Replacement of enzyme-bound calcium with strontium alters the kinetic properties of methanol dehydrogenase

Mediating role of host metabolites in strontium's effect on osteoporosis among older individuals: Findings from Wuhan, China

Strontium is receiving widespread attention due to its remarkable biological qualities in preventing bone resorption and fostering osteogenesis. However, the chemical processes behind strontium's dual activities on bone cells are not yet fully understood. This study used the metabolomic technique to identify and examine potential biomarkers between strontium exposure and osteoporosis (OP) risk. A total of 806 participants were recruited for the detection of plasma strontium content via inductively coupled plasma-mass spectrometry. Plasma metabolites were profiled in 254 participants through an untargeted metabolomics technique. Generalized linear models were primarily used to analyze associations among plasma strontium, metabolites, and OP. The mediating effects of metabolites on the strontium-OP association were further investigated. A total of 31 differential metabolites were observed, 10 of which were upregulated and 21 were downregulated in the OP group compared with the non-OP group. Five metabolites (3-phenoxybenzoic acid, Cer (t18:0/16:1), HexCer(t16:1/12:1(2OH)), HexCer(t14:2/18:1(2OH)), and TG(16:0-18:1-24:4)) were selected as potential mediators based on their significant association with OP risk and with femoral neck and lumbar spine bone mineral density (BMD). Moreover, all except TG(16:0-18:1-24:4) were involved in the OP discrimination model with excellent power combined with several traditional variables. 3-Phenoxybenzoic acid and Cer(t18:0/16:1) had significant indirect effects on the strontium-OP association. The five candidate metabolites mediated 83.79 % of the strontium-OP association. Plasma strontium level was associated with reduced OP risk in the Han population in Wuhan. Thus, plasma metabolite profiling revealed five BMD/OP-associated metabolites that acted as mediators in the strontium-OP association. Our findings provided evidence of the mediating role of host plasma metabolites in strontium's effect on OP pathology.

Metal mixture and osteoporosis risk: Insights from plasma metabolite profiling

The pathophysiology of osteoporosis (OP) is influenced by exposure to nonessential harmful metals and insufficient or excessive intake of necessary metals. Investigating multiple plasma metals, metabolites, and OP risk among older adults may reveal novel clues of underlying mechanisms for metal toxicity on bone mass. A total of 294 adults ≥ 55 years from Wuhan communities were included. Plasma concentrations of 23 metals and metabolites were measured via inductively coupled plasma-mass spectrometry and global metabolite detection. To investigate the relationships between plasma metals, OP risk, and OP-related metabolites, three different statistical techniques were used: generalized linear regression model, two-way orthogonal partial least-squares analysis (O2PLS), and weighted quantile sum (WQS). The mean ages were 66.82 and 66.21 years in OP (n = 115) and non-OP (n = 179) groups, respectively. Of all 2999 metabolites detected, 111 differential between-group members were observed. The OP risk decreased by 58.5% (OR=0.415, 95% CI: 0.237, 0.727) per quartile increment in the WQS index indicative of metal mixture exposure. Consistency remained for bone mineral density (BMD) measurements. The O2PLS model identified the top five OP-related metabolites, namely, DG(18:2_22:6), 3-phenoxybenzoic acid, TG(16:1_16:1_22:6), TG(16:0_16:0_20:4), and TG(14:1_18:2_18:3), contributing most to the joint covariation between the metal mixture and metabolites. Significant correlations between each of them and the metal mixture were found using WQS regression. Furthermore, the five metabolites mediated the associations of the metal mixtures, BMD, and OP risk. Our findings shed additional light on the mediation functions of plasma metabolites in the connection between multiple metal co-exposure and OP pathogenesis and offer new insights into the probable mechanisms underpinning the bone effects of the metal mixture.

Anaerobic microbial methanol conversion in marine sediments

Methanol is an ubiquitous compound that plays a role in microbial processes as a carbon and energy source, intermediate in metabolic processes or as end product in fermentation. In anoxic environments, methanol can act as the sole carbon and energy source for several guilds of microorganisms: sulfate-reducing microorganisms, nitrate-reducing microorganisms, acetogens and methanogens. In marine sediments, these guilds compete for methanol as their common substrate, employing different biochemical pathways. In this review, we will give an overview of current knowledge of the various ways in which methanol reaches marine sediments, the ecology of microorganisms capable of utilizing methanol and their metabolism. Furthermore, through a metagenomic analysis, we shed light on the unknown diversity of methanol utilizers in marine sediments which is yet to be explored.

