonstrate that AMP, ADP, and ATP bind in a positively cooperative manner to CBS-PPases from four bacteria: Desulfitobacterium hafniense, Clostridium novyi, Clostridium perfringens, and Eggerthella lenta. Enzyme interaction with substrate as characterized by the Michaelis constant (Km) also exhibited positive catalytic cooperativity that decreased in magnitude upon nucleotide binding. The degree of both types of cooperativity increased with increasing concentration of the cofactor Mg(2+) except for the C. novyi PPase where Mg(2+) produced the opposite effect on kinetic cooperativity. Further exceptions from these general rules were ADP binding to C. novyi PPase and AMP binding to E. lenta PPase, neither of which had any effect on activity. A genetically engineered deletion variant of D. hafniense PPase lacking the regulatory insert was fully active but differed from the wild-type enzyme in that it was insensitive to nucleotides and bound substrate non-cooperatively and with a smaller Km value. These results indicate that the regulatory insert acts as an internal inhibitor and confers dual positive cooperativity to CBS domain-containing PPases, making them highly sensitive regulators of the PPi level in response to the changes in cell energy status that control adenine nucleotide distribution. These regulatory features may be common among other CBS domain-containing proteins.

Membrane Na+-pyrophosphatases can transport protons at low sodium concentrations.

Membrane-bound Na(+)-pyrophosphatase (Na(+)-PPase), working in parallel with the corresponding ATP-energized pumps, catalyzes active Na(+) transport in bacteria and archaea. Each ~75-kDa subunit of homodimeric Na(+)-PPase forms an unusual funnel-like structure with a catalytic site in the cytoplasmic part and a hydrophilic gated channel in the membrane. Here, we show that at subphysiological Na(+) concentrations (<5 mM), the Na(+)-PPases of Chlorobium limicola, four other bacteria, and one archaeon additionally exhibit an H(+)-pumping activity in inverted membrane vesicles prepared from recombinant Escherichia coli strains. H(+) accumulation in vesicles was measured with fluorescent pH indicators. At pH 6.2-8.2, H(+) transport activity was high at 0.1 mM Na(+) but decreased progressively with increasing Na(+) concentrations until virtually disappearing at 5 mM Na(+). In contrast, (22)Na(+) transport activity changed little over a Na(+) concentration range of 0.05-10 mM. Conservative substitutions of gate Glu(242) and nearby Ser(243) and Asn(677) residues reduced the catalytic and transport functions of the enzyme but did not affect the Na(+) dependence of H(+) transport, whereas a Lys(681) substitution abolished H(+) (but not Na(+)) transport. All four substitutions markedly decreased PPase affinity for the activating Na(+) ion. These results are interpreted in terms of a model that assumes the presence of two Na(+)-binding sites in the channel: one associated with the gate and controlling all enzyme activities and the other located at a distance and controlling only H(+) transport activity. The inherent H(+) transport activity of Na(+)-PPase provides a rationale for its easy evolution toward specific H(+) transport.

Pyrophosphate-fueled Na+ and H+ transport in prokaryotes.

In its early history, life appeared to depend on pyrophosphate rather than ATP as the source of energy. Ancient membrane pyrophosphatases that couple pyrophosphate hydrolysis to active H(+) transport across biological membranes (H(+)-pyrophosphatases) have long been known in prokaryotes, plants, and protists. Recent studies have identified two evolutionarily related and widespread prokaryotic relics that can pump Na(+) (Na(+)-pyrophosphatase) or both Na(+) and H(+) (Na(+),H(+)-pyrophosphatase). Both these transporters require Na(+) for pyrophosphate hydrolysis and are further activated by K(+). The determination of the three-dimensional structures of H(+)- and Na(+)-pyrophosphatases has been another recent breakthrough in the studies of these cation pumps. Structural and functional studies have highlighted the major determinants of the cation specificities of membrane pyrophosphatases and their potential use in constructing transgenic stress-resistant organisms.

