More than an "inverted-U"? An exploratory study of the association between the catechol-o-methyltransferase gene polymorphism and executive functions in Parkinson's disease

Executive dysfunction is common in Parkinson's disease (PD) patients. The catechol-O-methyltransferase (COMT) Val158Met polymorphism has been proposed to affect executive functions (EFs) in the prefrontal cortex. The present study attempted to explore the influence of the COMT polymorphism on EFs in patients with PD. Fifty-four PD patients were recruited and underwent neuropsychological assessments for three core EFs. The COMT polymorphism was genotyped using the TaqMan SNP Genotyping Assay. Participants were divided into three study groups: Val homozygotes, heterozygotes, and Met homozygotes. The three COMT genotype groups had significantly different performances in set-shifting [χ2 (2, 54) = 9.717, p = 0.008] and working memory tasks [χ2 (2, 54) = 7.806, p = 0.020]. Post-hoc analyses revealed that PD Val homozygotes performed significantly poorer in the set-shifting task than did either the PD Met homozygotes (z = -2.628, p = 0.009) or PD heterozygotes (z = -2.212, p = 0.027). Our explorative results suggest that the putative level of prefrontal dopamine influenced set-shifting through a "cane-shaped" dopamine level-response relationship. Our results have clinical implications, which may influence PD treatment with dopamine in the future because the optimal dopamine level to maximize EFs may vary based on the clinical course and COMT polymorphism status. Further study recruiting a larger number of participants is needed to confirm our preliminary findings.

Emotion-Specific Affective Theory of Mind Impairment in Parkinson's Disease

The neuropathology of Parkinson’s disease (PD) involves the frontal-subcortical circuit, an area responsible for processing affective theory of mind (ToM). Patients with PD are expected to experience deficits in the affective ToM. This study aims to investigate whether the ability to infer emotion in others is affected in either young-onset Parkinson’s disease (YOPD) or middle-onset PD (MOPD) patients and to test whether the impairments in affective ToM are associated with the motor symptoms. The affective ToM, global mental abilities, and clinical symptoms were assessed in a total of 107 MOPD, 30 YOPD, and 30 normal controls (NCs). The MOPD patients exhibited deficits in affective ToM to the negative and neutral valences, when compared to the participants in the NCs and YOPD group. By conducting gender-stratified analysis, the deficits in affective ToM was only found in female participants. After adjusting for demographic variables, the multiple linear regression model revealed that affective ToM predicted motor symptoms, especially in female MOPD patients. The present study may aid in the development of medical care programs by advocating for a more comprehensive therapeutic plan that includes continuous disease progression monitoring and social skills training for female MOPD patients or their caregivers.

Characterization of the binding of Photobacterium phosphoreum P-flavin by Vibrio harveyi Luciferase

The isolated Photobacterium phosphoreum luciferase is associated with a bound flavin designated P-flavin and tentatively identified as 6-(3"-myristic acid)-FMN. Since FMN and myristic acid are products of the normal luciferase reaction, we explored the possibility that P-flavin can also be bound by luciferase from other luminous bacteria and serve as an active site probe. P-flavin has never been detected in Vibrio harveyi cells. We found that the V. harveyi luciferase binds P. phosphoreum P-flavin, at a ratio of 1 P-flavin per luciferase alphabeta dimer, and with concomitant absorption spectral perturbation of P-flavin, fluorescence quenching of P-flavin and luciferase, and activity inhibition of luciferase. Isolated P-flavin can be fully reduced photochemically. V. harveyi luciferase bound the oxidized P-flavin with a K(d) (or K(i) competitively against decanal) of 0.1-0.16 microM, which is three orders of magnitude lower than the K(d) for FMN binding but similar to that of reduced FMN binding. The reduced P-flavin exhibited a K(i) (competitively against the reduced FMN substrate) of 0.16 microM, also similar to the K(d) for reduced FMN. Hence, the covalent attachment of myristic acid to FMN greatly and preferentially enhanced the binding of oxidized P-flavin. The dissociation of P-flavin was slow in comparison with the binding of reduced FMN and decanal substrates. Modification of the alphaCys106 near the active site by N-ethylmaleimide can be retarded by P-flavin. These findings indicate that P-flavin is potentially a superb active site probe for luciferase. We hypothesize that P-flavin is a by-product of luciferase generated by a side reaction which is trivial with the V. harveyi luciferase but significant in the P. phosphoreum luciferase-catalyzed reaction.

