Methanococcus jannaschii ORF mj0608 codes for a class C inorganic pyrophosphatase protected by Co(2+) or Mn(2+) ions against fluoride inhibition

The inorganic pyrophosphatases of microorganisms: a structural and functional review

Pyrophosphatases (PPases) are enzymes that catalyze the hydrolysis of pyrophosphate (PPi), a byproduct of the synthesis and degradation of diverse biomolecules. The accumulation of PPi in the cell can result in cell death. Although the substrate is the same, there are variations in the catalysis and features of these enzymes. Two enzyme forms have been identified in bacteria: cytoplasmic or soluble pyrophosphatases and membrane-bound pyrophosphatases, which play major roles in cell bioenergetics. In eukaryotic cells, cytoplasmic enzymes are the predominant form of PPases (c-PPases), while membrane enzymes (m-PPases) are found only in protists and plants. The study of bacterial cytoplasmic and membrane-bound pyrophosphatases has slowed in recent years. These enzymes are central to cell metabolism and physiology since phospholipid and nucleic acid synthesis release important amounts of PPi that must be removed to allow biosynthesis to continue. In this review, two aims were pursued: first, to provide insight into the structural features of PPases known to date and that are well characterized, and to provide examples of enzymes with novel features. Second, the scientific community should continue studying these enzymes because they have many biotechnological applications. Additionally, in this review, we provide evidence that there are m-PPases present in fungi; to date, no examples have been characterized. Therefore, the diversity of PPase enzymes is still a fruitful field of research. Additionally, we focused on the roles of H+/Na+ pumps and m-PPases in cell bioenergetics. Finally, we provide some examples of the applications of these enzymes in molecular biology and biotechnology, especially in plants. This review is valuable for professionals in the biochemistry field of protein structure-function relationships and experts in other fields, such as chemistry, nanotechnology, and plant sciences.

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 β-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.

X-ray Crystallography and Electron Paramagnetic Resonance Spectroscopy Reveal Active Site Rearrangement of Cold-Adapted Inorganic Pyrophosphatase

Inorganic pyrophosphatase (PPase) catalyses the hydrolysis reaction of inorganic pyrophosphate to phosphates. Our previous studies showed that manganese (Mn) activated PPase from the psychrophilic bacterium Shewanella sp. AS-11 (Mn-Sh-PPase) has a characteristic temperature dependence of the activity with an optimum at 5 °C. Here we report the X-ray crystallography and electron paramagnetic resonance (EPR) spectroscopy structural analyses of Sh-PPase in the absence and presence of substrate analogues. We successfully determined the crystal structure of Mn-Sh-PPase without substrate and Mg-activated Sh-PPase (Mg-Sh-PPase) complexed with substrate analogue (imidodiphosphate; PNP). Crystallographic studies revealed a bridged water placed at a distance from the di-Mn centre in Mn-Sh-PPase without substrate. The water came closer to the metal centre when PNP bound. EPR analysis of Mn-Sh-PPase without substrate revealed considerably weak exchange coupling, whose magnitude was increased by binding of substrate analogues. The data indicate that the bridged molecule has weak bonds with the di-Mn centre, which suggests a 'loose' structure, whereas it comes closer to di-Mn centre by substrate binding, which suggests a 'well-tuned' structure for catalysis. Thus, we propose that Sh-PPase can rearrange the active site and that the 'loose' structure plays an important role in the cold adaptation mechanism.

Crystal structures of plant inorganic pyrophosphatase, an enzyme with a moonlighting autoproteolytic activity