Impact of the lanthanide contraction on the activity of a lanthanide-dependent methanol dehydrogenase - a kinetic and DFT study

Interest in the bioinorganic chemistry of lanthanides is growing rapidly as more and more lanthanide-dependent bacteria are being discovered. Especially the earlier lanthanides have been shown to be preferentially utilized by bacteria that need these Lewis acids as cofactors in their alcohol dehydrogenase enzymes. Here, we investigate the impact of the lanthanide ions lanthanum(iii) to lutetium(iii) (excluding Pm) on the catalytic parameters (vmax, KM, kcat/KM) of a methanol dehydrogenase (MDH) isolated from Methylacidiphilum fumariolicum SolV. Kinetic experiments and DFT calculations were used to discuss why only the earlier lanthanides (La-Gd) promote high MDH activity. Impact of Lewis acidity, coordination number preferences, stability constants and other properties that are a direct result of the lanthanide contraction are discussed in light of the two proposed mechanisms for MDH.

Similar but Not the Same: First Kinetic and Structural Analyses of a Methanol Dehydrogenase Containing a Europium Ion in the Active Site

Since the discovery of the biological relevance of rare earth elements (REEs) for numerous different bacteria, questions concerning the advantages of REEs in the active sites of methanol dehydrogenases (MDHs) over calcium(II) and of why bacteria prefer light REEs have been a subject of debate. Here we report the cultivation and purification of the strictly REE-dependent methanotrophic bacterium Methylacidiphilum fumariolicum SolV with europium(III), as well as structural and kinetic analyses of the first methanol dehydrogenase incorporating Eu in the active site. Crystal structure determination of the Eu-MDH demonstrated that overall no major structural changes were induced by conversion to this REE. Circular dichroism (CD) measurements were used to determine optimal conditions for kinetic assays, whereas inductively coupled plasma mass spectrometry (ICP-MS) showed 70 % incorporation of Eu in the enzyme. Our studies explain why bacterial growth of SolV in the presence of Eu3+ is significantly slower than in the presence of La3+ /Ce3+ /Pr3+ : Eu-MDH possesses a decreased catalytic efficiency. Although REEs have similar properties, the differences in ionic radii and coordination numbers across the series significantly impact MDH efficiency.

Oharaeibacter diazotrophicus gen. nov., sp. nov., a diazotrophic and facultatively methylotrophic bacterium, isolated from rice rhizosphere

A novel facultatively methanol-utilizing bacterial strain, SM30T, was isolated from rice rhizosphere. Strain SM30T was Gram-stain-negative, aerobic, motile, short rods, and grew optimally at pH 7 and at 28 °C. It could tolerate 0 to 2 % (w/v) NaCl. Based on 16S rRNA gene sequence comparisons, strain SM30T was most closely related to Pleomorphomonas oryzae DSM 16300T, with a low similarity of 94.17 %. One of the lanthanide metals, lanthanum, could enhance its growth slightly on methanol. Phylogenetic trees, based on the mxaF, xoxF and cpn60 genes of SM30T showed its distinct phylogenetic position with respect to species with validly published names. Polymerase chain reaction (PCR) amplification of the nifH and growth on nitrogen-free medium indicated that strain SM30T is a diazotroph. The major cellular fatty acids were summed feature 8 (containing 18 : 1ω7c and 18 : 1ω6c) and cyclo 19 : 0ω8c. The major quinone was ubiquinone 10. The DNA G+C content was 74.6 mol%. Based on the genotypic and phenotypic characteristics, strain SM30T represents a novel genus and species, for which the name Oharaeibacter diazotrophicus gen. nov., sp. nov. is proposed with the type strain SM30T (=NBRC 111955T=DSM 102969T).