Reduced flavin: donor and acceptor enzymes and mechanisms of channeling

Although mechanisms of metabolite channeling have been extensively studied, the nature of reduced flavin transfer from donor to acceptor enzymes remains essentially unexplored. In this review, identities and properties of reduced flavin-producing enzymes (namely flavin reductases) and reduced flavin-requiring processes and enzymes are summarized. By using flavin reductase-luciferase enzyme couples from luminous bacteria, two types of reduced flavin channeling were observed involving the differential transfers of the reduced flavin cofactor and the reduced flavin product of reductase to luciferase. The exact mode of transfer is controlled by the specific makeup of the constituent enzymes within the reductase-luciferase couple. The plausible physiological significance of the monomer-dimer equilibrium of the NADPH-specific flavin reductase from Vibrio harveyi is also discussed.

Flavin specificity and subunit interaction of Vibrio fischeri general NAD(P)H-flavin oxidoreductase FRG/FRase I

Apoenzyme of the major NAD(P)H-utilizing flavin reductase FRG/FRase I from Vibrio fischeri was prepared. The apoenzyme bound one FMN cofactor per enzyme monomer to yield fully active holoenzyme. The FMN cofactor binding resulted in substantial quenching of both the flavin and the protein fluorescence intensities without any significant shifts in the emission peaks. In addition to FMN binding (K(d) 0.5 microM at 23 degrees C), the apoenzyme also bound 2-thioFMN, FAD and riboflavin as a cofactor with K(d) values of 1, 12, and 37 microM, respectively, at 23 degrees C. The 2-thioFMN containing holoenzyme was about 40% active in specific activity as compared to the FMN-containing holoenzyme. The FAD- and riboflavin-reconstituted holoenzymes were also catalytically active but their specific activities were not determined. FRG/FRase I followed a ping-pong kinetic mechanism. It is proposed that the enzyme-bound FMN cofactor shuttles between the oxidized and the reduced form during catalysis. For both the FMN- and 2-thioFMN-containing holoenzymes, 2-thioFMN was about 30% active as compared to FMN as a substrate. FAD and riboflavin were also active substrates. FRG/FRase I was shown by ultracentrifugation at 4 degrees C to undergo a monomer-dimer equilibrium, with K(d) values of 18.0 and 13.4 microM for the apo- and holoenzymes, respectively. All the spectral, ligand equilibrium binding, and kinetic properties described above are most likely associated with the monomeric species of FRG/FRase I. Many aspects of these properties are compared with a structurally and functionally related Vibrio harveyi NADPH-specific flavin reductase FRP.

Differential transfers of reduced flavin cofactor and product by bacterial flavin reductase to luciferase

It is believed that the reduced FMN substrate required by luciferase from luminous bacteria is provided in vivo by NAD(P)H-FMN oxidoreductases (flavin reductases). Our earlier kinetic study indicates a direct flavin cofactor transfer from Vibrio harveyi NADPH-preferring flavin reductase P (FRP(H)) to the luciferase (L(H)) from the same bacterium in the in vitro coupled luminescence reaction. Kinetic studies were carried out in this work to characterize coupled luminescence reactions using FRP(H) and the Vibrio fischeri NAD(P)H-utilizing flavin reductase G (FRG(F)) in combination with L(H) or luciferase from V. fischeri (L(F)). Comparisons of K(m) values of reductases for flavin and pyridine nucleotide substrates in single-enzyme and luciferase-coupled assays indicate a direct transfer of reduced flavin, in contrast to free diffusion, from reductase to luciferase by all enzyme couples tested. Kinetic mechanisms were determined for the FRG(F)-L(F) and FRP(H)-L(F) coupled reactions. For these two and the FRG(F)-L(H) coupled reactions, patterns of FMN inhibition and effects of replacement of the FMN cofactor of FRP(H) and FRG(F) by 2-thioFMN were also characterized. Similar to the FRP(H)-L(H) couple, direct cofactor transfer was detected for FRG(F)-L(F) and FRP(H)-L(F). In contrast, despite the structural similarities between FRG(F) and FRP(H) and between L(F) and L(H), direct flavin product transfer was observed for the FRG(F)-L(H) couple. The mechanism of reduced flavin transfer appears to be delicately controlled by both flavin reductase and luciferase in the couple rather than unilaterally by either enzyme species.