Inorganic pyrophosphatases (PPases, EC 3.6.1.1), which hydrolyze inorganic pyrophosphate to phosphate in the presence of divalent metal cations, play a key role in maintaining phosphorus homeostasis in cells. DNA coding inorganic pyrophosphatases from Arabidopsis thaliana (AtPPA1) and Medicago truncatula (MtPPA1) were cloned into a bacterial expression vector and the proteins were produced in Escherichia coli cells and crystallized. In terms of their subunit fold, AtPPA1 and MtPPA1 are reminiscent of other members of Family I soluble pyrophosphatases from bacteria and yeast. Like their bacterial orthologs, both plant PPases form hexamers, as confirmed in solution by multi-angle light scattering and size-exclusion chromatography. This is in contrast with the fungal counterparts, which are dimeric. Unexpectedly, the crystallized AtPPA1 and MtPPA1 proteins lack ∼30 amino acid residues at their N-termini, as independently confirmed by chemical sequencing. In vitro, self-cleavage of the recombinant proteins is observed after prolonged storage or during crystallization. The cleaved fragment corresponds to a putative signal peptide of mitochondrial targeting, with a predicted cleavage site at Val31-Ala32. Site-directed mutagenesis shows that mutations of the key active site Asp residues dramatically reduce the cleavage rate, which suggests a moonlighting proteolytic activity. Moreover, the discovery of autoproteolytic cleavage of a mitochondrial targeting peptide would change our perception of this signaling process.

Inorganic pyrophosphatases of Family II-two decades after their discovery

Inorganic pyrophosphatases (PPases) convert pyrophosphate (PP_i ) 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 (k_cat ≈ 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 β-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, PP_i levels, in response to changes in cell energy status (ATP/ADP ratio).

Structure of inorganic pyrophosphatase from Staphylococcus aureus reveals conformational flexibility of the active site

Cytoplasmic inorganic pyrophosphatase (PPiase) is an enzyme essential for survival of organisms, from bacteria to human. PPiases are divided into two structurally distinct families: family I PPiases are Mg(2+)-dependent and present in most archaea, eukaryotes and prokaryotes, whereas the relatively less understood family II PPiases are Mn(2+)-dependent and present only in some archaea, bacteria and primitive eukaryotes. Staphylococcus aureus (SA), a dangerous pathogen and a frequent cause of hospital infections, contains a family II PPiase (PpaC), which is an attractive potential target for development of novel antibacterial agents. We determined a crystal structure of SA PpaC in complex with catalytic Mn(2+) at 2.1Å resolution. The active site contains two catalytic Mn(2+) binding sites, each half-occupied, reconciling the previously observed 1:1 Mn(2+):enzyme stoichiometry with the presence of two divalent metal ion sites in the apo-enzyme. Unexpectedly, despite the absence of the substrate or products in the active site, the two domains of SA PpaC form a closed active site, a conformation observed in structures of other family II PPiases only in complex with substrate or product mimics. A region spanning residues 295-298, which contains a conserved substrate binding RKK motif, is flipped out of the active site, an unprecedented conformation for a PPiase. Because the mutant of Arg295 to an alanine is devoid of activity, this loop likely undergoes an induced-fit conformational change upon substrate binding and product dissociation. This closed conformation of SA PPiase may serve as an attractive target for rational design of inhibitors of this enzyme.

Cystathionine β-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 β-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.

Acetate activation in Methanosaeta thermophila: characterization of the key enzymes pyrophosphatase and acetyl-CoA synthetase

The thermophilic methanogen Methanosaeta thermophila uses acetate as sole substrate for methanogenesis. It was proposed that the acetate activation reaction that is needed to feed acetate into the methanogenic pathway requires the hydrolysis of two ATP, whereas the acetate activation reaction in Methanosarcina sp. is known to require only one ATP. As these organisms live at the thermodynamic limit that sustains life, the acetate activation reaction in Mt. thermophila seems too costly and was thus reevaluated. It was found that of the putative acetate activation enzymes one gene encoding an AMP-forming acetyl-CoA synthetase was highly expressed. The corresponding enzyme was purified and characterized in detail. It catalyzed the ATP-dependent formation of acetyl-CoA, AMP, and pyrophosphate (PP_i) and was only moderately inhibited by PP_i. The breakdown of PP_i was performed by a soluble pyrophosphatase. This enzyme was also purified and characterized. The pyrophosphatase hydrolyzed the major part of PP_i (K_M = 0.27 ± 0.05 mM) that was produced in the acetate activation reaction. Activity was not inhibited by nucleotides or PP_i. However, it cannot be excluded that other PP_i-dependent enzymes take advantage of the remaining PP_i and contribute to the energy balance of the cell.