Gene cloning and characterization of an aldehyde dehydrogenase from long-chain alkane-degrading Geobacillus thermoleovorans B23

Geobacillus thermoleovorans B23 is capable of degrading long-chain alkanes at 70 degrees C. Bt-aldh, an aldehyde dehydrogenase gene in B23, was located in the upstream region of p21 whose expression level was dramatically increased when alkane degradation was started (Kato et al. 2009, BMC Microbiol 9:60). Like p21, transcription level of Bt-aldh was also increased upon alkane degradation. Bt-Aldh (497 aa, MW = 53,886) was overproduced in Escherichia coli, purified, and characterized biochemically. Bt-Aldh acted as an octamer, required NAD(+) as a coenzyme, and showed high activity against aliphatic long-chain aldehydes such as tetradecanal. The optimum condition for activity was 50-55 degrees C and pH 10.0. The activity was elevated to two- to threefold in the presence of 2 mM Ba(2+), Ca(2+), or Sr(2+), while Mg(2+) and Zn(2+) inhibited the enzyme activity. Bt-Aldh represents thermophilic aldehyde dehydrogenases responsible for degradation of long-chain alkanes.

Crystal Structures of Cytochrome cL and Methanol Dehydrogenase from Hyphomicrobium denitrificans: Structural and Mechanistic Insights into Interactions between the Two Proteins,

Methanol dehydrogenase (Hd-MDH) and its physiological electron acceptor, cytochrome c(L) (Hd-Cyt c(L)), isolated from a methylotrophic denitrifying bacterium, Hyphomicrobium denitrificans A3151, have been kinetically and structurally characterized; the X-ray structures of Hd-MDH and Hd-Cyt c(L) have been determined using molecular replacement at 2.5 and 2.0 A resolution, respectively. To explain the mechanism for electron transfer between these proteins, the dependence of MDH activity on the concentration of Hd-Cyt c(L) has been investigated at pH 4.5-7.0. The Michaelis constant for Hd-Cyt c(L) shows the smallest value (approximately 0.3 microM) at pH 5.5. The pseudo-first-order rate constant (k(obs)) of the reduction of Hd-Cyt c(L) exhibits a hyperbolic concentration dependence of Hd-MDH at pH 5.5, although k(obs) linearly increases at pH 6.5. These findings indicate formation of a transient complex between these proteins during an electron transfer event. Hd-MDH (148 kDa) is a large tetrameric protein with an alpha(2)beta(2) subunit composition, showing a high degree of structural similarity with other MDHs. Hd-Cyt c(L) (19 kDa) exhibiting the alpha-band at 550.7 nm has a unique C-terminal region involving a disulfide bond between Cys47 and Cys165. Moreover, there is a pair of Hd-Cyt c(L) monomers related with a pseudo-2-fold axis of symmetry in the asymmetric unit, and the two monomers tightly interact with each other through three hydrogen bonds. This configuration is the first example in the studies of cytochrome c as the physiological electron acceptor for MDH. The docking simulation between the coupled Hd-Cyt c(L) molecules and the heterotetrameric Hd-MDH molecule has been carried out.

Site-directed mutagenesis and X-ray crystallography of the PQQ-containing quinoprotein methanol dehydrogenase and its electron acceptor, cytochrome c(L)

Two proteins specifically involved in methanol oxidation in the methylotrophic bacterium Methylobacterium extorquens have been modified by site-directed mutagenesis. Mutation of the proposed active site base (Asp303) to glutamate in methanol dehydrogenase (MDH) gave an active enzyme (D303E-MDH) with a greatly reduced affinity for substrate and with a lower activation energy. Results of kinetic and deuterium isotope studies showed that the essential mechanism in the mutant protein was unchanged, and that the step requiring activation by ammonia remained rate limiting. No spectrally detectable intermediates could be observed during the reaction. The X-ray structure, determined to 3 A resolution, of D303E-MDH showed that the position and coordination geometry of the Ca2+ ion in the active site was altered; the larger Glu303 side chain was coordinated to the Ca2+ ion and also hydrogen bonded to the O5 atom of pyrroloquinoline quinone (PQQ). The properties and structure of the D303E-MDH are consistent with the previous proposal that the reaction in MDH is initiated by proton abstraction involving Asp303, and that the mechanism involves a direct hydride transfer reaction. Mutation of the two adjacent cysteine residues that make up the novel disulfide ring in the active site of MDH led to an inactive enzyme, confirming the essential role of this remarkable ring structure. Mutations of cytochrome c(L), which is the electron acceptor from MDH was used to identify Met109 as the sixth ligand to the heme.

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.

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.