Stability and peptide binding specificity of Btk SH2 domain: molecular basis for X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is caused by mutations in the Bruton's tyrosine kinase (Btk). The absence of functional Btk leads to failure of B-cell development that incapacitates antibody production in XLA patients leading to recurrent bacterial infections. Btk SH2 domain is essential for phospholipase C-gamma phosphorylation, and mutations in this domain were shown to cause XLA. Recently, the B-cell linker protein (BLNK) was found to interact with the SH2 domain of Btk, and this association is required for the activation of phospholipase C-gamma. However, the molecular basis for the interaction between the Btk SH2 domain and BLNK and the cause of XLA remain unclear. To understand the role of Btk in B-cell development, we have determined the stability and peptide binding affinity of the Btk SH2 domain. Our results indicate that both the structure and stability of Btk SH2 domain closely resemble with other SH2 domains, and it binds with phosphopeptides in the order pYEEI > pYDEP > pYMEM > pYLDL > pYIIP. We expressed the R288Q, R288W, L295P, R307G, R307T, Y334S, Y361C, L369F, and 1370M mutants of the Btk SH2 domain identified from XLA patients and measured their binding affinity with the phosphopeptides. Our studies revealed that mutation of R288 and R307 located in the phosphotyrosine binding site resulted in a more than 200-fold decrease in the peptide binding compared to L295, Y334, Y361, L369, and 1370 mutations in the pY + 3 hydrophobic binding pocket (approximately 3- to 17-folds). Furthermore, mutation of the Tyr residue at the betaD5 position reverses the binding order of Btk SH2 domain to pYIIP > pYLDL > pYDEP > pYMEM > pYEEI. This altered binding behavior of mutant Btk SH2 domain likely leads to XLA.

Vibrio harveyi NADPH-FMN oxidoreductase arg203 as a critical residue for NADPH recognition and binding

Luminous bacteria contain three types of NAD(P)H-FMN oxidoreductases (flavin reductases) with different pyridine nucleotide specificities. Among them, the NADPH-specific flavin reductase from Vibrio harveyi exhibits a uniquely high preference for NADPH. In comparing the substrate specificity, crystal structure, and primary sequence of this flavin reductase with other structurally related proteins, we hypothesize that the conserved Arg203 residue of this reductase is critical to the specific recognition of NADPH. The mutation of this residue to an alanine resulted in only small changes in the binding and reduction potential of the FMN cofactor, the K(m) for the FMN substrate, and the k(cat). In contrast, the K(m) for NADPH was increased 36-fold by such a mutation. The characteristic perturbation of the FMN cofactor absorption spectrum upon NADP(+) binding by the wild-type reductase was abolished by the same mutation. While the k(cat)/K(m,NADPH) was reduced from 1990 x 10(5) to 46 x 10(5) M(-1) min(-1) by the mutation, the mutated variant showed a k(cat)/K(m,NADH) of 4 x 10(5) M(-1) min(-1), closely resembling that of the wild-type reductase. The deuterium isotope effects (D)V and (D)(V/K) for (4R)-[4-(2)H]-NADPH were 1.7 and 1.4, respectively, for the wild-type reductase but were increased to 3.8 and 4.0, respectively, for the mutated variant. Such a finding indicates that the rates of NADPH and NADP(+) dissociation in relation to the isotope-sensitive redox steps were both increased as a result of the mutation. These results all provide support to the critical role of the Arg203 in the specific recognition and binding of NADPH.