Membrane-integral pyrophosphatase subfamily capable of translocating both Na+ and H+.

One of the strategies used by organisms to adapt to life under conditions of short energy supply is to use the by-product pyrophosphate to support cation gradients in membranes. Transport reactions are catalyzed by membrane-integral pyrophosphatases (PPases), which are classified into two homologous subfamilies: H(+)-transporting (found in prokaryotes, protists, and plants) and Na(+)-transporting (found in prokaryotes). Transport activities have been believed to require specific machinery for each ion, in accordance with the prevailing paradigm in membrane transport. However, experiments using a fluorescent pH probe and (22)Na(+) measurements in the current study revealed that five bacterial PPases expressed in Escherichia coli have the ability to simultaneously translocate H(+) and Na(+) into inverted membrane vesicles under physiological conditions. Consistent with data from phylogenetic analyses, our results support the existence of a third, dual-specificity bacterial Na(+),H(+)-PPase subfamily, which apparently evolved from Na(+)-PPases. Interestingly, genes for Na(+),H(+)-PPase have been found in the major microbes colonizing the human gastrointestinal tract. The Na(+),H(+)-PPases require Na(+) for hydrolytic and transport activities and are further activated by K(+). Based on ionophore effects, we conclude that the Na(+) and H(+) transport reactions are electrogenic and do not result from secondary antiport effects. Sequence comparisons further disclosed four Na(+),H(+)-PPase signature residues located outside the ion conductance channel identified earlier in PPases using X-ray crystallography. Our results collectively support the emerging paradigm that both Na(+) and H(+) can be transported via the same mechanism, with switching between Na(+) and H(+) specificities requiring only subtle changes in the transporter structure.

The CBS domain: a protein module with an emerging prominent role in regulation.

Regulatory CBS (cystathionine ß-synthase) domains exist as two or four tandem copies in thousands of cytosolic and membrane-associated proteins from all kingdoms of life. Mutations in the CBS domains of human enzymes and membrane channels are associated with an array of hereditary diseases. Four CBS domains encoded within a single polypeptide or two identical polypeptides (each having a pair of CBS domains at the subunit interface) form a highly conserved disk-like structure. CBS domains act as autoinhibitory regulatory units in some proteins and activate or further inhibit protein function upon binding to adenosine nucleotides (AMP, ADP, ATP, S-adenosyl methionine, NAD, diadenosine polyphosphates). As a result of the differential effects of the nucleotides, CBS domain-containing proteins can sense cell energy levels. Significant conformational changes are induced in CBS domains by bound ligands, highlighting the structural basis for their effects.

Na+-translocating membrane pyrophosphatases are widespread in the microbial world and evolutionarily precede H+-translocating pyrophosphatases.

Membrane pyrophosphatases (PPases), divided into K(+)-dependent and K(+)-independent subfamilies, were believed to pump H(+) across cell membranes until a recent demonstration that some K(+)-dependent PPases function as Na(+) pumps. Here, we have expressed seven evolutionarily important putative PPases in Escherichia coli and estimated their hydrolytic, Na(+) transport, and H(+) transport activities as well as their K(+) and Na(+) requirements in inner membrane vesicles. Four of these enzymes (from Anaerostipes caccae, Chlorobium limicola, Clostridium tetani, and Desulfuromonas acetoxidans) were identified as K(+)-dependent Na(+) transporters. Phylogenetic analysis led to the identification of a monophyletic clade comprising characterized and predicted Na(+)-transporting PPases (Na(+)-PPases) within the K(+)-dependent subfamily. H(+)-transporting PPases (H(+)-PPases) are more heterogeneous and form at least three independent clades in both subfamilies. These results suggest that rather than being a curious rarity, Na(+)-PPases predominantly constitute the K(+)-dependent subfamily. Furthermore, Na(+)-PPases possibly preceded H(+)-PPases in evolution, and transition from Na(+) to H(+) transport may have occurred in several independent enzyme lineages. Site-directed mutagenesis studies facilitated the identification of a specific Glu residue that appears to be central in the transport mechanism. This residue is located in the cytoplasm-membrane interface of transmembrane helix 6 in Na(+)-PPases but shifted to within the membrane or helix 5 in H(+)-PPases. These results contribute to the prediction of the transport specificity and K(+) dependence for a particular membrane PPase sequence based on its position in the phylogenetic tree, identity of residues in the K(+) dependence signature, and position of the membrane-located Glu residue.

