PQQ and quinoprotein enzymes in microbial oxidations

Abstract Pyrroloquinoline quinone (PQQ) is found in a wide range of microorganisms, and several bacteria even excrete this compound into their culture medium when grown on alcohols. The existence of different classes of quinoprotein (PQQ-containing) enzymes is now well established (alcohol dehydrogenases, aldose (glucose) dehydrogenases, amine dehydrogenases and amine oxidases) while several other enzymes are suspected to be quinoproteins. In addition, many bacteria produce a quinoprotein apoenzyme, e.g., Escherichia coli and Pseudomonas testosteroni , producing glucose and ethanol dehydrogenase apoenzyme, respectively. It is unclear why these bacteria do not produce the holoenzyme form, but the apoenzymes have the ability to become functional, as was shown when the organisms were provided with PQQ. With this approach it could be demonstrated that E. coli has a non-phosphorylative route of glucose dissimilation via gluconate. Also, results with mixed cultures indicate that PQQ is a growth factor for certain bacteria under certain conditions. Despite the relatively high redox potential of the PQQ/PQQH₂ couple, quinoproteins transfer electrons to a variety of natural electron acceptors. Depending on the type of quinoprotein enzyme, the following components of the respiratory chain appear to be active: cytochrome c (sometimes with a copper protein as an intermediate), cytochrome b , and NADH dehydrogenase. PQQ is not restricted to a particular group of organisms, and reactions catalysed by quinoproteins can also be performed by NAD(P)-dependent or flavoprotein enzymes. Thus, these observations do not provide arguments for the view that quinoproteins have a unique role in microbial oxidations. Further comparative studies on oxidoreductases are necessary to reveal the special features of this novel group of enzymes.     

Quinoproteins: structure, function, and biotechnological applications

A new class of oxidoreductase containing an amino acid-derived o-quinone cofactor, of which the most typical is pyrroloquinoline quinone (PQQ), is called quinoproteins, and has been recognized as the third redox enzyme following pyridine nucleotide- and flavin-dependent dehydrogenases. Some quinoproteins include a heme c moiety in addition to the quinone cofactor in the molecule and are called quinohemoproteins. PQQ-containing quinoproteins and quinohemoproteins have a common structural basis, in which PQQ is deeply embedded in the center of the unique superbarrel structure. Increased evidence for the structure and function of quinoproteins has revealed their unique position within the redox enzymes with respect to catalytic and electron transfer properties, and also to physiological and energetic function. The peculiarities of the quinoproteins, together with their unique substrate specificity, have encouraged their biotechnological application in the fields of biosensing and bioconversion of useful compounds, and also to environmental treatment.     

PQQ and quinoprotein research--the first decade

On the occasion of the first international symposium on pyrroloquinoline quinone (PQQ) and quinoproteins (Delft, September 1988), a review of this novel field in enzymology is presented. Quinoproteins (PQQ-containing enzymes) are widespread, from bacteria to mammalian organisms (including man), and occur in several classes of enzymes. Indications already exist that PQQ is a versatile cofactor, involved not only in oxidation but also in hydroxylation, transamination, decarboxylation and hydration reactions. The current list of quinoproteins shows that it was overlooked in several well-studied enzymes where the presence of a common cofactor had already been established. Up until now, all eukaryotic quinoproteins have covalently bound PQQ (or perhaps pro-PQQ), while free PQQ occurs exclusively in a number of (bacterial) dehydrogenases and in the culture fluid of certain Gram-negative bacteria. Biosynthesis of free PQQ in methylotrophic bacteria starts with tyrosine and glutamic acid as precursors while intermediates in the route have not been detected and the presence of free PQQ is not required for synthesis of the covalently bound form of the cofactor in glutamic acid decarboxylase from Escherichia coli. Therefore, the assembly of covalently bound cofactor might occur in situ, i.e. in the quinoproteins themselves. If the latter also applies to mammalian quinoproteins, this implies that PQQ is not a vitamin. On the other hand, positive effects have been reported upon administration of PQQ to test animals. Methods suited to detach and to detect PQQ with a derivatized o-quinone moiety may answer questions on the uptake and processing of the compound.     

Physiologic importance of pyrroloquinoline quinone.