Probing the mechanisms of the biological intermolecular transfer of reduced flavin

NAD(P)H-flavin oxidoreductases [flavin reductases (FR)] are a class of enzymes capable of producing reduced flavin for bacterial bioluminescence and other biological processes. Bacterial luciferase utilizes oxygen, reduced FMN (FMNH2) and a long-chain aliphatic aldehyde as substrates for light emission. The Vibrio harveyi luciferase and FRP (for which we have cloned the gene and determined the crystal structure) is a model for the elucidation of the reduced flavin transfer mechanism using both a flavin reductase single-enzyme assay monitoring the NADPH oxidation and a flavin reductase-luciferase coupled assay measuring bioluminescence intensity or quantum output. The FRP exhibits a ping-pong kinetic pattern in the single-enzyme assay but changes to a sequential pattern in the coupled assay. Furthermore, FMN at >2x10(-6) mol/L reduced both the light intensity and quantum yield of the coupled reaction by noncompetitively inhibiting NADPH and competitively inhibiting luciferase. These results support a scheme in which the luciferase forms specific complex(es) with FRP. Indeed, such complexes were shown by fluorescence anisotropy to exist between luciferase and monomeric FRP either in the holo- or apoenzyme form. Furthermore, the reduced flavin cofactor of FRP is transferred directly to luciferase for bioluminescence, whereas the reduced flavin product of FRP is inefficient in supporting the luminescence reaction. The mechanism of reduced flavin transfer is apparently flavin and flavin reductase specific.

Action mechanism of antitubercular isoniazid. Activation by Mycobacterium tuberculosis KatG, isolation, and characterization of inha inhibitor

Activation of the antitubercular isoniazid (INH) by the Mycobacterium tuberculosis KatG produces an inhibitor for enoyl reductase (InhA). The mechanism for INH activation remains poorly understood, and the inhibitor has never been isolated. We have purified the InhA-inhibitor complex generated in the M. tuberculosis KatG-catalyzed INH activation. The complex exhibited a 278-nm absorption peak and a shoulder around 326 nm with a characteristic A(326)/A(278) ratio of 0.16. The complex was devoid of enoyl reductase activity. The inhibitor noncovalently binds to InhA with a K(d) < 0.4 nM and can be dissociated from denatured InhA for chromatographic isolation. The free inhibitor showed absorption peaks at 326 (epsilon(326) 6900 M(-1) cm(-1)) and 260 nm (epsilon(260) 27,000 M(-1) cm(-1)). The inactive complex can be reconstituted from InhA and the isolated inhibitor. The InhA inhibitor from the KatG-catalyzed INH activation was identical to that from a slow, KatG-independent, Mn(2+)-mediated reaction based on high pressure liquid chromatography analysis and absorption and mass spectral characteristics. By monitoring the formation of the InhA-inhibitor complex, we have found that manganese is not essential to the INH activation by M. tuberculosis KatG. Furthermore, the formation of the InhA inhibitor in the KatG reaction was independent of InhA.

Relationship between the conserved alpha subunit arginine 107 and effects of phosphate on the activity and stability of Vibrio harveyi luciferase

The Arg107 of the alpha subunit is a conserved residue for all known bacterial luciferases. The phosphate moiety of the reduced flavin mononucleotide (FMNH(2)) side chain has been hypothesized to be anchored at this site (A. J. Fisher, F. M. Raushel, T. O. Baldwin, and I. Rayment Biochemistry 34, 6581-6586, 1995). Mutations of alphaArg107 of the Vibrio harveyi luciferase to alanine, serine, and glutamate were carried out to test such a hypothesis. These variants were characterized and compared with the wild-type luciferase with respect to their K(m) for decanal, FMNH(2), and reduced riboflavin in both low- (0.01 or 0.05 M) and high- (0.3 M) phosphate buffers at pH 7.0. Results are consistent with the hypothesized binding of the FMNH(2) phosphate group by alphaArg107. Moreover, the alphaArg107 residue was apparently important in the expression of the luciferase maximal activity and aldehyde binding. Phosphate ion is also known to have other effects on luciferase stability. We compared the three luciferase variants with the native enzyme with respect to the decay rate of the FMN 4a-hydroperoxide intermediate II, and rates of inactivation by trypsin digestion, modification by N-ethylmaleimide, and heat treatment in low- and high-phosphate buffers. On the basis of patterns of the phosphate effects, alphaArg107 appeared to be important to the enhancement of luciferase stability against trypsin proteolysis at high phosphate but was not involved in regulating the intermediate II decay or sensitivity to N-ethylmaleimide modification. Differential effects of mutations on luciferase thermal stability were observed. It is uncertain whether alphaArg107 is involved in the enhanced thermal stability of the native luciferase in high phosphate buffer.

