RESUMEN
We present a case of Streptococcus equi subsp. equi meningitis in a young boy. This case represents the first report in the literature of meningitis caused by this organism, as far as we know.
Asunto(s)
Meningitis Bacterianas/etiología , Infecciones Estreptocócicas/etiología , Streptococcus equi/aislamiento & purificación , Adolescente , Humanos , Masculino , Meningitis Bacterianas/tratamiento farmacológico , Penicilina G/uso terapéutico , Infecciones Estreptocócicas/tratamiento farmacológicoRESUMEN
BACKGROUND: Treatment strategies involving dose intensification have recently demonstrated improvements in cure compared with older trials. However, dose-intensive therapy is associated with increased acute and long-term toxicities, particularly in pediatric patients. The Children's Cancer Group initiated this pilot study to assess the feasibility and toxicity of a moderate dose-intensive regimen, BEACOPP (bleomycin, etoposide, doxorubicin, cyclophosphamide, vincristine, procarbazine and prednisone), in children and adolescents with advanced-stage Hodgkin's lymphoma (HL). PATIENTS AND METHODS: Children with stage IIB or IIIB with bulk disease, or stage IV were eligible. Induction consisted of four cycles of escalated dose BEACOPP. The rapidity of response, defined as >70% reduction in disease burden, was assessed after two and four cycles. Rapid responders then received consolidation therapy as per gender-specific guidelines to reduce the risk of gender-specific long-term toxicities of therapy, i.e. females received four cycles of COPP/ABV (cyclophosphamide, vincristine, procarbazine, prednisone, doxorubicin, bleomycin and vinblastine) without radiation therapy and males received two cycles of ABVD (doxurubicin, bleomycin, vinblastine and dacarbazine) with involved field radiation therapy (IFRT). Slow responders received four cycles of BEACOPP and IFRT. RESULTS: Ninety-nine patients were enrolled. Myelosuppression was frequent. Non-hematological grade 4 toxicities included allergic reaction (two patients), hypotension (one), mucositis (four), infection (three), seizure (one) and elevated transaminases (one). Typhlitis developed in four patients; three recovered and completed dose-modified chemotherapy, while one died of sepsis associated with grade 4 neutropenia. A rapid response was achieved by 45 and 72% of patients after two and four cycles, respectively. There are no disease progressions or secondary malignancies to date. There is only one reported relapse to date. Median follow-up for the cohort is 6 months. CONCLUSIONS: BEACOPP chemotherapy is feasible and generally well tolerated in children with advanced-stage HL. The absence of reported progressive disease and only one relapse to date is encouraging.
Asunto(s)
Protocolos de Quimioterapia Combinada Antineoplásica/uso terapéutico , Bleomicina/uso terapéutico , Ciclofosfamida/uso terapéutico , Dacarbazina/uso terapéutico , Doxorrubicina/uso terapéutico , Etopósido/uso terapéutico , Enfermedad de Hodgkin/tratamiento farmacológico , Prednisona/uso terapéutico , Procarbazina/uso terapéutico , Vinblastina/uso terapéutico , Vincristina/uso terapéutico , Adolescente , Adulto , Niño , Preescolar , Estudios de Factibilidad , Femenino , Humanos , Masculino , Resultado del TratamientoRESUMEN
Three residues in the active site of the flavoprotein phenol hydroxylase (PHHY) were independently changed by site-directed mutagenesis. One of the mutant forms of PHHY, Tyr289Phe, is reduced by NADPH much slower than is the wild-type enzyme, although it has a slightly higher redox potential than the wild-type enzyme. In the structure of the wild-type enzyme, residue Tyr289 is hydrogen-bonded with the FAD when the latter is at the "out" position but has no direct contact with the flavin when it is "in". The oxidative half-reaction of PHHY is not significantly affected by this mutation, contrary to the concept that Tyr289 is a critical residue in the hydroxylation reaction [Enroth, C., Neujahr, H., Schneider, G., and Lindqvist, Y. (1998) Structure 6, 605-617; Ridder, L., Mullholland, A. J., Rietjens, I. M. C. M., and Vervoort, J. (2000) J. Am. Chem. Soc. 122, 8728-8738]. Tyr289 may help stabilize the FAD in the out conformation where it can be reduced by NADPH. For the Asp54Asn mutant form of PHHY, the initial step of the oxidative half-reaction is significantly slower than for the wild-type enzyme. Asp54Asn utilizes less than 20% of the reduced flavin for hydroxylating the substrate with the remainder forming H(2)O(2). Similar changes are observed when Arg281, a residue between Asp54 and the solvent, is mutated to Met. These two residues are suggested to be part of the active site environment the enzyme provides for the flavin cofactor to function optimally in the oxidative half-reaction. In the construction of the mutant forms of PHHY, it was determined that 11 of the previously reported amino acid residues in the sequence of PHHY were incorrect.
