Studies on the structure and mechanism of Streptococcus faecium L-alpha-glycerophosphate oxidase

Glycerol 3-phosphate dehydrogenases (1 and 2) in cancer and other diseases

The glycerol 3-phosphate shuttle (GPS) is composed of two different enzymes: cytosolic NAD+-linked glycerol 3-phosphate dehydrogenase 1 (GPD1) and mitochondrial FAD-linked glycerol 3-phosphate dehydrogenase 2 (GPD2). These two enzymes work together to act as an NADH shuttle for mitochondrial bioenergetics and function as an important bridge between glucose and lipid metabolism. Since these genes were discovered in the 1960s, their abnormal expression has been described in various metabolic diseases and tumors. Nevertheless, it took a long time until scientists could investigate the causal relationship of these enzymes in those pathophysiological conditions. To date, numerous studies have explored the involvement and mechanisms of GPD1 and GPD2 in cancer and other diseases, encompassing reports of controversial and non-conventional mechanisms. In this review, we summarize and update current knowledge regarding the functions and effects of GPS to provide an overview of how the enzymes influence disease conditions. The potential and challenges of developing therapeutic strategies targeting these enzymes are also discussed.

Evidence for the Cytoplasmic Localization of the L-α-Glycerophosphate Oxidase in Members of the "Mycoplasma mycoides Cluster"

Members of the "Mycoplasma mycoides cluster" are important animal pathogens causing diseases including contagious bovine pleuropneumonia and contagious caprine pleuropneumonia, which are of utmost importance in Africa or Asia. Even if all existing vaccines have shortcomings, vaccination of herds is still considered the best way to fight mycoplasma diseases, especially with the recent and dramatic increase of antimicrobial resistance observed in many mycoplasma species. A new generation of vaccines will benefit from a better understanding of the pathogenesis of mycoplasmas, which is very patchy up to now. In particular, surface-exposed virulence traits are likely to induce a protective immune response when formulated in a vaccine. The candidate virulence factor L-α-glycerophosphate oxidase (GlpO), shared by many mycoplasmas including Mycoplasma pneumoniae, was suggested to be a surface-exposed enzyme in Mycoplasma mycoides subsp. mycoides responsible for the production of hydrogen peroxide directly into the host cells. We produced a glpO isogenic mutant GM12::YCpMmyc1.1-ΔglpO using in-yeast synthetic genomics tools including the tandem-repeat endonuclease cleavage (TREC) technique followed by the back-transplantation of the engineered genome into a mycoplasma recipient cell. GlpO localization in the mutant and its parental strain was assessed using scanning electron microscopy (SEM). We obtained conflicting results and this led us to re-evaluate the localization of GlpO using a combination of in silico and in vitro techniques, such as Triton X-114 fractionation or tryptic shaving followed by immunoblotting. Our in vitro results unambiguously support the finding that GlpO is a cytoplasmic protein throughout the "Mycoplasma mycoides cluster." Thus, the use of GlpO as a candidate vaccine antigen is unlikely to induce a protective immune response.

Kinetic mechanism of L-α-glycerophosphate oxidase from Mycoplasma pneumoniae

L-α-glycerophosphate oxidase is an FAD-dependent enzyme that catalyzes the oxidation of L-α-glycerophosphate (Glp) by molecular oxygen to generate dihydroxyacetone phosphate (DHAP) and hydrogen peroxide (H2O2). The catalytic properties of recombinant His6-GlpO from Mycoplasma pneumoniae (His6-MpGlpO) were investigated through transient and steady-state kinetics and ligand binding studies. The results indicate that the reaction mechanism of His6-MpGlpO follows a ping-pong model. Double-mixing mode stopped-flow experiments show that, after flavin-mediated substrate oxidation, DHAP leaves rapidly prior to the oxygen reaction. The values determined for the individual rate constants and kcat (4.2 s(-1) at 4 °C), in addition to the finding that H2 O2 binds to the oxidized enzyme, suggest that H2O2 release is the rate-limiting step for the overall reaction. The results indicate that His6 -MpGlpO contains mixed populations of fast- and slow-reacting species. It is predominantly the fast-reacting species that participates in turnover. In contrast to other GlpO enzymes previously described, His6-MpGlpO is able to catalyze the reverse reaction of reduced enzyme and DHAP. This result may be explained by the standard reduction potential value of His6-MpGlpO (-167 ± 1 mV), which is lower than those of GlpO from other species. We found that D,L-glyceraldehyde 3-phosphate (GAP) may be used as a substrate in the His6-MpGlpO reaction, although it exhibited an approximately 100-fold lower kcat value in comparison with the reaction of Glp. These results also imply involvement of GlpO in glycolysis, as well as in lipid and glycerol metabolism. The kinetic models and distinctive properties of His6-MpGlpO reported here should be useful for future drug development against Mycoplasma pneumoniae infection.