Pyrroloquinoline quinone (PQQ, methoxatin) is a dissociable cofactor for a number of bacterial dehydrogenases. The compound is unusual because of its ability to catalyze redox cycling reactions at a high rate of efficiency and it has the potential of catalyzing various carbonyl amine reactions as well. In methylotrophic bacteria, PQQ is derived from the condensation of L-tyrosine with L-glutamic acid. Whether or not PQQ serves as a cofactor in higher plants and animals remains controversial. Nevertheless, a strong case may be made that PQQ and related quinoids have nutritional and pharmacologic importance. In highly purified, chemically defined diets, PQQ stimulates animal growth. Furthermore, PQQ deprivation appears to impair connective tissue maturation, particularly when initiated in utero and throughout perinatal development.     

The organic cofactor in plasma amine oxidase: evidence for pyrroloquinoline quinone and against pyridoxal phosphate.

Plasma amine oxidases (EC 1.4.3.6) are classified as containing the organic cofactor pyridoxal phosphate. Biochemical and bioassays on the pig plasma amine oxidase fail to reveal the presence of pyridoxal phosphate and 31P n.m.r. evidence is also inconsistent with pyridoxal phosphate in the enzyme. Resonance Raman spectral studies on phenylhydrazone derivatives of the pig and bovine plasma enzymes have been carried out and comparisons made with the corresponding derivatives of pyridoxal phosphate and pyrroloquinoline quinone (PQQ). The resonance Raman evidence indicates that the cofactor in both plasma amine oxidases is PQQ or a closely related species and not pyridoxal phosphate. The results substantiate earlier reports concerning the identity of the organic cofactor.     

Factors relevant in the reaction of pyrroloquinoline quinone with amino acids. Analytical and mechanistic implications

In order to reveal the stability of pyrroloquinoline quinone (PQQ) in complex samples, its reaction on incubation with amino acids was followed spectrophotometrically by monitoring oxygen consumption, and with a biological assay. For several alpha-amino acids, the formation of a yellow coloured compound (lambda max = 420 nm) was accompanied by oxygen uptake and disappearance of biological activity from the reaction mixture. The yellow product appeared to be an oxazole of PQQ, the exact structure depending on the amino acid used. Oxazole formation also occurred under anaerobic conditions with concomitant formation of PQQH2, suggesting that PQQ is able to oxidize the presumed oxazoline to the oxazole. Besides the condensation reaction, there is also a catalytic cycle in which an aldimine adduct of PQQ and the amino acid is converted into the aminophenol form of the cofactor and an aldehyde resulting from oxidative decarboxylation of the amino acid. Addition of NH4+ salts, as well as that of certain divalent cations, greatly stimulated both the cyclic and the linear reaction. With basic amino acids, oxazole formation scarcely occurred. However, as oxygen consumption was observed (provided that certain divalent cations were present), conversion of these compounds took place. A reaction scheme is proposed accounting for the products formed and the effects observed. Since NH4+ ions activate several quinoproteins (PQQ-containing enzymes) and divalent cations (Ca2+, Fe2+, and Cu2+) are additional (co)factors in certain metallo quinoproteins, the effects of metal ions observed here could be related to the mechanistic features of these enzymes. Although all oxazoles were converted to PQQ by acid hydrolysis, PQQ was not detected when hydrolysis was carried out in the presence of tryptophan, a compound which appeared to have a deleterious effect on the cofactor under this condition. The results here described explain why analysis methods for free PQQ in complex samples fail in certain cases, or are not quantitative.     