The biochemistry, physiology and genetics of PQQ and PQQ-containing enzymes

Pyrrolo-quinoline quinone (PQQ) is the non-covalently bound prosthetic group of many quinoproteins catalysing reactions in the periplasm of Gram-negative bacteria. Most of these involve the oxidation of alcohols or aldose sugars. PQQ is formed by fusion of glutamate and tyrosine, but details of the biosynthetic pathway are not known; a polypeptide precursor in the cytoplasm is probably involved, the completed PQQ being transported into the periplasm. In addition to the soluble methanol dehydrogenase of methylotrophs, there are three classes of alcohol dehydrogenases; type I is similar to methanol dehydrogenase; type II is a soluble quinohaemoprotein, having a C-terminal extension containing haem C; type III is similar but it has two additional subunits (one of which is a multihaem cytochrome c), bound in an unusual way to the periplasmic membrane. There are two types of glucose dehydrogenase; one is an atypical soluble quinoprotein which is probably not involved in energy transduction. The more widely distributed glucose dehydrogenases are integral membrane proteins, bound to the membrane by transmembrane helices at the N-terminus. The structures of the catalytic domains of type III alcohol dehydrogenase and membrane glucose dehydrogenase have been modelled successfully on the methanol dehydrogenase structure (determined by X-ray crystallography). Their mechanisms are likely to be similar in many ways and probably always involve a calcium ion (or other divalent cation) at the active site. The electron transport chains involving the soluble alcohol dehydrogenases usually consist only of soluble c-type cytochromes and the appropriate terminal oxidases. The membrane-bound quinohaemoprotein alcohol dehydrogenases pass electrons to membrane ubiquinone which is then oxidized directly by ubiquinol oxidases. The electron acceptor for membrane glucose dehydrogenase is ubiquinone which is subsequently oxidized directly by ubiquinol oxidases or by electron transfer chains involving cytochrome bc1, cytochrome c and cytochrome c oxidases. The function of most of these systems is to produce energy for growth on alcohol or aldose substrates, but there is some debate about the function of glucose dehydrogenases in those bacteria which contain one or more alternative pathways for glucose utilization. Synthesis of the quinoprotein respiratory systems requires production of PQQ, haem and the dehydrogenase subunits, transport of these into the periplasm, and incorporation together with divalent cations, into active quinoproteins and quinohaemoproteins. Six genes required for regulation of synthesis of methanol dehydrogenase have been identified in Methylobacterium, and there is evidence that two, two-component regulatory systems are involved.

The structure and function of the PQQ-containing quinoprotein dehydrogenases

Bacterial methanol and glucose dehydrogenases containing a novel type of prosthetic group, subsequently identified as pyrrolo-quinoline quinone (PQQ), were first described about 30 years ago. Quinoproteins were originally defined as proteins containing PQQ but this definition has since been broadened to include those proteins containing other types of quinone-containing prosthetic groups, and the X-ray structures of representatives of each type of quinoprotein have recently been published. This review is mainly concerned with the structure and function of the PQQ-containing methanol dehydrogenase, whose structure has been determined at high resolution, and related proteins. Their basic structure consists of a 'propeller' fold superbarrel made up of 8-sheet 'propeller blades' which are held together by novel tryptophan-docking motifs. In methanol dehydrogenase the PQQ in the active site is coordinated to a Ca2+ ion and is maintained in position by a stacked tryptophan and a novel 8-membered ring structure made up of a disulphide bridge between adjacent cysteine residues. This review describes these features and discusses them in relation to previously proposed mechanisms for this enzyme.

Conformation of coenzyme pyrroloquinoline quinone and role of Ca2+ in the catalytic mechanism of quinoprotein methanol dehydrogenase

The ab initio structures of 2,7,9-tricarboxypyrroloquinoline quinone (PQQ), semiquinone (PQQH), and dihydroquinone (PQQH2) have been determined and compared with ab initio structures of the (PQQ)Ca2+, (PQQH)Ca2+, and (PQQH2)Ca2+ complexes as well as the x-ray structure of (PQQ)Ca2+ bound at the active site of the methanol dehydrogenase (MDH) of methyltropic bacteria. Plausible mechanisms for the MDH oxidation of methanol involving the (PQQ)Ca2+ complex are explored via ab initio computations and discussed. Considering the reaction of methanol with PQQ in the absence of Ca2+, nucleophilic addition of methanol to the PQQ C-5 carbonyl followed by a retro-ene elimination is deemed unlikely due to large energy barrier. A much more favorable disposition of the methanol C-5 adduct to provide formaldehyde involves proton ionization of the intermediate followed by elimination of methoxide concerted with hydride transfer to the oxygen of the C-4 carbonyl. Much the same transition state is reached if one searches for the transition state beginning with Asp-303-CO2-general-base removal of the methanol proton of the (PQQ)Ca2+O(H)CH3 complex concerted with hydride transfer to the oxygen at C-4. For such a mechanism the role of the Ca2+ moiety would be to (i) contribute to the formation of the ES complex (ii) provide a modest decrease in the pKa of methanol substrate,; and (iii) polarize the oxygen at C-5.