Bioelectrocatalytic sensor for triglycerides in human skin sebum based on enzymatic cascade reaction of lipase, glycerol kinase and glycerophosphate oxidase

We report the development of an electrochemical biosensor for the quantification of triglycerides in human skin sebum, based on a multienzyme cascade reaction. The presence of excessive triglycerides in human sebum is one of the leading causes of various skin ailments. However, to the best of our knowledge, no bioelectrocatalytic approach for the quantification of sebum triglycerides has been made. In order to develop triglyceride biosensor, we fabricated a multienzyme-associated electrode incorporating lipase, glycerol kinase, and glycerophosphate oxidase. Enzymes were deposited by electrostatic force and further stabilized via crosslinking between enzymes and polymer matrices. The enzyme-modified biosensing electrode maintained its bioelectrocatalytic activity for five days. An additional constraint was the limited solubility of sebum triglycerides in aqueous electrolytes, impeding the analysis. To address this issue, triglyceride samples were prepared in the form of micelles, enabling efficient sample preparation for biosensor signaling. Calibration tests revealed that the designed assay had a detection range of 15-200mg/dL of micellar triglyceride, which covered the required determination range. The developed biosensing approach was successfully used to determine triglyceride concentrations in real sebum samples of unknown triglyceride content.

Identification of a novel pneumococcal vaccine antigen preferentially expressed during meningitis in mice

Streptococcus pneumoniae is the most common cause of severe bacterial meningitis in children, the elderly, and immunocompromised individuals. To identify virulence factors preferentially expressed during meningitis, we conducted niche-specific genome-wide in vivo transcriptomic analysis after intranasal infection of mice with serotype 4 or 6A pneumococci. The expression of 34 bacterial genes was substantially altered in brain tissue of mice infected with either of the 2 strains. Ten upregulated genes were common to both strains, 7 of which were evaluated for their role in the development of meningitis. One previously uncharacterized protein, α-glycerophosphate oxidase (GlpO), was cytotoxic for human brain microvascular endothelial cells (HBMECs) via generation of H(2)O(2). A glpO deletion mutant was defective in adherence to HBMECs in vitro as well as in progression from the blood to the brain in vivo. Mutant bacteria also induced markedly reduced meningeal inflammation and brain pathology compared with wild type, despite similar levels of bacteremia. Immunization of mice with GlpO protected against invasive pneumococcal disease and provided additive protection when formulated with pneumolysin toxoid. Our results provide the basis of a strategy that can be adapted to identify genes that contribute to the development of meningitis caused by other pathogens.

Effect of IPTG amount on apo- and holo- forms of glycerophosphate oxidase expressed in Escherichia coli

Escherichia coli has proved to be a successful host for the expression of many heterologous proteins, and much efforts have been made toward improving recombinant protein expression including the usage of strong promoters and co-expression with chaperones. But little attention was paid on the relation between expression level and function of the target protein. Glycerophosphate oxidase (GPO) is a protein with FAD cofactor (without free cysteine and disulfide bonds).It was observed that the specific activity of GPO dramatically decreased with the increase of inducer IPTG. In addition, the stability of it decreased correspondingly. The structural difference of samples expressed under varying IPTG was investigated using size-exclusion and reverse-phase high performance liquid chromatography, together with CD spectrum. It was found that the conformation of peptide and organization of subunits were not affected. The loss of specific activity and stability were correlated to incomplete attachment of FAD onto GPO. These results revealed that synthesis speed should be controlled either by reduction of IPTG amount or using weak promoters in the production of GPO.