PQQ-dependent methanol dehydrogenases: rare-earth elements make a difference

Methanol dehydrogenase (MDH) catalyzes the first step in methanol use by methylotrophic bacteria and the second step in methane conversion by methanotrophs. Gram-negative bacteria possess an MDH with pyrroloquinoline quinone (PQQ) as its catalytic center. This MDH belongs to the broad class of eight-bladed β propeller quinoproteins, which comprise a range of other alcohol and aldehyde dehydrogenases. A well-investigated MDH is the heterotetrameric MxaFI-MDH, which is composed of two large catalytic subunits (MxaF) and two small subunits (MxaI). MxaFI-MDHs bind calcium as a cofactor that assists PQQ in catalysis. Genomic analyses indicated the existence of another MDH distantly related to the MxaFI-MDHs. Recently, several of these so-called XoxF-MDHs have been isolated. XoxF-MDHs described thus far are homodimeric proteins lacking the small subunit and possess a rare-earth element (REE) instead of calcium. The presence of such REE may confer XoxF-MDHs a superior catalytic efficiency. Moreover, XoxF-MDHs are able to oxidize methanol to formate, rather than to formaldehyde as MxaFI-MDHs do. While structures of MxaFI- and XoxF-MDH are conserved, also regarding the binding of PQQ, the accommodation of a REE requires the presence of a specific aspartate residue near the catalytic site. XoxF-MDHs containing such REE-binding motif are abundantly present in genomes of methylotrophic and methanotrophic microorganisms and also in organisms that hitherto are not known for such lifestyle. Moreover, sequence analyses suggest that XoxF-MDHs represent only a small part of putative REE-containing quinoproteins, together covering an unexploited potential of metabolic functions.     

Properties of copper-free pig kidney amine oxidase: role of topa quinone

Copper removal from pig kidney amine oxidase containing Cu/topaquinone (TPQ) has been obtained using CN(-) in the presence of the poor substrate p-(dimethylamino)benzylamine. Upon removal of copper, the enzyme loses its activity while the TPQ cofactor remains in its oxidized form. The addition of copper to the apo-form fully restores the active enzyme. The CN(-) treatment in the presence of sodium dithionite or good substrates (cadaverine or benzylamine) also removes copper but the TPQ cofactor is irreversibly reduced and the addition of copper does not regenerate the active enzyme. Ni(II) and Zn(II) do not bind the apo-protein in contrast to Co(II) which is incorporated to the same extent as Cu(II). However, Co-reconstituted enzyme only shows a very low activity. These results demonstrate that copper is essential for the catalytic mechanism because it maintains the correct active site geometry.     

Structure of the pyrroloquinoline quinone radical in quinoprotein ethanol dehydrogenase

Quinoprotein alcohol dehydrogenases use the pyrroloquinoline quinone (PQQ) cofactor to catalyze the oxidation of alcohols. The catalytic cycle is thought to involve a hydride transfer from the alcohol to the oxidized PQQ, resulting in the generation of aldehyde and reduced PQQ. Reoxidation of the cofactor by cytochrome proceeds in two sequential steps via the PQQ radical. We have used a combination of electron nuclear double resonance and density functional theory to show that the PQQ radical is not protonated at either O-4 or O-5, a result that is at variance with the general presumption of a singly protonated radical. The quantum mechanical calculations also show that reduced PQQ is unlikely to be protonated at O-5; rather, it is either singly protonated at O-4 or not protonated at either O-4 or O-5, a result that also challenges the common assumption of a reduced PQQ protonated at both O-4 and O-5. The reaction cycle of PQQ-dependent alcohol dehydrogenases is revised in light of these findings.     

On the biosynthesis of free and covalently bound PQQ. Glutamic acid decarboxylase from Escherichia coli is a pyridoxo-quinoprotein

Analysis of glutamic acid decarboxylase (GDC) (EC 4.1.1.15) from Escherichia coli ATCC 11246 revealed the presence of six pyridoxal phosphates (PLPs) as well as six covalently bound pyrroloquinoline quinones (PQQs) per hexameric enzyme molecule. This is the second example of a pyridoxo-quinoprotein, suggesting that other atypical pyridoxoproteins (PLP-containing enzymes) have similar cofactor composition. Since the organism did not produce free PQQ and its quinoprotein glucose dehydrogenase was present in the apo form, free PQQ is not used in the assemblage of GDC. Most probably, biosynthesis of covalently bound cofactor occurs in situ via a route which is different from that of free PQQ. Thus, organisms previously believed to be unable to synthesize (free) PQQ could in fact be able to produce quinoproteins with covalently bound cofactor. Implications for the role of PQQ in eukaryotic cells are discussed.     