Asunto(s)
Oxigenasas de Función Mixta/química , Oxigenasas de Función Mixta/genética , Secuencia de Aminoácidos , Sustitución de Aminoácidos , Dominio Catalítico/genética , Cinética , Oxigenasas de Función Mixta/metabolismo , Modelos Moleculares , Mutagénesis Sitio-Dirigida , NADP/metabolismo , Oxidación-Reducción , Trichosporon/enzimología , Trichosporon/genéticaRESUMEN
The PHOT1 (NPH1) gene from Avena sativa specifies the blue light receptor for phototropism, phototropin, which comprises two FMN-binding LOV domains and a serine/threonine protein kinase domain. Light exposure is conducive to autophosphorylation of the protein kinase domain. We have reconstituted a recombinant LOV2 domain of A. sativa phototropin with various (13)C/(15)N-labeled isotopomers of the cofactor, FMN. The reconstituted protein samples were analyzed by NMR spectroscopy under dark and light conditions. Blue light irradiation is shown to result in the addition of a thiol group (cysteine 450) to the 4a position of the FMN chromophore. The adduct reverts spontaneously in the dark by elimination. The light-driven flavin adduct formation results in conformational modification, which was diagnosed by (1)H and (31)P NMR spectroscopy. This conformational change is proposed to initiate the transmission of the light signal via conformational modulation of the protein kinase domain conducive to autophosphorylation of NPH1.
RESUMEN
Cyclohexanone monooxygenase (CHMO), a bacterial flavoenzyme, carries out an oxygen insertion reaction on cyclohexanone to form a seven-membered cyclic product, epsilon-caprolactone. The reaction catalyzed involves the four-electron reduction of O2 at the expense of a two-electron oxidation of NADPH and a two-electron oxidation of cyclohexanone to form epsilon-caprolactone. Previous studies suggested the participation of either a flavin C4a-hydroperoxide or a flavin C4a-peroxide intermediate during the enzymatic catalysis [Ryerson, C. C., Ballou, D. P., and Walsh, C. (1982) Biochemistry 21, 2644-2655]. However, there was no kinetic or spectral evidence to distinguish between these two possibilities. In the present work we used double-mixing stopped-flow techniques to show that the C4a-flavin-oxygen adduct, which is formed rapidly from the reaction of oxygen with reduced enzyme in the presence of NADP, can exist in two states. When the reaction is carried out at pH 7.2, the first intermediate is a flavin C4a-peroxide with maximum absorbance at 366 nm; this intermediate becomes protonated at about 3 s(-1) to form what is believed to be the flavin C4a-hydroperoxide with maximum absorbance at 383 nm. These two intermediates can be interconverted by altering the pH, with a pK(a) of 8.4. Thus, at pH 9.0 the flavin C4a-peroxide persists mainly in the deprotonated form. Further kinetic studies also demonstrated that only the flavin C4a-peroxide intermediate could oxygenate the substrate, cyclohexanone. The requirement in catalysis of the deprotonated flavin C4a-peroxide, a nucleophile, is consistent with a Baeyer-Villiger rearrangement mechanism for the enzymatic oxygenation of cyclohexanone. In the course of these studies, the Kd for cyclohexanone to the C4a-peroxyflavin form of CHMO was determined to be approximately 1 microM. The rate-determining step in catalysis was shown to be the release of NADP from the oxidized enzyme.
Asunto(s)
Caproatos/metabolismo , Ciclohexanonas/metabolismo , Flavoproteínas/metabolismo , Lactonas/metabolismo , Oxigenasas/metabolismo , Catálisis , Cinética , Modelos Químicos , NADP/metabolismo , Oxidación-Reducción , Oxígeno/metabolismo , Proteínas Recombinantes/metabolismo , EspectrofotometríaRESUMEN
The reaction of the old yellow enzyme and reduced flavins with organic nitrate esters has been studied. Reduced flavins have been found to react readily with glycerin trinitrate (GTN ) (nitroglycerin) and propylene dinitrate, with rate constants at pH 7.0, 25 degrees C of 145 M(-1)s(-1) and 5.8 M(-1)s(-1), respectively. With GTN, the secondary nitrate was removed reductively 6 times faster than the primary nitrate, with liberation of nitrite. With propylene dinitrate, on the other hand, the primary nitrate residue was 3 times more reactive than the secondary residue. In the old yellow enzyme-catalyzed NADPH-dependent reduction of GTN and propylene dinitrate, ping-pong kinetics are displayed, as found for all other substrates of the enzyme. Rapid-reaction studies of mixing reduced enzyme with the nitrate esters show that a reduced enzyme--substrate complex is formed before oxidation of the reduced flavin. The rate constants for these reactions and the apparent K(d) values of the enzyme--substrate complexes have been determined and reveal that the rate-limiting step in catalysis is reduction of the enzyme by NADPH. Analysis of the products reveal that with the enzyme-catalyzed reactions, reduction of the primary nitrate in both GTN and propylene dinitrate is favored by comparison with the free-flavin reactions. This preferential positional reactivity can be rationalized by modeling of the substrates into the known crystal structure of the enzyme. In contrast to the facile reaction of free reduced flavins with GTN, reduced 5-deazaflavins have been found to react some 4--5 orders of magnitude slower. This finding implies that the chemical mechanism of the reaction is one involving radical transfers.