Ca2+ and its substitutes have two different binding sites and roles in soluble, quinoprotein (pyrroloquinoline-quinone-containing) glucose dehydrogenase

To investigate the mode of binding and the role of Ca2+ in soluble, pyrroloquinoline-quinone (PQQ)-containing glucose dehydrogenase of the bacterium Acinetobacter calcoaceticus (sGDH), the following enzyme species were prepared and their interconversions studied: monomeric apoenzyme (M); monomer with one firmly bound Ca2+ ion (M*); dimer consisting of 2 M* (D); dimer consisting of 2 M and 2 PQQ (Holo-Y); dimer consisting of D with 2 PQQ (Holo-X); fully reconstituted enzyme consisting of Holo-X with two extra Ca2+ ions (Holo) or substitutes for Ca2+ (hybrid Holo-enzymes). D and Holo are very stable enzyme species regarding monomerization and inactivation by chelator, respectively, the bound Ca2+ being locked up in such a way that it is not accessible to chelator. D can be converted into M* by heat treatment and the tightly bound Ca2+ can be removed from M* with chelator, transforming it into M. Reassociation of M* to D occurs spontaneously at 20 degrees C; reassociation of M to D occurs by adding a stoichiometric amount of Ca2+. Synergistic effects were exerted by bound Ca2+ and PQQ, each increasing the affinity of the protein for the other component. Dimerization of M to D occurred with Ca2+, Cd2+, Mn2+, and Sr2+ (in decreasing order of effectiveness), but not with Mg2+, Ba2+, Co2+, Ni2+, Zn2+, or monovalent cations. Conversion of inactive Holo-X into active Holo, was achieved with Ca2+ or metal ions effective in dimerization. Although it is likely that activation of Holo-X involves binding of metal ion to PQQ, the spectral and enzymatic activity differences between normal Holo- and hybrid Holo-enzymes are relatively small. Titration experiments revealed that the two Ca2+ ions required for activation of Holo-X are even more firmly bound than the two required for dimerization of M and anchoring of PQQ. Although the two binding sites related with the dual function of Ca2+ show similar metal ion specificity, they are not identical. The presence of two different sites in sGDH appears to be unique because in other PQQ-containing dehydrogenases, the PQQ-containing subunit has only one site. Given the broad spectrum of bivalent metal ions effective in reconstituting quinoprotein dehydrogenase apoenzymes to active holoenzymes, but the limited spectrum for an individual enzyme, the specificity is not so much determined by PQQ but by the variable metal-ion-binding sites.

Quinoprotein-catalysed reactions

This review is concerned with the structure and function of the quinoprotein enzymes, sometimes called quinoenzymes. These have prosthetic groups containing quinones, the name thus being analogous to the flavoproteins containing flavin prosthetic groups. Pyrrolo-quinoline quinone (PQQ) is non-covalently attached, whereas tryptophan tryptophylquinone (TTQ), topaquinone (TPQ) and lysine tyrosylquinone (LTQ) are derived from amino acid residues in the backbone of the enzymes. The mechanisms of the quinoproteins are reviewed and related to their recently determined three-dimensional structures. As expected, the quinone structures in the prosthetic groups play important roles in the mechanisms. A second common feature is the presence of a catalytic base (aspartate) at the active site which initiates the reactions by abstracting a proton from the substrate, and it is likely to be involved in multiple reactions in the mechanism. A third common feature of these enzymes is that the first part of the reaction produces a reduced prosthetic group; this part of the mechanism is fairly well understood. This is followed by an oxidative phase involving electron transfer reactions which remain poorly understood. In both types of dehydrogenase (containing PQQ and TTQ), electrons must pass from the reduced prosthetic group to redox centres in a second recipient protein (or protein domain), whereas in amine oxidases (containing TPQ or LTQ), electrons must be transferred to molecular oxygen by way of a redox-active copper ion in the protein.