A CBS domain-containing pyrophosphatase of Moorella thermoacetica is regulated by adenine nucleotides

CBS (cystathionine beta-synthase) domains are found in proteins from all kingdoms of life, and point mutations in these domains are responsible for a variety of hereditary diseases in humans; however, the functions of CBS domains are not well understood. In the present study, we cloned, expressed in Escherichia coli, and characterized a family II PPase (inorganic pyrophosphatase) from Moorella thermoacetica (mtCBS-PPase) that has a pair of tandem 60-amino-acid CBS domains within its N-terminal domain. Because mtCBS-PPase is a dimer and requires transition metal ions (Co2+ or Mn2+) for activity, it resembles common family II PPases, which lack CBS domains. The mtCBS-PPase, however, has lower activity than common family II PPases, is potently inhibited by ADP and AMP, and is activated up to 1.6-fold by ATP. Inhibition by AMP is competitive, whereas inhibition by ADP and activation by ATP are both of mixed types. The nucleotides are effective at nanomolar (ADP) or micromolar concentrations (AMP and ATP) and appear to compete for the same site on the enzyme. The nucleotide-binding affinities are thus 100-10000-fold higher than for other CBS-domain-containing proteins. Interestingly, genes encoding CBS-PPase occur most frequently in bacteria that have a membrane-bound H+-translocating PPase with a comparable PP(i)-hydrolysing activity. Our results suggest that soluble nucleotide-regulated PPases act as amplifiers of metabolism in bacteria by enhancing or suppressing ATP production and biosynthetic reactions at high and low [ATP]/([AMP]+[ADP]) ratios respectively.

Kinetic and structural properties of inorganic pyrophosphatase from the pathogenic bacterium Helicobacter pylori

Inorganic pyrophosphatase (PPase) catalyzes the hydrolysis of pyrophosphate (PPi) to orthophosphate (Pi) and controls the level of PPi in cells. PPase plays an essential role in energy conservation and provides the energy for many biosynthetic pathways. The Helicobacter pylori pyrophosphatase (HpPPase) gene was cloned, expressed, purified, and found to have a molecular weight of 20 kDa. The K(m) and V (max) of HpPPase were determined as 214.4 microM and 594 micromol Pi min(-1) mg(-1), respectively. PPi binds Mg(2+) to form a true substrate that activates the enzyme. However, free PPi could be a potent inhibitor for HpPPase. The effects of the inhibitors NaF, ATP, iminodiphosphate, and N-ethylmaleimide on HpPPase activity were evaluated. NaF showed the highest inhibition of the enzyme. Crystal structures of HpPPase and the PPi-HpPPase complex were determined. HpPPase comprises three alpha-helices and nine beta-strands and folds as a barrel structure. HpPPase forms a hexamer in both the solution and crystal states, and each monomer has its own PPi-binding site. The PPi binding does not cause a significant conformational change in the PPi-HpPPase complex, which might represent an inhibition state for HpPPase in the absence of a divalent metal ion.

A Trimetal Site and Substrate Distortion in a Family II Inorganic Pyrophosphatase

We report the first crystal structures of a family II pyrophosphatase complexed with a substrate analogue, imidodiphosphate (PNP). These provide new insights into the catalytic reaction mechanism of this enzyme family. We were able to capture the substrate complex both by fluoride inhibition and by site-directed mutagenesis providing complementary snapshots of the Michaelis complex. Structures of both the fluoride-inhibited wild type and the H98Q variant of the PNP-Bacillus subtilis pyrophosphatase complex show a unique trinuclear metal center. Each metal ion coordinates a terminal oxygen on the electrophilic phosphate and a lone pair on the putative nucleophile, thus placing it in line with the scissile bond without any coordination by protein. The nucleophile moves further away from the electrophilic phosphorus site, to the opposite side of the trimetal plane, upon binding of substrate. In comparison with earlier product complexes, the side chain of Lys296 has swung in and so three positively charged side chains, His98, Lys205 and Lys296, now surround the bridging nitrogen in PNP. Finally, one of the active sites in the wild-type structure appears to show evidence of substrate distortion. Binding to the enzyme may thus strain the substrate and thus enhance the catalytic rate.

