(1/143) Characterization of the membrane quinoprotein glucose dehydrogenase from Escherichia coli and characterization of a site-directed mutant in which histidine-262 has been changed to tyrosine.
The requirements for substrate binding in the quinoprotein glucose dehydrogenase (GDH) in the membranes of Escherichia coli are described, together with the changes in activity in a site-directed mutant in which His262 has been altered to a tyrosine residue (H262Y-GDH). The differences in catalytic efficiency between substrates are mainly related to differences in their affinity for the enzyme. Remarkably, it appears that, if a hexose is able to bind in the active site, then it is also oxidized, whereas some pentoses are able to bind (and act as competitive inhibitors), but are not substrates. The activation energies for the oxidation of hexoses and pentoses are almost identical. In a previously published model of the enzyme, His262 is at the entrance to the active site and appears to be important in holding the prosthetic group pyrroloquinoline quinone (PQQ) in place, and it has been suggested that it might play a role in electron transfer from the reduced PQQ to the ubiquinone in the membrane. The H262Y-GDH has a greatly diminished catalytic efficiency for all substrates, which is mainly due to a marked decrease in their affinities for the enzyme, but the rate of electron transfer to oxygen is unaffected. During the processing of the PQQ into the apoenzyme to give active enzyme, its affinity is markedly dependent on the pH, four groups with pK values between pH7 and pH8 being involved. Identical results were obtained with H262Y-GDH, showing that His262 it is not directly involved in this process. (+info)
(2/143) Functions of amino acid residues in the active site of Escherichia coli pyrroloquinoline quinone-containing quinoprotein glucose dehydrogenase.
Several mutants of quinoprotein glucose dehydrogenase (GDH) in Escherichia coli, located around its cofactor pyrroloquinoline quinone (PQQ), were constructed by site-specific mutagenesis and characterized by enzymatic and kinetic analyses. Of these, critical mutants were further characterized after purification or by different amino acid substitutions. H262A mutant showed reduced affinities both for glucose and PQQ without significant effect on glucose oxidase activity, indicating that His-262 occurs very close to PQQ and glucose, but is not the electron acceptor from PQQH(2). W404A and W404F showed pronounced reductions of affinity for PQQ, and the latter rather than the former had equivalent glucose oxidase activity to the wild type, suggesting that Trp-404 may be a support for PQQ and important for the positioning of PQQ. D466N, D466E, and K493A showed very low glucose oxidase activities without influence on the affinity for PQQ. Judging from the enzyme activities of D466E and K493A, as well as their absorption spectra of PQQ during glucose oxidation, we conclude that Asp-466 initiates glucose oxidation reaction by abstraction of a proton from glucose and Lys-493 is involved in electron transfer from PQQH(2). (+info)
(3/143) Novel role for the NMDA receptor redox modulatory site in the pathophysiology of seizures.
Redox-active compounds modulate NMDA receptors (NMDARs) such that reduction of NMDAR redox sites increases, and oxidation decreases, NMDAR-mediated activity. Because NMDARs contribute to the pathophysiology of seizures, redox-active compounds also may modulate seizure activity. We report that the oxidant 5, 5'-dithio-bis(2-nitrobenzoic acid) (DTNB) and the redox cofactor pyrroloquinoline quinone (PQQ) suppressed low Mg(2+)-induced hippocampal epileptiform activity in vitro. Additionally, in slices exposed to 4-7 microM bicuculline, DTNB and PQQ reversed the potentiation of evoked epileptiform responses by the reductants dithiothreitol and Tris(2-carboxyethyl)phosphine (TCEP). NMDA-evoked whole-cell currents in CA1 neurons in slices were increased by TCEP and subsequently decreased by DTNB or PQQ at the same concentrations that modulated epileptiform activity. However, DTNB and PQQ had little effect on baseline NMDA-evoked currents in control medium, and PQQ did not alter NMDAR-dependent long-term potentiation. In contrast, in slices returned to control medium after low Mg(2+)-induced ictal activity, DTNB significantly inhibited NMDAR-mediated currents, indicating endogenous reduction of NMDAR redox sites under this epileptogenic condition. These data suggested that PQQ and DTNB suppressed spontaneous ictal activity by reversing pathological NMDAR redox potentiation without inhibiting physiological NMDAR function. In vivo, PQQ decreased the duration of chemoconvulsant-induced seizures in rat pups with no effect on baseline behavior. Our results reveal endogenous potentiation of NMDAR function via mass reduction of redox sites as a novel mechanism that may enhance epileptogenesis and facilitate the transition to status epilepticus. The results further suggest that redox-active compounds may have therapeutic use by reversing NMDAR-mediated pathophysiology without blocking physiological NMDAR function. (+info)
(4/143) Physiological importance of quinoenzymes and the O-quinone family of cofactors.