Reconstitution of the quinoprotein methanol dehydrogenase from inactive Ca(2+)-free enzyme with Ca2+, Sr2+ or Ba2+

The reconstitution of active holoenzyme containing calcium from inactive calcium-free methanol dehydrogenase, isolated from a moxA mutant of Methylobacterium extorquens, has a pH optimum of about pH 10, with a well defined pK for the process at pH 9.3. Two Ca2+ ions were irreversibly incorporated per alpha 2 beta 2 tetramer. Calcium could be replaced in the incorporation process by strontium or barium, the affinities for these ions being similar to that for Ca2+. Arrhenius plots for measurement of the activation energy of reconstitution were biphasic; the lower activation energy was typical of most biological processes, while the higher activation energy was at least three times greater, implying the involvement of a large conformational change during incorporation of the cations. The activation energy for incorporation of Ba2+ was considerably higher than that for incorporation of Ca2+. The novel disulphide bridge that is at the active site of the enzyme was not involved in the incorporation process. Studies of the time courses for incorporation of 45Ca2+, production of active enzyme and changes in absorption spectra failed to show any intermediates in the incorporation process.

Characterization of a novel methanol dehydrogenase containing a Ba2+ ion at the active site

The quinoprotein methanol dehydrogenase (MDH) contains a Ca2+ ion at the active site. Ca(2-)-free enzyme (from a processing mutant) was used to obtain enzyme containing Sr2+ or Ba2+, the Ba(2+)-MDH being the first enzyme to be described in which a Ba2+ ion functions at the active site. The activation energy for oxidation of methanol by Ba(2+)-MDH is less than half that of the reaction catalysed by Ca(2+)-MDH (a difference of 21.4 kJ/mol), and the Vmax value is 2-fold higher. The affinities of Ba(2+)-MDH for substrate and activator are very much less than those of Ca(2+)-MDH; the Km for methanol is 3.5 mM (compared with 3 microM) and the KA for ammonia is 52 mM (compared with 2 mM). The different activity of Ba(2+)-MDH is probably due to a change in the conformation of the active site, leading to a decrease in the free energy of substrate binding and hence a decrease in activation energy. The kinetic model for Ba(2+)-MDH with respect to substrate and activator is consistent with previous models for Ca(2+)-MDH. The pronounced deuterium isotope effect (6.0-7.6) is influenced by ammonia, and is consistent with activation of the pyrroloquinoline quinone reduction step by ammonia. Because of its low affinity for substrates, it is possible to prepare the oxidized form of Ba(2+)-MDH. No spectral intermediates could be detected during reduction by added substrate, and so it is not possible to distinguish between those mechanisms involving covalent substrate addition and those involving only hydride transfer.

Thermal stability of methanol dehydrogenase is altered by the replacement of enzyme-bound Ca2+ with Sr2+

Methanol dehydrogenase (MEDH) possesses tightly bound Ca2+ in addition to its pyrroloquinoline quinone prosthetic group. Ca2+ was replaced with Sr2+ by growing the host bacterium, Paracoccus denitrificans, in media in which Ca2+ was replaced with Sr2+. At temperatures in the transition region for stability, the rate constants for inactivation of MEDH purified from these cells (Sr-MEDH) were 2-fold lower than those for MEDH. However, Arrhenius plots yielded an activation energy (Ea) of 699 kJ (167 kcal)/mol for MEDH compared with 640 kJ (153 kcal)/mol for Sr-MEDH. Further analysis by transition-state theory yielded values for the activation enthalpy (delta H*) and activation entropy (delta S*) of 696 kJ (166 kcal)/mol and 1.73 kJ (414 cal)/mol per K for MEDH and 637 kJ (152 kcal)/mol and 1.55 kJ (371 cal)/mol per K for Sr-MEDH. The higher rate of inactivation of MEDH than Sr-MEDH at higher temperatures is a consequence of a more favourable net gain in entropy. This positive entropy contribution increases at high temperatures, and reduces the more favourable stability obtained from the enthalpy contribution for the free energy (delta G*) of inactivation. The differences in these thermodynamic data are discussed in relation to the recently determined crystal structure of MEDH as well as 1H electron-nuclear double resonance studies of the influence of Sr2+ substitution on the structure of the pyrroloquinoline quinone-derived radical in MEDH.