Streptococcus gordonii soluble inorganic pyrophosphatase: An important role for the interdomain region in enzyme activity

Streptococcus gordonii DL1(Challis) soluble inorganic pyrophosphatase was shown to be a homo dimer with a subunit molecular mass of 33407. In solution, in the presence of Mn(2+), the protein is ellipsoidal with an axial ratio of 3.37 and molecular mass of 67000. In the absence of the divalent cation, the molecular mass is unchanged but the axial ratio increases to 3.94. The enzyme, in the presence of 5 mM Mg(2+), at 25 degrees Celsius and pH 9.0, has K(m) and k(cat) values of 62 microM and 6290 s(-1), respectively. The free N- and C-terminal domains of Streptococcus gordonii PPase did not interact productively when mixed together. Replacing the interdomain region with that from Bacillus subtilis decreased the catalytic efficiency of the enzyme whereas inserting the same region from the Archaeglobus fulgidus thermophilic enzyme yielded an inactive protein. Substitution, deletion and insertion of amino acid residues in the interdomain region were found to affect the monomer dimer equilibrium in the absence of Mn(2+) ions. In the presence of these ions however the variant proteins were dimers. Proteins with altered interdomain regions also displayed a 2- to 625-fold decrease in catalytic efficiency. These data together with that of computer analysis show that the interdomain region has characteristics of a mechanical hinge. Modelling mutant proteins onto the wild type shows that the active site regions are not significantly perturbed. These results show that, although distant from the active site, the interdomain region plays a role in enzyme activity and both its length and composition are important. This supports the hypothesis that catalytic activity requires the N- and C terminal domains of the enzyme to open and close using the interdomain region as a hinge.

Characterization of the Family I inorganic pyrophosphatase from Pyrococcus horikoshii OT3

A gene encoding for a putative Family I inorganic pyrophosphatase (PPase, EC 3.6.1.1) from the hyperthermophilic archaeon Pyrococcus horikoshii OT3 was cloned and the biochemical characteristics of the resulting recombinant protein were examined. The gene (Accession No. 1907) from P. horikoshii showed some identity with other Family I inorganic pyrophosphatases from archaea. The recombinant PPase from P. horikoshii (PhPPase) has a molecular mass of 24.5 kDa, determined by SDS-PAGE. This enzyme specifically catalyzed the hydrolysis of pyrophosphate and was sensitive to NaF. The optimum temperature and pH for PPase activity were 70 degrees C and 7.5, respectively. The half-life of heat inactivation was about 50 min at 105 degrees C. The heat stability of PhPPase was enhanced in the presence of Mg2+. A divalent cation was absolutely required for enzyme activity, Mg2+ being most effective; Zn2+, Co2+ and Mn2+ efficiently supported hydrolytic activity in a narrow range of concentrations (0.05-0.5 mM). The K(m) for pyrophosphate and Mg2+ were 113 and 303 microM, respectively; and maximum velocity, V(max), was estimated at 930 U mg(-1).

Inhibition of Family II Pyrophosphatases by Analogs of Pyrophosphate and Phosphate

Imidodiphosphate (the pyrophosphate analog containing a nitrogen atom in the bridge position instead of oxygen) is a potent inhibitor of family II pyrophosphatases from Streptococcus mutans and Streptococcus gordonii (inhibition constant Ki approximately 10 microM), which is slowly hydrolyzed by these enzymes with a catalytic constant of approximately 1 min(-1). Diphosphonates with different substituents at the bridge carbon atom are much less effective (Ki = 1-6 mM). The value of Ki for sulfate (a phosphate analog) is only 12 mM. The inhibitory effect of the pyrophosphate analogs exhibits only a weak dependence on the nature of the metal ion (Mn, Mg, or Co) bound in the active site.