Characterisation of the PQQ cofactor radical in quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa by electron paramagnetic resonance spectroscopy

The binding pocket of the pyrroloquinoline quinone (PQQ) cofactor in quinoprotein alcohol dehydrogenases contains a characteristic disulphide ring formed by two adjacent cysteine residues. To analyse the function of this unusual structural motif we have investigated the wild-type and a double cysteine:alanine mutant of the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa by electron paramagnetic resonance (EPR) spectroscopy. Thus, we have obtained the principal values for the full rhombic g-tensor of the PQQ semiquinone radical by high-field (94 GHz) EPR necessary for a discrimination of radical species in dehydrogenases containing PQQ together with other redox-active cofactors. Our results show that the characteristic disulphide ring is no prerequisite for the formation of the functionally important semiquinone form of PQQ.     

Evidence for PQQ as cofactor in 3,4-dihydroxyphenylalanine (dopa) decarboxylase of pig kidney

Pig kidney 3,4-dihydroxyphenylalanine (dopa) decarboxylase (EC 4.1.1.28) was purified to homogeneity. Treatment of the enzyme with phenylhydrazine (PH) according to a procedure developed for analysis of quinoproteins gave products which were identified as the hydrazone of pyridoxal phosphate (PLP) and the C(5)-hydrazone of pyrroloquinoline quinone (PQQ). This method failed, however, in quantifying the amounts of cofactor. Direct hydrolysis of the enzyme by refluxing with hexanol and concentrated HCl led to detachment of PQQ from the protein in a quantity of 1 PQQ per enzyme molecule. In view of the reactivity of PQQ towards amines and amino acids, we postulate that it participates as a covalently bound cofactor in the catalytic cycle of the enzyme, in interplay with PLP. Since several other enzymes have been reported to show the atypical behaviour of dopa decarboxylase, it seems that the PLP-containing group of enzymes can be subdivided into pyridoxoproteins and pyridoxo-quinoproteins.     

Structural and enzyme activity studies demonstrate that aryl substituted 2,3-butadienamine analogs inactivate Arthrobacter globiformis amine oxidase (AGAO) by chemical derivatization of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor

Copper amine oxidases (CAOs) are a family of redox active enzymes containing a 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor generated from post translational modification of an active site tyrosine residue. The Arthrobacter globiformis amine oxidase (AGAO) has been widely used as a model to guide the design and development of selective inhibitors of CAOs. In this study, two aryl 2,3-butadienamine analogs, racemic 5-phenoxy-2,3-pentadienylamine (POPDA) and racemic 6-phenyl-2,3-hexadienylamine (PHDA), were synthesized and evaluated as mechanism-based inactivators of AGAO. Crystal structures show that both compounds form a covalent adduct with the amino group of the substrate-reduced TPQ, and that the chemical structures of the rac-PHDA and rac-POPDA modified TPQ differ by the allenic carbon that is attached to the cofactor. A chemical mechanism accounting for the formation of the respective TPQ derivative is proposed. Under steady-state conditions, no recovery of enzyme activity is detected when AGAO pre-treated with rac-PHDA or rac-POPDA is diluted with excess amount of the benzylamine substrate (100-fold K(m)). Comparing the IC(50) values further reveals that the phenoxy substituent in POPDA offers an approximately 4-fold increase in inhibition potency, which can be attributed to a favourable binding interaction between the oxygen atom in the phenoxy group and the active site of AGAO as revealed by crystallographic studies. This hypothesis is corroborated by the observed >3-fold higher partition ratio of PHDA compared to POPDA. Taken together, the results presented in this study reveal the mechanism by which aryl 2,3-butadienamines act as mechanism-based inhibitors of AGAO, and the potency of enzyme inactivation could be fine-tuned by optimizing binding interaction between the aryl substituent and the enzyme active site.     

Pyrroloquinoline quinone as cofactor in galactose oxidase (EC 1.1.3.9)

Galactose oxidase from Dactylium dendroides was shown to contain one molecule of covalently bound pyrroloquinoline quinone (PQQ/enzyme molecule. From the spectroscopic characteristics reported for the enzyme forms, a mechanistic role for PQQ could be deduced. In analogy with other quinoproteins, the initial formation of a PQQ-substrate adduct is proposed. Following internal hydrogen transfer, leading to aldehyde product and reduced pyrroloquinoline quinone, reoxidation of the organic cofactor with molecular oxygen could be mediated by the PQQ-liganded copper ion with concomitant formation of hydrogen peroxide. With PQQ as an additional (two-electron) redox center the occurrence of a "superoxidized" enzyme form must be considered. Possible consequences of this view, in relation to a physiological function of the enzyme and interpretation of ESR data, are discussed.     