Asunto(s)
NADPH Deshidrogenasa/metabolismo , Nitroglicerina/metabolismo , Glicoles de Propileno/metabolismo , Catálisis , Ésteres/metabolismo , Flavinas/metabolismo , Cinética , Nitratos/metabolismo , Nitroglicerina/síntesis química , Resonancia Magnética Nuclear Biomolecular/métodos , Oxidación-Reducción , Riboflavina/metabolismoRESUMEN
The oxygen transfer to p-hydroxybenzoate catalyzed by p-hydroxybenzoate hydroxylase (PHBH) has been shown to occur via a C4a-hydroperoxide of the flavin. Two factors are likely to be important in facilitating the transfer of oxygen from the C4a-hydroperoxide to the substrate. (a) The positive electrostatic potential of the active site partially stabilizes the negative charge centered on the oxygen of the flavin-C4a-alkoxide leaving group during the transition state [Ortiz-Maldonado, M., Ballou, D. P., and Massey, V. (1999) Biochemistry 38, 8124-8137]. (b) The hydrogen-bonding network ionizes the substrate to promote its nucleophilic attack on the electrophilic C4a-hydroperoxide intermediate [Entsch, B., Palfey, B. A., Ballou, D. P., and Massey, V. (1991) J. Biol. Chem. 266, 17341-17349]. This ionization is also aided by the positive electrostatic potential of the active site [Moran, G. R., Entsch, B., Palfey, B. A., and Ballou, D. P. (1997) Biochemistry 36, 7548-7556]. Substituents on the flavin can specifically affect the stability of the alkoxide leaving-group, whereas changes to specific enzyme residues can affect the charge in the active site and the hydrogen-bonding network. We have used wild-type (WT) PHBH and several mutant forms, all with normal FAD and with 8-Cl-FAD substituted for FAD, to assess the relative contributions of the two effects. Lys297Met and Asn300Asp have decreased positive charge in the active site, and these variants engender approximately 35-fold slower hydroxylation rates than the WT enzyme. Substitution of 8-Cl-FAD in these mutant forms gives approximately 1.8-fold increases in hydroxylation rates, compared with a > or =4.8-fold increase for WT with this flavin. The hydroxylation catalyzed by Tyr385Phe, a mutant enzyme form with a disrupted hydrogen-bonding network that compromises the ionization of the substrate without changing the positive charge of the active site, is stimulated 1.5-fold by substituting the enzyme with 8-Cl-FAD. The substrate, tetrafluoro-p-hydroxybenzoate, is fully ionized in WT PHBH, but this phenolate is a poor nucleophile because of the electron-withdrawing effects of the fluorine substituents. With tetrafluoro-p-hydroxybenzoate as the substrate, substitution of FAD with 8-Cl-FAD in the WT enzyme stabilizes the leaving alkoxide and leads to a 2.3-fold increase in the hydroxylation rate compared to that with FAD. Either the use of substrates that do not communicate with the proton network or the mutation of amino acid residues that perturb this interaction may prevent a necessary conformational change that allows proper orientation between reactants during the hydroxylation reaction or permits the essential protonation of the initially formed nascent flavin-C4a-peroxide anion. Thus, both activation of substrate by the proton network and stabilization of the leaving alkoxide appear to be important for oxygen transfer catalyzed by PHBH. The full effect of the substituents on the flavin (4.8-fold) can only be realized when the optimal transition state can be achieved, and this optimal state is not fully realized with the mutant forms.