Cloning and expression of the inorganic pyrophosphatase gene from the amino acid producer Brevibacterium lactofermentum ATCC 13869

A 20-kDa Brevibacterium lactofermentum protein was detected when purifying the His-tagged FtsZBL. The protein was identified by matrix-assisted laser desorption/ionisation time of flight as the inorganic pyrophosphatase encoded by the ppa gene, which is present as a single copy in the genome of Corynebacterium glutamicum. The ppa gene was cloned from B. lactofermentum chromosomal DNA by polymerase chain reaction; it seemed to be an essential gene and it might represent an attractive target for drug discovery. The cloned ppa gene complemented a ppa- Escherichia coli mutant and a ppa-gfp gene fusion revealed that the gene product mainly accumulated at the cell poles in both E. coli and B. lactofermentum.

Rhodobacter sphaeroides has a family II pyrophosphatase: comparison with other species of photosynthetic bacteria

The cytoplasmic pyrophosphatase from Rhodobacter sphaeroides was purified and characterized. The enzyme is a homodimer of 64 kDa. The N-terminus was sequenced and used to obtain the complete pyrophosphatase sequence from the preliminary genome sequence of Rba. sphaeroides, showing extensive sequence similarity to family II or class C pyrophosphatases. The enzyme hydrolyzes only Mg-PP(i) and Mn-PP(i) with a K(m) of 0.35 mM for both substrates. It is not activated by free Mg (2+), in contrast to the cytoplasmic pyrophosphatase from Rhodospirillum rubrum, and it is not inhibited by NaF, methylendiphosphate, or imidodiphosphate. This work shows that Rba. sphaeroides and Rhodobacter capsulatus cytoplasmic pyrophosphatases belong to family II, in contrast to Rsp. rubrum, Rhodopseudomonas palustris, Rhodopseudomonas gelatinosa, and Rhodomicrobium vannielii cytoplasmic pyrophosphatases which should be classified as members of family I. This is the first report of family II cytoplasmic pyrophosphatases in photosynthetic bacteria and in a gram-negative organism.

Fluoride and organic weak acids as modulators of microbial physiology

Fluoride is widely used as an anticaries agent in drinking water and a variety of other vehicles. This use has resulted in major health benefits. However, there are still open questions regarding the mechanisms of anticaries action and the importance of antimicrobial effects in caries reduction. Fluoride acts in multiple ways to affect the metabolism of cariogenic and other bacteria in the mouth. F(-)/HF can bind directly to many enzymes, for example, heme-containing enzymes or other metalloenzymes, to modulate metabolism. Fluoride is able also to form complexes with metals such as aluminum or beryllium, and the complexes, notably AlF(4)(-) and BeF(3)(-).H(2)O, can mimic phosphate with either positive or negative effects on a variety of enzymes and regulatory phosphatases. The fluoride action that appears to be most important for glycolytic inhibition at low pH in dental plaque bacteria derives from its weak-acid properties (pK(a)=3.15) and the capacity of HF to act as a transmembrane proton conductor. Since many of the actions of fluoride are related to its weak-acid character, it is reasonable to compare fluoride action to those of organic weak acids, including metabolic acids, food preservatives, non-steroidal anti-inflammatory agents and fatty acids, all of which act to de-energize the cell membrane by discharging DeltapH. Moreover, with the realization that the biofilm state is the common lifestyle for most microorganisms in nature, there is need to consider interactions of fluoride and organic weak acids with biofilm communities. Hopefully, this review will stimulate interest in the antimicrobial effects of fluoride or other weak acids and lead to more effective use of the agents for disease control and other applications.

The exopolyphosphatase gene from sulfolobus solfataricus: characterization of the first gene found to be involved in polyphosphate metabolism in archaea

Inorganic polyphosphate (polyP) polymers are widely distributed in all kinds of organisms. Although the presence of polyP in members of the domain Archaea has been described, at present nothing is known about the enzymology of polyP metabolism or the genes involved in this domain. We have cloned, sequenced, and overexpressed an exopolyphosphatase (PPX) gene (ppx) from thermophilic Sulfolobus solfataricus. The gene codes for a functional PPX and possesses an open reading frame for 417 amino acids (calculated mass, 47.9 kDa). The purified recombinant PPX was highly active, degrading long-chain polyP (700 to 800 residues) in vitro at 50 to 60 degrees C. The putative PPXs present in known archaeal genomes showed the highest similarity to yeast PPXs. In contrast, informatic analysis revealed that the deduced amino acid sequence of S. solfataricus PPX showed the highest similarity (25 to 45%) to sequences of members of the bacterial PPXs, possessing all of their conserved motifs. To our knowledge, this is the first report of an enzyme characterized to be involved in polyP metabolism in members of the Archaea.