Glycerol is metabolized in a complex and strain-dependent manner in Enterococcus faecalis

Enterococcus faecalis is equipped with two pathways of glycerol dissimilation. Glycerol can either first be phosphorylated by glycerol kinase and then oxidized by glycerol-3-phosphate oxidase (the glpK pathway) or first be oxidized by glycerol dehydrogenase and then phosphorylated by dihydroxyacetone kinase (the dhaK pathway). Both pathways lead to the formation of dihydroxyacetone phosphate, an intermediate of glycolysis. It was assumed that the glpK pathway operates during aerobiosis and that the dhaK pathway operates under anaerobic conditions. Because this had not been analyzed by a genetic study, we constructed mutants of strain JH2-2 affected in both pathways. The growth of these mutants on glycerol under aerobic and anaerobic conditions was monitored. In contrast to the former model, results strongly suggest that glycerol is catabolized simultaneously by both pathways in the E. faecalis JH2-2 strain in the presence of oxygen. In accordance with the former model, glycerol is metabolized by the dhaK pathway under anaerobic conditions. Comparison of different E. faecalis isolates revealed an impressive diversity of growth behaviors on glycerol. Analysis by BLAST searching and real-time reverse transcriptase PCR revealed that this diversity is based not on different gene contents but rather on differences in gene expression. Some strains used preferentially the glpK pathway whereas others probably exclusively the dhaK pathway under aerobic conditions. Our results demonstrate that the species E. faecalis cannot be represented by only one model of aerobic glycerol catabolism.

Functional and antigenic properties of GlpO from Mycoplasma mycoides subsp. mycoides SC: characterization of a flavin adenine dinucleotide-binding site deletion mutant

L-α-glycerophosphate oxidase (GlpO) plays a central role in virulence of Mycoplasma mycoides subsp. mycoides SC, a severe bacterial pathogen causing contagious bovine pleuropneumonia (CBPP). It is involved in production and translocation of toxic H₂O₂ into the host cell, causing inflammation and cell death. The binding site on GlpO for the cofactor flavin adenine dinucleotide (FAD) has been identified as Gly₁₂−Gly₁₃−Gly₁₄−Ile₁₅−Ile₁₆−Gly₁₇. Recombinant GlpO lacking these six amino acids (GlpOΔFAD) was unable to bind FAD and was also devoid of glycerophosphate oxidase activity, in contrast to non-modified recombinant GlpO that binds FAD and is enzymatically active. Polyclonal monospecific antibodies directed against GlpOΔFAD, similarly to anti-GlpO antibodies, neutralised H₂O₂ production of M. mycoides subsp. mycoides SC grown in the presence of glycerol, as well as cytotoxicity towards embryonic calf nasal epithelial (ECaNEp) cells. The FAD-binding site of GlpO is therefore suggested as a valuable target site for the future construction of deletion mutants to yield attenuated live vaccines of M. mycoides subsp. mycoides SC necessary to efficiently combat CBPP.

Blue light diminishes interaction of PAS/LOV proteins, putative blue light receptors in Arabidopsis thaliana, with their interacting partners

The light, oxygen, or voltage (LOV) domain that belongs to the Per-ARNT-Sim (PAS) domain superfamily is a blue light sensory module. The Arabidopsis thaliana PAS/LOV PROTEIN (PLP) gene encodes three putative blue light receptor proteins, PLPA, PLPB, and PLPC, because of its mRNA splicing variation. PLPA and PLPB each contain one PAS domain at the N-terminal region and one LOV domain at the C-terminal region, while the LOV domain is truncated in PLPC. RNA gel blot analysis showed that PLP mRNA was markedly expressed after exposure to salt or dehydration stress. Yeast two-hybrid screening led to the isolation of VITAMIN C DEFECTIVE 2 (VTC2), VTC2-LIKE (VTC2L), and BEL1-LIKE HOMEODOMAIN 10 proteins (BLH10A and BLH10B) as PLP-interacting proteins. The molecular interaction of PLPA with VTC2L, BLH10A or BLH10B, and that of PLPB with VTC2L were diminished when yeasts were grown under blue light illumination. Furthermore, the possible binding of flavin chromophore to PLPA and PLPB was demonstrated. These results imply that the LOV domain of PLPA and PLPB functions as a blue light sensor, and suggest the applicability of these interactions to blue light-dependent switching in transcriptional regulation in yeast or other organisms.