Unusual folded conformation of nicotinamide adenine dinucleotide bound to flavin reductase P

The 2.1 A resolution crystal structure of flavin reductase P with the inhibitor nicotinamide adenine dinucleotide (NAD) bound in the active site has been determined. NAD adopts a novel, folded conformation in which the nicotinamide and adenine rings stack in parallel with an inter-ring distance of 3.6 A. The pyrophosphate binds next to the flavin cofactor isoalloxazine, while the stacked nicotinamide/adenine moiety faces away from the flavin. The observed NAD conformation is quite different from the extended conformations observed in other enzyme/NAD(P) structures; however, it resembles the conformation proposed for NAD in solution. The flavin reductase P/NAD structure provides new information about the conformational diversity of NAD, which is important for understanding catalysis. This structure offers the first crystallographic evidence of a folded NAD with ring stacking, and it is the first enzyme structure containing an FMN cofactor interacting with NAD(P). Analysis of the structure suggests a possible dynamic mechanism underlying NADPH substrate specificity and product release that involves unfolding and folding of NADP(H).

Computational analysis of the oxygen addition at the C4a site of reduced flavin in the bacterial luciferase bioluminescence reaction

The energetic characteristics of selected reaction steps in the bacterial luciferase-catalyzed luminescence reaction were examined by computation using the MNDO-PM3 method. Specifically, a three-step model was proposed to account for the reaction between oxygen and reduced riboflavin 5'-phosphate (1,5H2-FMN) to generate first the 5-hydroFMN-4a-peroxide (5H-FMN-4aOO-) and then the 5-hydro-4a-hydroperoxyFMN (5H-FMN-4aOOH) intermediates. Lysine (Lys-H+) and aspartate (Asp-) were chosen as representative catalytic residues involved in the protonation and deprotonation processes. Results show that deprotonation at the N1 site of 1,5H2-FMN by a basic amino acid residue at the luciferase active site would efficiently accelerate the reaction rate of O2 addition to form 5H-FMN-4aOO-. The most favored site of oxygen attack is at the flavin C4a. With the aid of a catalytic acid group, the 5H-FMN-4aOO- so formed tends to undergo a spontaneous protonation reaction to yield the 5H-FMN-4aOOH.

Effects of mutations of the alpha His45 residue of Vibrio harveyi luciferase on the yield and reactivity of the flavin peroxide intermediate

This work was undertaken to investigate the functional consequences of mutations of the essential alpha His45 residue of Vibrio harveyi luciferase, especially with respect to the yield and reactivity of the flavin 4a-hydroperoxide intermediate II. A total of 14 luciferase variants, each with a different single-residue replacement for the alpha His45, were examined. These variants showed changes, mostly slight, in their light decay rates of the nonturnover luminescence reaction and in their Km values for decanal and reduced riboflavin 5'-phosphate (FMNH2). All alpha His45 mutants, however, showed markedly reduced bioluminescence activities, the magnitude of the reduction ranging from about 300-fold to 6 orders of magnitude. Remarkably, a good correlation was obtained for the wild-type luciferase, 12 alpha His45-mutated luciferases, and six additional variants with mutations of other alpha-subunit histidine residues between the degrees of luminescence activity reduction and the dark decay rates of intermediate II. Such a correlation further indicates that the activation of the O-O bond fission is an important function of the flavin 4a-hydroperoxide intermediate II. Both alpha H45G and alpha H45W were found to bind near-stoichiometric amounts of FMNH2. Moreover, each variant catalyzed the oxidation of bound FMNH2 by two mechanisms, with a minor pathway leading to the formation of a luminescence-active intermediate II and a major dark pathway not involving any detectable flavin 4a-hydroperoxide species. This latter pathway mimics that in the normal catalysis by flavooxidases, and its elicitation in luciferase was demonstrated for the first time by single-residue mutations.