Bacillus subtilis inorganic pyrophosphatase: the C-terminal signature sequence is essential for enzyme activity and conformational integrity

Bacillus subtilis inorganic pyrophosphatase is the first member of a newly identified Family II of PPases. To examine the role of a signature sequence found near the C-terminus, two truncated variants and a series of site-specific mutants were produced. A truncation of 17 residues (17AATR) but also single alanine substitutions, R295A and K296A, produced inactive enzyme. Removal of 5 nonconserved terminal residues (5AATR) markedly affected enzyme stability. Replacing S294 with A, T, C, or V decreased activity, the latter two mutations showing the greatest effect. Substitutions V299I and V300I had no or minor effects, whereas V300W and V299G/V300W significantly reduced activity. The sizes of truncated proteins and the full-length PPase were indistinguishable by gel-filtration. We conclude that the C-terminus has no role in multimerization, while both its conserved and nonconserved regions are essential for full enzyme activity. The signature sequence is required for both the conformation and composition of the active site.

Quaternary structure and metal ion requirement of family II pyrophosphatases from Bacillus subtilis, Streptococcus gordonii, and Streptococcus mutans

Pyrophosphatase (PPase) from Bacillus subtilis has recently been found to be the first example of a family II soluble PPase with a unique requirement for Mn2+. In the present work, we cloned and overexpressed in Escherichia coli putative genes for two more family II PPases (from Streptococcus mutans and Streptococcus gordonii), isolated the recombinant proteins, and showed them to be highly specific and active PPases (catalytic constants of 1700-3300 s(-)1 at 25 degrees C in comparison with 200-400 s(-)1 for family I). All three family II PPases were found to be dimeric manganese metalloenzymes, dissociating into much less active monomers upon removal of Mn2+. The dimers were found to have one high affinity manganese-specific site (K(d) of 0.2-3 nm for Mn2+ and 10-80 microm for Mg2+) and two or three moderate affinity sites (K(d) approximately 1 mm for both cations) per subunit. Mn2+ binding to the high affinity site, which occurs with a half-time of less than 10 s at 1.5 mm Mn2+, dramatically shifts the monomer <--> dimer equilibrium in the direction of the dimer, further activates the dimer, and allows substantial activity (60-180 s(-)1) against calcium pyrophosphate, a potent inhibitor of family I PPases.

Inorganic pyrophosphate generation and disposition in pathophysiology

Inorganic pyrophosphate (PP(i)) regulates certain intracellular functions and extracellular crystal deposition. PP(i) is produced, degraded, and transported by specialized mechanisms. Moreover, dysregulated cellular PP(i) production, degradation, and transport all have been associated with disease, and PP(i) appears to directly mediate specific disease manifestations. In addition, natural and synthetic analogs of PP(i) are in use or currently under evaluation as prophylactic agents or therapies for disease. This review summarizes recent developments in the understanding of how PP(i) is made and disposed of by cells and assesses the body of evidence for potentially significant physiological functions of intracellular PP(i) in higher organisms. Major topics addressed are recent lines of molecular evidence that directly link decreased and increased extracellular PP(i) levels with diseases in which connective tissue matrix calcification is disordered. To illustrate in depth the effects of disordered PP(i) metabolism, this review weighs the roles in matrix calcification of the transmembrane protein ANK, which regulates intracellular to extracellular movement of PP(i), and the PP(i)-generating phosphodiesterase nucleotide pyrophosphatase family isoenzyme plasma cell membrane glycoprotein-1 (PC-1).