Asunto(s)
4-Hidroxibenzoato-3-Monooxigenasa/genética , 4-Hidroxibenzoato-3-Monooxigenasa/metabolismo , Sustitución de Aminoácidos/genética , Flavina-Adenina Dinucleótido/análogos & derivados , Flavina-Adenina Dinucleótido/metabolismo , Mutagénesis Sitio-Dirigida , 4-Hidroxibenzoato-3-Monooxigenasa/química , Alanina/genética , Alanina/metabolismo , Anaerobiosis , Asparagina/genética , Asparagina/metabolismo , Ácido Aspártico/genética , Ácido Aspártico/metabolismo , Catálisis , Hidroxilación , Lisina/genética , Lisina/metabolismo , Metionina/genética , Metionina/metabolismo , Oxidación-Reducción , Parabenos/metabolismo , Fenilalanina/genética , Fenilalanina/metabolismo , Riboflavina/metabolismo , Serina/genética , Serina/metabolismo , Espectrofotometría , Especificidad por Sustrato/genética , Tirosina/genética , Tirosina/metabolismoRESUMEN
OBJECTIVE: To assess the clinical utility of an enzyme-linked immunosorbent assay method for the quantitative detection of Candida species antigen (Candigen; Biomerica Inc, Newport Beach, Calif) in patients with suspected disseminated candidiasis. METHODS: Specimens of blood or cerebrospinal fluid from 75 patients with suspected disseminated candidiasis were analyzed by the Candigen test. Results were compared with those obtained by culture and polymerase chain reaction (PCR) analysis. RESULTS: Thirty-seven patients had specimens positive for Candida species by either culture or PCR. Of these specimens, 4 were positive by both culture and PCR, 21 were culture positive but PCR negative, and 12 were PCR positive but culture negative. Five specimens were positive by the Candigen test, all of which were PCR positive but culture negative. The sensitivity, specificity, and positive and negative predictive values of the Candigen test compared to culture plus PCR were 13.5%, 100%, 100%, and 54.3%, respectively. Turnaround time for the Candigen test was approximately 3 hours. CONCLUSION: The Candigen test showed excellent specificity and turnaround time, but its poor sensitivity coupled with its inability to provide species information or susceptibility data make its clinical utility questionable.
Asunto(s)
Antígenos Fúngicos/análisis , Candida/inmunología , Candidiasis/diagnóstico , Ensayo de Inmunoadsorción Enzimática/métodos , Adolescente , Adulto , Candida/genética , Candida/aislamiento & purificación , Candidiasis/microbiología , Humanos , Reacción en Cadena de la Polimerasa , Valor Predictivo de las Pruebas , Sensibilidad y Especificidad , Factores de TiempoRESUMEN
Amphibacillus xylanus and Sporolactobacillus inulinus NADH oxidases belonging to the peroxiredoxin oxidoreductase family show extremely high peroxide reductase activity for hydrogen peroxide and alkyl hydroperoxides in the presence of the small disulfide redox protein, AhpC (peroxiredoxin). In order to investigate the distribution of this enzyme system in bacteria, 15 bacterial strains were selected from typical aerobic, facultatively anaerobic, and anaerobic bacteria. AhpC-linked alkyl hydroperoxide reductase activities were detected in most of the tested strains, and especially high activities were shown in six bacterial species that grow well under aerobic conditions, including aerobic bacteria (Alcaligenes faecalis and Bacillus licheniformis) and facultatively anaerobic bacteria (Amphibacillus xylanus, Sporolactobacillus inulinus, Escherichia coli, and Salmonella enterica serovar Typhimurium). In the absence of AhpC, the purified enzymes from A. xylanus and S. inulinus catalyze the NADH-linked reduction of oxygen to hydrogen peroxide. Similar activities were observed in the cell extracts from each of these six strains. The cell extract of B. licheniformis revealed the highest AhpC-linked alkyl hydroperoxide reductase activity in the four strains, with V(max) values for hydrogen peroxide and alkyl hydroperoxides being similar to those for the enzymes from A. xylanus and S. inulinus. Southern blot analysis of the three strains probed with the A. xylanus peroxiredoxin reductase gene revealed single strong bands, which are presumably derived from the individual peroxiredoxin reductase genes. Single bands were also revealed in other strains which show high AhpC-linked reductase activities, suggesting that the NADH oxidases belonging to the peroxiredoxin oxidoreductase family are widely distributed and possibly play an important role both in the peroxide-scavenging systems and in an effective regeneration system for NAD in aerobically growing bacteria.