Mutational analysis of residues in the regulatory CBS domains of Moorella thermoacetica pyrophosphatase corresponding to disease-related residues of human proteins.

mtCBS-PPase [CBS (cystathionine ß-synthase) domain-containing pyrophosphatase from Moorella thermoacetica] contains a pair of CBS domains that strongly bind adenine nucleotides, thereby regulating enzyme activity. Eight residues associated with the CBS domains of mtCBS-PPase were screened to explore possible associations with regulation of enzyme activity. The majority of the substitutions (V99A, R168A, Y169A, Y169F, Y188A and H189A) enhanced the catalytic activity of mtCBS-PPase, two substitutions (R170A and R187G) decreased activity, and one substitution (K100G) had no effect. AMP-binding affinity was markedly decreased in the V99A, R168A and Y169A mutant proteins, and elevated in the R187G and H189A mutant proteins. Remarkably, the R168A and Y169A substitutions changed the effect of AMP from inhibition to activation. The stoichiometry of AMP binding increased from one to two AMP molecules per CBS domain pair in the Y169F, R170A, R187G and Y188A variants. The ADP-binding affinity decreased in three and increased in four mutant proteins. These findings identify residues determining the strength and selectivity of nucleotide binding, as well as the direction (inhibition or activation) of the subsequent effect. The data suggest that mutations in human CBS domain-containing proteins can be translated into a bacterial context. Furthermore, our data support the hypothesis that the CBS domains act as an 'internal inhibitor' of mtCBS-PPase.

Nucleotide- and substrate-induced conformational transitions in the CBS domain-containing pyrophosphatase of Moorella thermoacetica.

In contrast to all other known pyrophosphatases, Moorella thermoacetica pyrophosphatase (mtCBS-PPase) contains nucleotide-binding CBS domains and is thus strongly regulated by adenine nucleotides. Stopped-flow measurements using a fluorescent AMP analogue, 2'(3')-O-(N-methylanthranoyl)-AMP (Mant-AMP), reveal that nucleotide binding to mtCBS-PPase involves a three-step increase in Mant-AMP fluorescence with relaxation times from 0.01 to 100 s, implying conformational changes in the complex. This effect is reversed by AMP. Metal cofactors (Co(2+) and Mg(2+)) enhance the fluorescence signal but are not absolutely required, unlike what is seen when the catalytic reaction is examined. The relaxation times and amplitudes of the fluorescence signals depend on Mant-AMP concentration in a manner suggestive of the presence of a second binding site for Mant-AMP on the protein. Equilibrium fluorescence titration experiments additionally support the presence of two types of AMP binding sites with different affinities, whereas equilibrium dialysis and membrane filtration measurements reveal binding of one AMP molecule per enzyme monomer, implying negative cooperativity in nucleotide binding. The substrate (PP(i)) modulates Mant-AMP binding, leading to a further conformational change in the enzyme-Mant-AMP complex, and stimulates mtCBS-PPase in alkaline medium within a time scale of minutes, via conversion to a more active form. This active form initially comprises only a third of the enzyme, as estimated from kinetic titration with ADP. AMP inhibits both enzyme forms but is unable to independently induce interconversion. Our results collectively suggest that nucleotides and the substrate induce multiple conformational changes in mtCBS-PPase occurring over a wide time scale; the changes are distinct and almost independent.