Mechanism of reduced flavin transfer from Vibrio harveyi NADPH-FMN oxidoreductase to luciferase

The mechanisms of reduced flavin transfer in biological systems are poorly understood at the present. The Vibrio harveyi NADPH-FMN oxidoreductase (FRP) and the luciferase pair were chosen as a model for the delineation of the reduced flavin transfer mechanism. FRP, which uses FMN as a cofactor to mediate the reduction of the flavin substrate by NADPH, exhibited a ping-pong kinetic pattern with a Km, FMN of 8 microM and a Km,NADPH of 20 microM in a single-enzyme spectrophotometric assay monitoring the NADPH oxidation. However, the kinetic mechanism of FRP was changed to a sequential pattern with a Km,FMN of 0.3 microM and a Km,NADPH of 0.02 microM in a luciferase-coupled assay measuring light emission. In contrast, the Photobacterium fischeri NAD(P)H-FMN oxidoreductase FRG showed the same ping-pong mechanism in both the single-enzyme spectrophotometric and the luciferase-coupled assays. Moreover, for the FRP, FMN at concentrations over 2 microM significantly inhibited the coupled reaction in both light intensity and quantum yield, and showed apparent noncompetitive and competitive inhibition patterns against NADPH and luciferase, respectively. No inhibition of the NADPH oxidation was detected under identical conditions. These results are consistent with a scheme that the reduced flavin cofactor of FRP is preferentially utilized by luciferase for light emission, the reduced flavin product generated by the reductase is primarily channeled into a dark oxidation, and luciferase competes against flavin substrate in reacting with the FRP reduced flavin cofactor. An FRP derivative containing 2-thioFMN as the cofactor was also used to further examine the mechanism of flavin transfer. Results again indicate a preferential utilization of the reductase reduced flavin cofactor by luciferase for the bioluminescence reaction.

Mechanism of reduced flavin transfer from Vibrio harveyi NADPH-FMN oxidoreductase to luciferase

The mechanisms of reduced flavin transfer in biological systems are poorly understood at the present. The Vibrio harveyi NADPH-FMN oxidoreductase (FRP) and the luciferase pair were chosen as a model for the delineation of the reduced flavin transfer mechanism. FRP, which uses FMN as a cofactor to mediate the reduction of the flavin substrate by NADPH, exhibited a ping-pong kinetic pattern with a Km, FMN of 8 microM and a Km,NADPH of 20 microM in a single-enzyme spectrophotometric assay monitoring the NADPH oxidation. However, the kinetic mechanism of FRP was changed to a sequential pattern with a Km,FMN of 0.3 microM and a Km,NADPH of 0.02 microM in a luciferase-coupled assay measuring light emission. In contrast, the Photobacterium fischeri NAD(P)H-FMN oxidoreductase FRG showed the same ping-pong mechanism in both the single-enzyme spectrophotometric and the luciferase-coupled assays. Moreover, for the FRP, FMN at concentrations over 2 microM significantly inhibited the coupled reaction in both light intensity and quantum yield, and showed apparent noncompetitive and competitive inhibition patterns against NADPH and luciferase, respectively. No inhibition of the NADPH oxidation was detected under identical conditions. These results are consistent with a scheme that the reduced flavin cofactor of FRP is preferentially utilized by luciferase for light emission, the reduced flavin product generated by the reductase is primarily channeled into a dark oxidation, and luciferase competes against flavin substrate in reacting with the FRP reduced flavin cofactor. An FRP derivative containing 2-thioFMN as the cofactor was also used to further examine the mechanism of flavin transfer. Results again indicate a preferential utilization of the reductase reduced flavin cofactor by luciferase for the bioluminescence reaction.