Investigation of Cu(I)-dependent 2,4,5-trihydroxyphenylalanine quinone biogenesis in Hansenula polymorpha amine oxidase

Copper, a mediator of redox chemistries in biology, is often found in enzymes that bind and reduce dioxygen. Among these, the copper amine oxidases catalyze the oxidative deamination of primary amines utilizing a type(II) copper center and 2,4,5-trihydroxyphenylalanine quinone (TPQ), a covalent cofactor derived from the post-translational modification of an active site tyrosine. Previous studies established the dependence of TPQ biogenesis on Cu(II); however, the dependence of cofactor formation on the biologically relevant Cu(I) ion has remained untested. In this study, we demonstrate that the apoform of the Hansenula polymorpha amine oxidase readily binds Cu(I) under anaerobic conditions and produces the quinone cofactor at a rate of 0.28 h(-1) upon subsequent aeration to yield a mature enzyme with kinetic properties identical to the protein product of the Cu(II)-dependent reaction. Because of the change in magnetic properties associated with the oxidation of copper, electron paramagnetic resonance spectroscopy was employed to investigate the nature of the rate-limiting step of Cu(I)-dependent cofactor biogenesis. Upon aeration of the unprocessed enzyme prebound with Cu(I), an axial Cu(II) electron paramagnetic resonance signal was found to appear at a rate equivalent to that for the cofactor. These data provide strong evidence for a rate-limiting release of superoxide from a Cu(II)(O(2)(.)) complex as a prerequisite for the activation of the precursor tyrosine and its transformation for TPQ. As copper is trafficked to intracellular protein targets in the reduced, Cu(I) state, these studies offer possible clues as to the physiological significance of the acquisition of Cu(I) by nascent H. polymorpha amine oxidase.     

Menaquinone as well as ubiquinone as a bound quinone crucial for catalytic activity and intramolecular electron transfer in Escherichia coli membrane-bound glucose dehydrogenase

Escherichia coli membrane-bound glucose dehydrogenase (mGDH), which is one of quinoproteins containing pyrroloquinoline quinone (PQQ) as a coenzyme, is a good model for elucidating the function of bound quinone inside primary dehydrogenases in respiratory chains. Enzymatic analysis of purified mGDH from cells defective in synthesis of ubiquinone (UQ) and/or menaquinone (MQ) revealed that Q-free mGDH has very low levels of activity of glucose dehydrogenase and UQ2 reductase compared with those of UQ-bearing mGDH, and both activities were significantly increased by reconstitution with UQ1. On the other hand, MQ-bearing mGDH retains both catalytic abilities at the same levels as those of UQ-bearing mGDH. A radiolytically generated hydrated electron reacted with the bound MQ to form a semiquinone anion radical with an absorption maximum at 400 nm. Subsequently, decay of the absorbance at 400 nm was accompanied by an increase in the absorbance at 380 nm with a first order rate constant of 5.7 x 10(3) s(-1). This indicated that an intramolecular electron transfer from the bound MQ to the PQQ occurred. EPR analysis revealed that characteristics of the semiquinone radical of bound MQ are similar to those of the semiquinone radical of bound UQ and indicated an electron flow from PQQ to MQ as in the case of UQ. Taken together, the results suggest that MQ is incorporated into the same pocket as that for UQ to perform a function almost equivalent to that of UQ and that bound quinone is involved at least partially in the catalytic reaction and primarily in the intramolecular electron transfer of mGDH.     

Adduct formation of pyrroloquinoline quinone and amino acid

When pyrroloquinoline quinone (PQQ) is mixed with an amino acid, a corresponding Schiff base PQQ adduct is readily formed between carbonyl groups of PQQ and the primary amino group. A potent growth stimulating effect for microorganisms was observed with the PQQ adduct when it was administered in a culture medium. Although PQQ itself shows a marked growth stimulating effect, PQQ adducts appeared to be more active than authentic PQQ when compared on a molar basis. Conversely, unlike authentic PQQ, PQQ adducts were shown to be less active (greater than or equal to 100-fold) as the prosthetic group for a quinoprotein apo-glucose dehydrogenase when examined by holoenzyme formation by exogenous addition of PQQ or PQQ adducts. These observations suggested that PQQ adduct formation readily occurs during isolation procedures for PQQ from biological materials or PQQ - chromophore from quinoproteins. Therefore, the presence of such adducts gives a PQQ estimation much lower than theoretically expected. As an example, formation, isolation and characterization of PQQ - serine are described.     