Asunto(s)
Bacterias/enzimología , Peróxido de Hidrógeno/metabolismo , Complejos Multienzimáticos/clasificación , Complejos Multienzimáticos/metabolismo , NADH NADPH Oxidorreductasas/clasificación , NADH NADPH Oxidorreductasas/metabolismo , Peroxidasas/metabolismo , Aerobiosis , Bacterias/genética , Bacterias/crecimiento & desarrollo , Bacterias Aerobias/enzimología , Bacterias Aerobias/crecimiento & desarrollo , Bacterias Anaerobias/enzimología , Bacterias Anaerobias/crecimiento & desarrollo , Southern Blotting , ADN Bacteriano/análisis , Proteínas de Escherichia coli , Oxidación-Reducción , PeroxirredoxinasRESUMEN
The FAD of p-hydroxybenzoate hydroxylase (PHBH) is known to exist in two conformations. The FAD must be in the in-position for hydroxylation of p-hydroxybenzoate (pOHB), whereas the out-position is essential for reduction of the flavin by NADPH. In these investigations, we have used 8-mercapto-FAD and 8-hydroxy-FAD to probe the movement of the flavin in catalysis. Under the conditions employed, 8-mercapto-FAD (pK(a) = 3.8) and 8-hydroxy-FAD (pK(a) = 4.8) are mainly anionic. The spectral characteristics of the anionic forms of these flavins are very sensitive to their environment, making them sensitive probes for detecting movement of the flavin during catalysis. With these flavin analogues, the enzyme hydroxylates pOHB efficiently, but at a rate much slower than that of enzyme with FAD. Reaction of oxygen with reduced forms of these modified enzymes in the absence of substrate appears to proceed through the formation of the flavin-C4a-hydroperoxide intermediate, as with normal enzyme, but the decay of this intermediate is so fast compared to its formation that very little accumulates during the reaction. However, after elimination of H2O2 from the flavin-C4a-hydroperoxide, a perturbed oxidized enzyme spectrum is observed (Eox*), and this converts slowly to the spectrum of the resting oxidized form of the enzyme (Eox). In the presence of pOHB, PHBH reconstituted with 8-mercapto-FAD also shows the additional oxidized intermediate (Eox*) after the usual oxygenated C4a-intermediates have formed and decayed in the course of the hydroxylation reaction. This Eox* to Eox step is postulated to be due to flavin movement. Furthermore, binding of pOHB to resting (Eox) follows a three-step equilibrium mechanism that is also consistent with flavin movement being the rate-limiting step. The rate for the slowest step during pOHB binding is similar to that observed for the conversion of Eox* to Eox during the oxygen reaction in the absence or presence of substrate. Steady-state kinetic analysis of PHBH substituted with 8-mercapto-FAD demonstrated that the apparent k(cat) is also similar to the rate of Eox* conversion to Eox. Presumably, the protein environment surrounding the flavin in Eox* differs slightly from that of the final resting form of the enzyme (Eox).
Asunto(s)
4-Hidroxibenzoato-3-Monooxigenasa/química , Flavina-Adenina Dinucleótido/análogos & derivados , Flavina-Adenina Dinucleótido/química , 4-Hidroxibenzoato-3-Monooxigenasa/metabolismo , Unión Competitiva , Brevibacterium/enzimología , Catálisis , Flavina-Adenina Dinucleótido/metabolismo , Concentración de Iones de Hidrógeno , Cinética , Ligandos , Oxidación-Reducción , Unión Proteica , Conformación ProteicaRESUMEN
Two arginine residues, Arg-181 and Arg-268, are conserved throughout the known family of FMN-containing enzymes that catalyze the oxidation of alpha-hydroxyacids. In the lactate oxidase from Aerococcus viridans, these residues have been changed to lysine in two single mutations and in a double mutant form. In addition, Arg-181 has been replaced by methionine to determine the effect of removing the positive charge on the residue. The effects of these replacements on the kinetic and thermodynamic properties are reported. With all mutant forms, there are only small effects on the reactivity of the reduced flavin with oxygen. On the other hand, the efficiency of reduction of the oxidized flavin by l-lactate is greatly reduced, particularly with the R268K mutant forms. The results demonstrate the importance of the two arginine residues in the binding of substrate and its interaction with the flavin, and are consistent with a previous hypothesis that they also play a role of charge neutralization in the transition state of substrate dehydrogenation. The replacement of Arg-268 by lysine also results in a slow conversion of the 8-CH(3)- substituent of FMN to yield 8-formyl-FMN, still tightly bound to the enzyme, and with significantly different physical and chemical properties from those of the FMN-enzyme.