Identification and characterization of a catalytic base in bacterial luciferase by chemical rescue of a dark mutant

Mutation of the His44 residue of the alpha subunit of Vibrio harveyi luciferase to an alanine was known to reduce the enzyme bioluminescence activity by five orders of magnitude [Xin, X., Xi, L., and Tu, S.-C. (1991) Biochemistry 30, 11255-11262]. We found that the residual activity of the alpha H44A luciferase was markedly enhanced by exogenously added imidazole and other simple amines. The peak luminescence intensity in nonturnover assays was linearly proportional to levels of alpha H44A and the rescue agent, indicating a lack of significant binding under our experimental conditions. The rescue effect of imidazole was pH dependent and quantitatively correlated well with the amount of imidazole base. The rescue efficiencies of imidazole and amines were found to be regulated by both their molecular volume and pKa. A Brønsted analysis revealed a beta value of 0.8 +/- 0.1. The enhancement of alpha H44A activity by imidazole took place after the formation of the flavin 4a-hydroperoxide intermediate. The predominant form of the flavin 4a-hydroperoxide intermediate generated by alpha H44A was inactive in bioluminescence, but was reactive with the aldehyde substrate for bioluminescence in the presence of imidazole. These findings, taken together, provide evidence for assigning a role for the alpha His44 imidazole as a catalytic base in the luciferase reaction. This study provides the first characterization of a catalytic residue for bacterial luciferase and the first demonstration of the rescue of a histidine-mutated enzyme by exogenous imidazole and amines.

Structure of bacterial luciferase beta 2 homodimer: implications for flavin binding

The crystal structure of the beta 2 homodimer of Vibrio harveyi luciferase has been determined to 2.5 A resolution by molecular replacement. Crystals were grown serendipitously using the alpha beta form of the enzyme. The subunits of the homodimer share considerable structural homology to the beta subunit of the alpha beta luciferase heterodimer. The four C-terminal residues that are disordered in the alpha beta structure are fully resolved in our structure. Four peptide bonds have been flipped relative to their orientations in the beta subunit of the alpha beta structure. The dimer interface of the homodimer is smaller than the interface of the heterodimer in terms of buried surface area and number of hydrogen bonds and salt links. Inspection of the subunits of our structure suggests that FMNH2 cannot bind to the beta 2 enzyme at the site that has been proposed for the alpha beta enzyme. However, we do uncover a potential FMNH2 binding pocket in the dimer interface, and we model FMN into this site. This proposed flavin binding motif is consistent with several lines of biochemical and structural evidence and leads to several conclusions. First, only one FMNH2 binds per homodimer. Second, we predict that reduced FAD and riboflavin should be poor substrates for beta 2. Third, the reduced activity of beta 2 compared to alpha beta is due to solvent exposure of the isoalloxazine ring in the beta 2 active site. Finally, we raise the question of whether our proposed flavin binding site could also be the binding site for flavin in the alpha beta enzyme.

Vibrio harveyi NADPH:FMN oxidoreductase: preparation and characterization of the apoenzyme and monomer-dimer equilibrium

A rapid chromatography method was developed for the preparation of apoenzyme of Vibrio harveyi NADPH:FMN oxidoreductase with > or =80% yields. The apoenzyme bound one FMN per enzyme monomer with a dissociation constant of 0.2 microM at 23 degrees C. The reconstituted holoenzyme was catalytically as active as the native enzyme. FMN binding resulted in 87 and 92% of quenching of protein and flavin fluorescence, respectively, indicating a conformational difference between the apoprotein and the holoenzyme. Neither riboflavin nor FAD showed any appreciable binding to the cofactor site of the apoenzyme but both flavins were active substrates for the FMN-containing holoenzyme. 2-ThioFMN bound to the cofactor site of the apoenzyme with an affinity similar to that for FMN binding. The holoenzyme reconstituted with 2-thioFMN showed a 509-nm absorption peak, which represents a 19-nm red shift from the corresponding peak of the free flavin, and was catalytically active in using either FMN or 2-thioFMN as a substrate. The holoenzyme showed a concentration dependence in molecular sieve chromatography corresponding to higher apparent molecular weights at higher concentrations. Both the holoenzyme and the apoenzyme was shown at 4 degrees C by equilibrium ultracentrifugation to undergo dimerization with dissociation constants of 1.8 and 3.3 microM, respectively.