O-quinone cofactors derived from tyrosine and tryptophan are involved in novel biological reactions that range from oxidative deaminations to free-radical redox reactions. The formation of each of these cofactors appears to involve post-translational modifications of either tyrosine or tryptophan residues. The modifications result in cofactors, such as topaquinone (TPQ), tryptophan tryptophylquinone (TTQ), lysine tyrosylquinone (LTQ) or the copper-complexed cysteinyl-tyrosyl radical from metal-catalyzed reactions. Pyrroloquinoline quinone (PQQ) appears to be formed from the annulation of peptidyl glutamic acid and tyrosine residues stemming from their modification as components of a precursor peptide substrate. PQQ, a primary focus of this review, has invoked considerable interest because of its presence in foods, antioxidant properties and role as a growth-promoting factor. Although no enzymes in animals have been identified that exclusively utilize PQQ, oral supplementation of PQQ in nanomolar amounts increases the responsiveness of B- and T-cells to mitogens and improves neurologic function and reproductive outcome in rodents. Regarding TPQ and LTQ, a case may be made that the formation of TPQ and LTQ is also influenced by nutritional status, specifically dietary copper. For at least one of the amine oxidases, lysyl oxidase, enzymatic activity correlates directly with copper intake. TPQ and LTQ are generated following the incorporation of copper by a process that involves the two-step oxidation of a specified tyrosyl residue to first peptidyl dopa and then peptidyl topaquinone to generate active enzymes, generally classed as "quinoenzymes." Limited attention is also paid to TTQ and the copper-complexed cysteinyl-tyrosyl radical, cofactors important to fungal and bacterial redox processes. (+info)
(5/143) Structural requirements of pyrroloquinoline quinone dependent enzymatic reactions.
On the basis of crystal structures of the pyrroloquinoline quinone (PQQ) dependent enzymes methanol dehydrogenase (MDH) and soluble glucose dehydrogenase (s-GDH), different catalytic mechanisms have been proposed. However, several lines of biochemical and kinetic evidence are strikingly similar for both enzymes. To resolve this discrepancy, we have compared the structures of these enzymes in complex with their natural substrates in an attempt to bring them in line with a single reaction mechanism. In both proteins, PQQ is located in the center of the molecule near the axis of pseudo-symmetry. In spite of the absence of significant sequence homology, the overall binding of PQQ in the respective active sites is similar. Hydrogen bonding interactions are made with polar protein side chains in the plane of the cofactor, whereas hydrophobic stacking interactions are important below and above PQQ. One Arg side chain and one calcium ion are ligated to the ortho-quinone group of PQQ in an identical fashion in either active site, in agreement with their proposed catalytic function of polarizing the PQQ C5-O5 bond. The substrates are bound in a similar position above PQQ and within hydrogen bond distance of the putative general bases Asp297 (MDH) and His144 (s-GDH). On the basis of these similarities, we propose that MDH and s-GDH react with their substrates through an identical mechanism, comprising general base-catalyzed hydride transfer from the substrate to PQQ and subsequent tautomerization of the PQQ intermediate to reduced PQQ. (+info)
(6/143) Synthesis of [(14)C]pyrroloquinoline quinone (PQQ) in E. coli using genes for PQQ synthesis from K. pneumoniae.