Asunto(s)
Arginina , Mononucleótido de Flavina/metabolismo , Oxigenasas de Función Mixta/química , Oxigenasas de Función Mixta/metabolismo , Streptococcaceae/enzimología , Secuencia de Aminoácidos , Sustitución de Aminoácidos , Sitios de Unión , Secuencia Conservada , Mononucleótido de Flavina/análogos & derivados , Flavoproteínas/química , Flavoproteínas/metabolismo , Cinética , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Alineación de Secuencia , Homología de Secuencia de Aminoácido , TermodinámicaRESUMEN
The Old Yellow Enzyme has been shown to catalyze efficiently the NADPH-linked reduction of nitro-olefins. The reduction of the nitro-olefin proceeds in a stepwise fashion, with formation of a nitronate intermediate that is freely dissociable from the enzyme. The first step involves hydride transfer from the enzyme-reduced flavin to carbon 2 of the nitro-olefin. The protonation of the nitronate at carbon 1 to form the final nitroalkane product also is catalyzed by the enzyme and involves Tyr-196 as an active site acid/base. This residue also is involved in aci-nitro tautomerization of nitroalkanes, the first example of a nonredox reaction catalyzed by the enzyme.
Asunto(s)
Alquenos/metabolismo , NADPH Deshidrogenasa/metabolismo , Proteínas de Plantas/metabolismo , Catálisis , Cinética , Oxidación-Reducción , Especificidad por SustratoRESUMEN
Since their discovery and chemical characterization in the 1930s, flavins have been recognized as being capable of both one- and two-electron transfer processes, and as playing a pivotal role in coupling the two-electron oxidation of most organic substrates to the one-electron transfers of the respiratory chain. In addition, they are now known as versatile compounds that can function as electrophiles and nucleophiles, with covalent intermediates of flavin and substrate frequently being involved in catalysis. Flavins are thought to contribute to oxidative stress through their ability to produce superoxide, but at the same time flavins are frequently involved in the reduction of hydroperoxides, products of oxygen-derived radical reactions. Flavoproteins play an important role in soil detoxification processes via the hydroxylation of many aromatic compounds, and a simple flavoprotein in liver microsomes catalyses many reactions similar to those carried out by cytochrome P450 enzymes. Flavins are involved in the production of light in bioluminescent bacteria, and are intimately connected with light-initiated reactions such as plant phototropism and nucleic acid repair processes. Recent reports also link them to programmed cell death. The chemical versatility of flavoproteins is clearly controlled by specific interactions with the proteins with which they are bound. One of the main thrusts of current research is to try to define the nature of these interactions, and to understand in chemical terms the various steps involved in catalysis by flavoprotein enzymes.
Asunto(s)
Flavinas/química , Riboflavina/química , Riboflavina/metabolismo , Animales , Catálisis , Disulfuros , Transporte de Electrón , Flavinas/fisiología , Microsomas Hepáticos/metabolismo , Modelos Químicos , NADPH Deshidrogenasa/metabolismo , NADPH-Ferrihemoproteína Reductasa/metabolismo , Estrés Oxidativo , Oxígeno/metabolismo , Riboflavina/fisiología , SuperóxidosRESUMEN
The Amphibacillus xylanus NADH oxidase, which catalyzes the reduction of oxygen to hydrogen peroxide with beta-NADH, can also reduce hydrogen peroxide to water in the presence of free flavin adenine dinucleotide (FAD) or the small disulfide-containing Salmonella enterica AhpC protein. The enzyme has two disulfide bonds, Cys128-Cys131 and Cys337-Cys340, which can act as redox centers in addition to the enzyme-bound FAD (K. Ohnishi, Y. Niimura, M. Hidaka, H. Masaki, H. Suzuki, T. Uozumi, and T. Nishino, J. Biol. Chem. 270:5812-5817, 1995). The NADH-FAD reductase activity was directly dependent on the FAD concentration, with a second-order rate constant of approximately 2.0 x 10(6) M(-1) s(-1). Rapid-reaction studies showed that the reduction of free flavin occurred through enzyme-bound FAD, which was reduced by NADH. The peroxidase activity of NADH oxidase in the presence of FAD resulted from reduction of peroxide by free FADH(2) reduced via enzyme-bound FAD. This peroxidase activity was markedly decreased in the presence of oxygen, since the free FADH(2) is easily oxidized by oxygen, indicating that this enzyme system is unlikely to be functional in aerobic growing cells. The A. xylanus ahpC gene was cloned and overexpressed in Escherichia coli. When the NADH oxidase was coupled with A. xylanus AhpC, the peroxidase activity was not inhibited by oxygen. The V(max) values for hydrogen peroxide and cumene hydroperoxide reduction were both approximately 150 s(-1). The K(m) values for hydrogen peroxide and cumene hydroperoxide were too low to allow accurate determination of their values. Both AhpC and NADH oxidase were induced under aerobic conditions, a clear indication that these proteins are involved in the removal of peroxides under aerobic growing conditions.