Recent progress in studies on the health benefits of pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ), an aromatic tricyclic o-quinone, was identified initially as a redox cofactor for bacterial dehydrogenases. Although PQQ is not biosynthesized in mammals, trace amounts of PQQ have been found in human and rat tissues because of its wide distribution in dietary sources. Importantly, nutritional studies in rodents have revealed that PQQ deficiency exhibits diverse systemic responses, including growth impairment, immune dysfunction, and abnormal reproductive performance. Although PQQ is not currently classified as a vitamin, PQQ has been implicated as an important nutrient in mammals. In recent years, PQQ has been receiving much attention owing to its physiological importance and pharmacological effects. In this article, we review the potential health benefits of PQQ with a focus on its growth-promoting activity, anti-diabetic effect, anti-oxidative action, and neuroprotective function. Additionally, we provide an update of its basic pharmacokinetics and safety information in oral ingestion.     

Reinvestigation of metal ion specificity for quinone cofactor biogenesis in bacterial copper amine oxidase

The topa quinone (TPQ) cofactor of copper amine oxidase is generated by copper-assisted self-processing of the precursor protein. Metal ion specificity for TPQ biogenesis has been reinvestigated with the recombinant phenylethylamine oxidase from Arthrobacter globiformis. Besides Cu2+ ion, some divalent metal ions such as Co2+, Ni2+, and Zn2+ were also bound to the metal site of the apoenzyme so tightly that they were not replaced by excess Cu2+ ions added subsequently. Although these noncupric metal ions could not initiate TPQ formation under the atmospheric conditions, we observed slow spectral changes in the enzyme bound with Co2+ or Ni2+ ion under the dioxygen-saturating conditions. Resonance Raman spectroscopy and titration with phenylhydrazine provided unambiguous evidence for TPQ formation by Co2+ and Ni2+ ions. Steady-state kinetic analysis showed that the enzymes activated by Co2+ and Ni2+ ions were indistinguishable from the corresponding metal-substituted enzymes prepared from the native copper enzyme (Kishishita, S., Okajima, T., Kim, M., Yamaguchi, H., Hirota, S., Suzuki, S., Kuroda, S., Tanizawa, K., and Mure, M. (2003) J. Am. Chem. Soc. 125, 1041-1055). X-ray crystallographic analysis has also revealed structural identity of the active sites of Co- and Ni-activated enzymes with Cu-enzyme. Thus Cu2+ ion is not the sole metal ion assisting TPQ formation. Co2+ and Ni2+ ions are also capable of forming TPQ, though much less efficiently than Cu2+.     

Chemical rescue of a site-specific mutant of bacterial copper amine oxidase for generation of the topa quinone cofactor

The topa quinone (TPQ) cofactor of copper amine oxidase is produced by posttranslational modification of a specific tyrosine residue through the copper-dependent, self-catalytic process. We have site-specifically mutated three histidine residues (His431, His433, and His592) involved in binding of the copper ion in the recombinant phenylethylamine oxidase from Arthrobacter globiformis. The mutant enzymes, in which each histidine was replaced by alanine, were purified in the Cu/TPQ-free precursor form and analyzed for their Cu-binding and TPQ-generating activities by UV-visible absorption, resonance Raman, and electron paramagnetic resonance spectroscopies. Among the three histidine-to-alanine mutants, only H592A was found to show a weak activity to form TPQ upon aerobic incubation with Cu(2+) ions. Also for H592A, exogenous imidazole rescued binding of copper and markedly promoted the TPQ formation. Accommodation of a free imidazole molecule within the cavity created in the active site of H592A was suggested by X-ray crystallography. Although the TPQ cofactor in H592A mutant was readily reduced with substrate, its catalytic activity was very low even in the presence of imidazole. Combined with the crystal structures of the mutant enzymes, these results demonstrate the importance of the three copper-binding histidine residues for both TPQ biogenesis and catalytic activity, fine-tuning the position of the essential metal.