Structure of alpha-glycerophosphate oxidase from Streptococcus sp.: a template for the mitochondrial alpha-glycerophosphate dehydrogenase

The FAD-dependent alpha-glycerophosphate oxidase (GlpO) from Enterococcus casseliflavus and Streptococcus sp. was originally studied as a soluble flavoprotein oxidase; surprisingly, the GlpO sequence is 30-43% identical to those of the alpha-glycerophosphate dehydrogenases (GlpDs) from mitochondrial and bacterial sources. The structure of a deletion mutant of Streptococcus sp. GlpO (GlpODelta, lacking a 50-residue insert that includes a flexible surface region) has been determined using multiwavelength anomalous dispersion data and refined at 2.3 A resolution. Using the GlpODelta structure as a search model, we have also determined the intact GlpO structure, as refined at 2.4 A resolution. The first two domains of the GlpO fold are most closely related to those of the flavoprotein glycine oxidase, where they function in FAD binding and substrate binding, respectively; the GlpO C-terminal domain consists of two helix bundles and is not closely related to any known structure. The flexible surface region in intact GlpO corresponds to a segment of missing electron density that links the substrate-binding domain to a betabetaalpha element of the FAD-binding domain. In accordance with earlier biochemical studies (stabilizations of the covalent FAD-N5-sulfite adduct and p-quinonoid form of 8-mercapto-FAD), Ile430-N, Thr431-N, and Thr431-OG are hydrogen bonded to FAD-O2alpha in GlpODelta, stabilizing the negative charge in these two modified flavins and facilitating transfer of a hydride to FAD-N5 (from Glp) as well. Active-site overlays with the glycine oxidase-N-acetylglycine and d-amino acid oxidase-d-alanine complexes demonstrate that Arg346 of GlpODelta is structurally equivalent to Arg302 and Arg285, respectively; in both cases, these residues interact directly with the amino acid substrate or inhibitor carboxylate. The structural and functional divergence between GlpO and the bacterial and mitochondrial GlpDs is also discussed.

Photoreaction cycle of the light, oxygen, and voltage domain in FKF1 determined by low-temperature absorption spectroscopy

Flavin-binding Kelch repeat F-box (FKF1) protein plays important roles in the photoregulation of flowering in Arabidopsis. FKF1 has a light, oxygen, and voltage (LOV) sensing domain binding a flavin mononucleotide (FMN) as a chromophore noncovalently. Photoreaction of the FKF1-LOV polypeptide was studied by low-temperature absorption spectroscopy. Upon blue light irradiation, a ground state, D(450), is converted to S(390) known as a cysteinyl-flavin adduct intermediate in the photoreaction of phototropin. Below 150 K, bleaching of D(450) was much reduced and a new photoproduct, Z(370), appeared as well as S(390) formation. The calculated absorption spectrum for Z(370) is very similar to those of flavoproteins in an anion radical state. On the basis of the results that S(390) formation proceeds to Z(370) formation and that Z(370) formed at low temperatures reverts to D(450) upon temperature increase, Z(370) is concluded to be not an intermediate from D(450) to S(390). Z(370) is suggested to be formed from the biradical triplet-excited state after relaxing to the ground state with the FMN anion radical trapped at the low temperature, in which the SH of the cysteine is in the wrong position that is able to produce a radical pair but unable to form the cysteinyl-flavin adduct. The counter SH in the cationic radical state may revert to the ground state by extracting an electron from the unidentified amino acid residue. Interestingly, S(390) that has been thought to be irreversible to D(450) was revealed to revert to D(450) very slowly with a half-life time of 62.5 h in solution at 298 K. The photoreaction mechanism is discussed in reference to the calculated activation energy of the reaction processes.