Asunto(s)
Bacterias Anaerobias/enzimología , Bacterias Anaerobias/genética , Complejos Multienzimáticos/genética , Complejos Multienzimáticos/metabolismo , NADH NADPH Oxidorreductasas/genética , NADH NADPH Oxidorreductasas/metabolismo , Peroxidasas/metabolismo , Secuencia de Aminoácidos , Anaerobiosis , Clonación Molecular , Disulfuros/análisis , Proteínas de Escherichia coli , Flavina-Adenina Dinucleótido/metabolismo , Peróxido de Hidrógeno/metabolismo , Cinética , Modelos Químicos , Datos de Secuencia Molecular , Complejos Multienzimáticos/química , NADH NADPH Oxidorreductasas/química , Peroxirredoxinas , Proteínas Recombinantes/metabolismo , Salmonella enterica/enzimología , Superóxido Dismutasa/metabolismoRESUMEN
An "artificial flavinylation" approach was developed to replace a native noncovalent flavin prosthetic group with a covalently attached flavin analogue in recombinant human d-amino acid oxidase. The protein residue Gly-281 was replaced with Cys by site-directed mutagenesis, followed by reaction between mutated apoenzyme and the thiol-reactive flavin analogue, 8-methylsulfonyl FAD. The stoichiometric process of flavin attachment was accompanied by gain in enzymatic activity, reaching up to 26% activity of the recombinant native enzyme. The steady-state kinetic data together with the results of limited proteolysis and benzoate-binding studies suggest that, although mutation perturbs protein structural and catalytic properties, the flavinylation alone does not have any negative impact. We conclude that, despite the implemented restraints on its mobility, the covalently attached flavin is properly positioned within the protein active site and acts efficiently during d-amino acid oxidase catalytic turnover.
Asunto(s)
D-Aminoácido Oxidasa/metabolismo , Flavinas/metabolismo , Secuencia de Bases , Benzoatos/metabolismo , Catálisis , D-Aminoácido Oxidasa/química , D-Aminoácido Oxidasa/genética , D-Aminoácido Oxidasa/aislamiento & purificación , Cartilla de ADN , Humanos , Hidrólisis , Cinética , Mutagénesis Sitio-Dirigida , Oxidación-Reducción , Análisis EspectralRESUMEN
The native flavin, FMN, has been removed from the l-lactate oxidase of Aerococcus viridans, and the apoprotein reconstituted with 12 FMN derivatives with various substituents at the flavin 6- and 8-positions. Impressive linear relationships are exhibited between the sum of the Hammett final sigma(para) and final sigma(ortho) parameters and the redox potentials of the free flavins, and between the redox potentials of the free and enzyme-bound flavins. Rapid reaction kinetics studies of the reconstituted enzymes with the substrates l-lactate and l-mandelate show an increase in the reduction rate constant with increasing redox potential, except that, with lactate, a limiting rate constant of approximately 700 s(-1) is obtained with flavins of high potential. Similar breakpoints are found in plots of the rate constants for flavin N5-sulfite adduct formation and for the reaction of the reduced enzymes with molecular oxygen. These results are interpreted in terms of a two-step equilibrium preceding the chemical reaction step, in which the second equilibrium step provides an upper limit to the rate with which the particular substrate or ligand is positioned with the flavin in the correct fashion for the observed chemical reaction to occur. The relationship of rate constants for flavin reduction and N5-sulfite adduct formation with flavin redox potential below the observed breakpoint indicate development of significant negative charge in the transition states of the reactions. In the case of reduction by substrate, the results are consistent either with a hydride transfer mechanism or with the so called "carbanion" mechanism, in which the substrate alpha-proton is abstracted by an enzyme base protected from exchange with solvent. These conclusions are supported by substrate alpha-deuterium isotope effects and by solvent viscosity effects on sulfite binding.
Asunto(s)
Oxigenasas de Función Mixta/química , Apoenzimas , Mononucleótido de Flavina/análogos & derivados , Cinética , Ácido Láctico/metabolismo , Ácidos Mandélicos/metabolismo , Oxigenasas de Función Mixta/metabolismo , Oxidación-Reducción , Unión Proteica , Venenos de Serpiente , Espectrofotometría , Streptococcaceae/química , Relación Estructura-ActividadRESUMEN
The National Committee for Clinical Laboratory Standards has recently changed the oxacillin breakpoint from >/=4 mg/liter to >/=0. 5 mg/liter to detect methicillin-resistant coagulase-negative staphylococci (CoNS) because the previous breakpoint lacked sensitivity. To determine the correlation between the new oxacillin breakpoint and the presence of the mecA gene, 493 CoNS of 11 species were tested. The presence of the mecA gene was determined by PCR, and oxacillin susceptibility was determined by the agar dilution method with Mueller-Hinton agar containing 2% NaCl and oxacillin (0. 125 to 4.0 mg/liter). The new breakpoint correctly classified all CoNS strains with mecA as methicillin resistant and strains of Staphylococcus epidermidis, S. haemolyticus, and S. hominis without mecA as methicillin susceptible. The breakpoint of >/=0.5 mg/liter was not specific for S. cohnii, S. lugdunensis, S. saprophyticus, S. warneri, and S. xylosus, in that it categorized 70 of 74 strains of these species without mecA (94.6%) as methicillin resistant. The results of this study indicate that the new oxacillin breakpoint accurately identifies strains of CoNS with mecA but is not specific for strains of certain species of CoNS without mecA.