Radiochemical forms of pyrroloquinoline quinone (PQQ) are of utility in studies to determine the metabolic role and fate of PQQ in biological systems. Accordingly, we have synthesized [(14)C]PQQ using a tyrosine auxotrophic strain of Escherichia coli (AT2471). A construct containing the six genes required for PQQ synthesis from Klebsiella pneumoniae was used to transform the auxotrophic strain of E. coli. E. coli were then grown in minimal M9 medium containing 3.7x10(9) Bq/mmol [(14)C]tyrosine. At confluence, the medium was collected and applied to a DEAE A-25 anionic exchange column; [(14)C]PQQ was eluted using a KCl gradient (0-2 M in 0.1 M potassium phosphate buffer, pH 7.0). Radioactivity co-eluting as PQQ was next pooled, acidified and passed through a C-18 column; [(14)C]PQQ was eluted with a phosphate buffer (0.1 M, pH 7.0). Reverse phase HPLC (C-18) using either the ion-pairing agent trifluoroacetic acid (0. 1%) and an acetonitrile gradient or phosphoric acid and a methanol gradient were used to isolate [(14)C]PQQ. Fractions were collected and analyzed by liquid scintillation counting. (14)C-labelled compounds isolated from the medium eluted corresponding to the elution of various tyrosine-derived products or PQQ. The radioactive compound corresponding to PQQ was also reacted with acetone to form 5-acetonyl-PQQ, which co-eluted with a 5-acetonyl-PQQ standard, as a validation of [(14)C]PQQ synthesis. The specific activity of synthesized [(14)C]PQQ was 3.7x10(9) Bq/mmol [(14)C]PQQ, equal to that of [U-(14)C]tyrosine initially added to the medium. (+info)
(7/143) Ca(2+) stabilizes the semiquinone radical of pyrroloquinoline quinone.
Spectroelectrochemical studies were performed on the interaction between Ca(2+) and pyrroloquinoline quinone (PQQ) in soluble glucose dehydrogenase (sGDH) and in the free state by applying a mediated continuous-flow column electrolytic spectroelectrochemical technique. The enzyme forms used were holo-sGDH (the holo-form of sGDH from Acinetobacter calcoaceticus) and an incompletely reconstituted form of this, holo-X, in which the PQQ-activating Ca(2+) is lacking. The spectroelectrochemical and ESR data clearly demonstrated the generation of the semiquinone radical of PQQ in holo-sGDH and in the free state in the presence of Ca(2+). In contrast, in the absence of Ca(2+) no semiquinone was observed, either for PQQ in the free state (at pH 7.0) or in the enzyme (holo-X). Incorporation of Ca(2+) into the active site of holo-X, yielding holo-sGDH, caused not only stabilization of the semiquinone form of PQQ but also a negative shift (of 26.5 mV) of the two-electron redox potential, indicating that the effect of Ca(2+) is stronger on the oxidized than on the reduced PQQ. Combining these data with the observations on the kinetic and chemical mechanisms, it was concluded that the strong stimulating effect of Ca(2+) on the activity of sGDH can be attributed to facilitation of certain kinetic steps, and not to improvement of the thermodynamics of substrate oxidation. The consequences of this conclusion are discussed for the oxidative as well as for the reductive part of the reaction of sGDH. (+info)
(8/143) Membrane-associated quinoprotein formaldehyde dehydrogenase from Methylococcus capsulatus Bath.
A membrane-associated, dye-linked formaldehyde dehydrogenase (DL-FalDH) was isolated from the obligate methylotroph Methylococcus capsulatus Bath. The enzyme was the major formaldehyde-oxidizing enzyme in cells cultured in high (above 1 micromol of Cu per mg of cell protein) copper medium and expressing the membrane-associated methane monooxygenase. Soluble NAD(P)(+)-linked formaldehyde oxidation was the major activity in cells cultured in low-copper medium and expressing the soluble methane monooxygenase (Tate and Dalton, Microbiology 145:159-167, 1999; Vorholt et al., J. Bacteriol. 180:5351-5356, 1998). The membrane-associated enzyme is a homotetramer with a subunit molecular mass of 49,500 Da. UV-visible absorption, electron paramagnetic resonance, and electrospray mass spectrometry suggest the redox cofactor of the DL-FalDH is pyrroloquinoline quinone (PQQ), with a PQQ-to-subunit stochiometry of approximately 1:1. The enzyme was specific for formaldehyde, oxidizing formaldehyde to formate, and utilized the cytochrome b(559/569) complex as the physiological electron acceptor. (+info)