Characterization of the FAD-containing N-methyltryptophan oxidase from Escherichia coli

N-Methyltryptophan oxidase (MTOX) is a flavoenzyme that catalyzes the oxidative demethylation of N-methyl-L-tryptophan and other N-methyl amino acids, including sarcosine, which is a poor substrate. The Escherichia coli gene encoding MTOX (solA) was isolated on the basis of its sequence homology with monomeric sarcosine oxidase, a sarcosine-inducible enzyme found in many bacteria. These studies show that MTOX is expressed as a constitutive enzyme in a wild-type E. coli K-12 strain, providing the first evidence that solA is a functional gene. MTOX expression is enhanced 3-fold by growth on minimal media but not induced by N-methyl-L-tryptophan, L-tryptophan, or 3-indoleacrylate. MTOX forms an anionic flavin semiquinone and a reversible, covalent flavin-sulfite complex (K(d) = 1.7 mM), properties characteristic of flavoprotein oxidases. Rates of formation (k(on) = 5.4 x 10(-3) M(-1) s(-1)) and dissociation (k(off) = 1.3 x 10(-5) s(-1)) of the MTOX-sulfite complex are orders of magnitude slower than observed with most other flavoprotein oxidases. The pK(a) for ionization of oxidized FAD at N(3)H in MTOX (8.36) is two pH units lower than that observed for free FAD. The MTOX active site was probed by characterization of various substrate analogues that act as competitive inhibitors with respect to N-methyl-L-tryptophan. Qualitatively similar perturbations of the MTOX visible absorption spectrum are observed for complexes formed with various aromatic carboxylates, including benzoate, 3-indole-(CH(2))(n)-CO(2)(-) and 2-indole-CO(2)(-). The most stable complex with 3-indole-(CH(2))(n)-CO(2)(-) is formed with 3-indolepropionate (K(d) = 0.79 mM), a derivative with the same side chain length as N-methyl-L-tryptophan. Benzoate binding is enhanced upon protonation of a group in the enzyme-benzoate complex (pK(EL) = 6.87) but blocked by ionization of a group in the free enzyme (pK(E) = 8.41), which is attributed to N(3)H of FAD. Difference spectra observed for the aromatic carboxylate complexes are virtually mirror images of those observed with sarcosine analogues (N,N'-dimethylglycine, N-benzylglycine). Charge-transfer complexes are formed with 3-indoleacrylate, pyrrole-2-carboxylate, and CH(3)XCH(2)CO(2)(-) (X = S, Se, Te).

Limited proteolysis as a structural probe of the soluble alpha-glycerophosphate oxidase from Streptococcus sp

As reported previously [Parsonage, D., Luba, J., Mallett, T. C., and Claiborne, A. (1998) J. Biol. Chem. 273, 23812-23822], the flavoprotein alpha-glycerophosphate oxidases (GlpOs) from a number of enterococcal and streptococcal sources contain a conserved 50-52 residue insert that is completely absent in the homologous alpha-glycerophosphate dehydrogenases. On limited proteolysis with trypsin, the GlpO from Streptococcus sp. (m = 67.6 kDa) is readily converted to two major fragments corresponding to masses of approximately 40 and 23 kDa. The combined application of sequence and mass spectrometric analyses demonstrates that the 40-kDa fragment represents the N-terminus of intact GlpO (Met1-Lys368; 40.5 kDa), while the 23-kDa band represents a C-terminal fragment (Ala405-Lys607; 22.9 kDa). Hence, limited proteolysis in effect excises most of the GlpO insert (Ser355-Lys404), indicating that this represents a flexible region on the protein surface. The active-site and other spectroscopic properties of the enzyme, including both flavin and tryptophan fluorescence spectra, titration behavior with both dithionite and sulfite, and preferential binding of the anionic form of the oxidized flavin, were largely unaffected by proteolysis. Enzyme-monitored turnover analyses of the intact and nicked streptococcal GlpOs (at [GlpO] approximately 10 microM) demonstrate that the single major catalytic defect in the nicked enzyme corresponds to a 20-fold increase in K(m)(Glp); the basis for this altered kinetic behavior is derived from an 8-fold decrease in the second-order rate constant for reduction of the nicked enzyme, as measured in anaerobic stopped-flow experiments. These results indicate that the flexible surface region represented by elements of the GlpO insert plays an important role in mediating efficient flavin reduction.