Asunto(s)
Resistencia a la Meticilina/genética , Oxacilina/farmacología , Penicilinas/farmacología , Staphylococcus/efectos de los fármacos , Coagulasa/metabolismo , Genes Bacterianos , Humanos , Pruebas de Sensibilidad Microbiana/métodos , Pruebas de Sensibilidad Microbiana/normas , Infecciones Estafilocócicas/microbiología , Staphylococcus/genéticaRESUMEN
Structural and kinetic studies have revealed two flavin conformations in p-hydroxybenzoate hydroxylase (PHBH), the in-position and the out-position. Conversion between these two conformations is believed to be essential during catalysis. Although substrate hydroxylation occurs while the flavin in PHBH is in the in-conformation, the position of the flavin during reduction by NADPH is uncertain. To investigate the catalytic importance of the out-conformation of the flavin and to clarify the mechanism of flavin reduction in PHBH, we report quantitative structure-reactivity relationships (QSAR) using PHBH substituted separately with nine derivatives of FAD modified in the 8-position and four dihydronicotinamide analogues as reducing agents. The 8-position of the FAD isoalloxazine ring was chosen for modification because in PHBH it has minimal interactions with the protein and is accessible to solvent. The chemical sequence of events during catalysis by PHBH was not altered when using any of the modified flavins, and normal products were obtained. Although the rate of reduction of PHBH reconstituted with flavin derivatives is expected to be dependent on the redox potential of the flavin, no strict correlation was observed. Instead, the rate of reduction correlated with the kappa-substituent constant, which is based on size and hydrophobicity of the 8-substituent on the FAD. Substituents that sterically hinder attainment of the out-conformation decreased the rate of flavin reduction much more than expected on the basis of the redox potential of the flavin. The results of this QSAR analysis are consistent with the hypothesis that the flavin in PHBH must move to the out-conformation for proper formation of the charge-transfer complex between NADPH and FAD that is necessary for rapid flavin reduction.
Asunto(s)
4-Hidroxibenzoato-3-Monooxigenasa/química , Flavina-Adenina Dinucleótido/análogos & derivados , Flavina-Adenina Dinucleótido/química , Niacinamida/análogos & derivados , Cristalografía por Rayos X , Flavinas/química , NADP/química , Niacinamida/química , Oxidación-Reducción , Espectrofotometría Ultravioleta , Relación Estructura-Actividad , Especificidad por SustratoRESUMEN
We report Hammett correlations, using 8-substituted flavins, to clarify the mechanism of hydroxylation by p-hydroxybenzoate hydroxylase (PHBH). The 8-position of the FAD isoalloxazine ring was chosen for modifications, because in PHBH it has minimal interactions with the protein, and it is accessible to solvent and away from the site of hydroxylation. Although two intermediates, a flavin-C4a-hydroperoxide and a flavin-C4a-hydroxide, are known to participate in hydroxylation, the mechanism of oxygen transfer remains controversial. Mechanisms as diverse as electrophilic aromatic substitution, diradical formation, and isoalloxazine ring opening have been proposed. In the studies reported here, it was possible to monitor spectrally each of the individual steps involved in hydroxylation, because the FAD cofactor acts as a reporter group. Thus, with PHBH, substituted separately with nine derivatives of FAD altered in the 8-position, quantitative structure-reactivity relationships (QSAR) have been applied to probe the mechanisms of formation of the flavin-C4a-hydroperoxide, the conversion to the flavin-C4a-hydroxide with concomitant oxygen transfer to the substrate, and the dehydration of the flavin-C4a-hydroxide to form oxidized FAD. The individual chemical steps in the mechanism of PHBH were not altered when using any of the modified flavins, and normal products were obtained; however, the rates of individual steps were affected, and depended on the electronic properties of the 8-substituent. Increased hydroxylation rates were observed when a more electrophilic flavin-C4a-hydroperoxide (i.e., with an electron-withdrawing substituent at the 8-position) is bound to PHBH. On the basis of QSAR analysis, we conclude that the mechanism of the hydroxylation step is best described by electrophilic aromatic substitution.