The soluble alpha-glycerophosphate oxidase from Enterococcus casseliflavus. Sequence homology with the membrane-associated dehydrogenase and kinetic analysis of the recombinant enzyme

The soluble flavoprotein alpha-glycerophosphate oxidase from Enterococcus casseliflavus catalyzes the oxidation of a "non-activated" secondary alcohol, in contrast to the flavin-dependent alpha-hydroxy- and alpha-amino acid oxidases. Surprisingly, the alpha-glycerophosphate oxidase sequence is 43% identical to that of the membrane-associated alpha-glycerophosphate dehydrogenase from Bacillus subtilis; only low levels of identity (17-22%) result from comparisons with other FAD-dependent oxidases. The recombinant alpha-glycerophosphate oxidase is fully active and stabilizes a flavin N(5)-sulfite adduct, but only small amounts of intermediate flavin semiquinone are observed during reductive titrations. Direct determination of the redox potential for the FAD/FADH2 couple yields a value of -118 mV; the protein environment raises the flavin potential by 100 mV in order to provide for a productive interaction with the reducing substrate. Steady-state kinetic analysis, using the enzyme-monitored turnover method, indicates that a ping-pong mechanism applies and also allows the determination of the corresponding kinetic constants. In addition, stopped-flow studies of the reductive half-reaction provide for the measurement of the dissociation constant for the enzyme. alpha-glycerophosphate complex and the rate constant for reduction of the enzyme flavin. These and other results demonstrate that this enzyme offers a very promising paradigm for examining the protein determinants for flavin reactivity and mechanism in the energy-yielding metabolism of alpha-glycerophosphate.

The genetic basis of colony opacity in Streptococcus pneumoniae: evidence for the effect of box elements on the frequency of phenotypic variation

Streptococcus pneumoniae undergoes spontaneous phase variation in colony morphology. Differences in colony opacity have previously been shown to correlate with differences in the ability of organisms to colonize the mucosal surface of the nasopharynx in an animal model. The genetic basis of opacity variation was identified in transformation experiments. A DNA library, from a strain that varies at high frequency, was screened to identify a single clone capable of transforming a transparent recipient strain which varies at low frequency to an opaque phenotype. Analysis of this opacity locus revealed two genes, glpD and glpF, with similarity to genes required for glycerol metabolism in other bacteria. Following the pneumococcal glpF, repetitive intergenic elements, boxes A and C, were identified. These stem-loop-forming elements were not present in the same locus of the recipient strain. Although not required for phase variation in colony opacity, the box element was necessary for expression of phase variation at high frequency. Introduction of the box elements during transformation affected colony morphology, possibly by altering expression of a putative regulatory gene downstream from the box element. Mutagenesis within this region confirmed the contribution of the putative regulatory gene to the expression of colony opacity. Growth characteristics of strains generated in this study provide additional evidence for an association of differences in cell wall autolysis and variation in colony opacity.

Molecular cloning and analysis of the gene encoding the NADH oxidase from Streptococcus faecalis 10C1. Comparison with NADH peroxidase and the flavoprotein disulfide reductases

The gene encoding the streptococcal flavoprotein NADH oxidase (NOXase), which catalyzes the four-electron reduction of O2-->2H2O, has been cloned and sequenced from the genome of Streptococcus (Enterococcus) faecalis 10C1 (ATCC 11700). The deduced NOXase protein sequence corresponds to a molecular mass of 48.9 kDa and contains three previously sequenced cysteinyl peptides obtained with the purified enzyme. In Escherichia coli, the expressed nox gene produced a catalytically active product, which retained its immunoreactivity to affinity-purified NOXase antisera. Alignment of the NOXase protein sequence with that of streptococcal NADH peroxidase (NPXase) revealed that the proteins are 44% identical. Among the most highly conserved segments is a sequence containing Cys42; this residue is known to exist as a stabilized cysteine-sulfenic acid (Cys-SOH) in NPXase and serves as the non-flavin redox center. In addition, three previously identified NPXase segments, known to be involved in FAD and NAD(P)-binding in other pyridine nucleotide-linked flavoprotein oxidoreductases, are strongly conserved in NOXase. Overall, the extensive homology observed between NOXase and NPXase suggests that the monomer chain fold of the oxidase closely resembles that of the peroxidase. Both sequences share limited but significant homology to those of glutathione reductase and other members of the flavoprotein disulfide reductase family. These and other considerations suggest that these two unusual streptococcal flavoproteins constitute a distinct class of FAD-dependent oxidoreductases, the flavoprotein peroxide reductases, easily contrasted with enzymes such as glutathione reductase and thioredoxin reductase.