Flavin reductase P: structure of a dimeric enzyme that reduces flavin

We report the structure of an NADPH:FMN oxidoreductase (flavin reductase P) that is involved in bioluminescence by providing reduced FMN to luciferase. The 1.8 A crystal structure of flavin reductase P from Vibrio harveyi was solved by multiple isomorphous replacement and reveals that the enzyme is a unique dimer of interlocking subunits, with 9352 A2 of surface area buried in the dimer interface. Each subunit comprises two domains. The first domain consists of a four-stranded antiparallel beta-sheet flanked by helices on either side. The second domain reaches out from one subunit and embraces the other subunit and is responsible for interlocking the two subunits. Our structure explains why flavin reductase P is specific for FMN as cofactor. FMN is recognized and tightly bound by a network of 16 hydrogen bonds, while steric considerations prevent the binding of FAD. A flexible loop containing a Lys and an Arg could account for the NADPH specificity. The structure reveals information about several aspects of the catalytic mechanism. For example, we show that the first step in catalysis, which is hydride transfer from C4 of NADPH to cofactor FMN, involves addition to the re face of the FMN, probably at the N5 position. The limited accessibility of the FMN binding pocket and the extensive FMN-protein hydrogen bond network are consistent with the observed ping-pong bisubstrate--biproduct reaction kinetics. Finally, we propose a model for how flavin reductase P might shuttle electrons between NADPH and luciferase.

Gene overexpression, purification, and identification of a desulfurization enzyme from Rhodococcus sp. strain IGTS8 as a sulfide/sulfoxide monooxygenase

The oxidation of dibenzothiophene to dibenzothiophene sulfone has been linked to the enzyme encoded by the sox/dszC gene from Rhodococcus sp. strain IGTS8 (S. A. Denome, C. Oldfield, L. J. Nash, and K. D. Young, J. Bacteriol. 176:6707-6717, 1994; C. S. Piddington, B. R. Kovacevich, and J. Rambosek, Appl. Environ. Microbiol. 61:468-475, 1995). However, this enzyme has not been characterized, and the type of its catalytic activity remains unclassified. In this work, the sox/dszC gene was overexpressed in Escherichia coli, a procedure for the purification of the expressed enzyme was developed, and the properties of and the reactions catalyzed by the purified enzyme were characterized. This enzyme binds one flavin mononucleotide (Kd, 7 micrometers) or reduced flavin mononucleotide (FMNH2) (Kd < 10(-8) M) per 90,200-Da homodimer, and FMNH2 is an essential cosubstrate for its activity. Patterns of product formation were examined under different FMNH2 availabilities, and results indicate that this enzyme catalyzes a stepwise conversion of dibenzothiophene to the corresponding sulfoxide and subsequently to the sulfone. On the basis of isotope labeling patterns with H2(18)O and 18O2, dibenzothiophene sulfoxide and sulfone obtained their oxygen atom(s) from molecular oxygen rather than water in their formation from dibenzothiophene. The enzyme also utilizes benzyl sulfide and benzyl sulfoxide as substrates. Hence, it is identified as a sulfide/sulfoxide monooxygenase. This monooxygenase is similar to the microsomal flavin-containing monooxygenase but is unique among microbial flavomonooxygenases in its ability to catalyze two consecutive monooxygenation reactions.

Cloning, direct expression, and purification of a snake venom cardiotoxin in Escherichia coli

The cardiotoxin analogue III (CTX III), isolated from the Taiwan cobra (Naja naja atra) venom, is a sixty-amino acid, all beta-sheet protein. We report the direct expression of CTX III from its synthetic gene as inclusion bodies in Escherichia coli. The yield of the expressed protein is about 40 mg/liter of the culture. CTX III trapped as inclusion bodies is dissolved and refolded by the slow refolding technique. The refolded protein is purified by reverse phase high performance liquid chromatography. The purified and refolded CTX III sample is further characterized by SDS-PAGE, circular dichroism, two-dimensional NMR spectroscopy and haemolytic activity. To our knowledge, this is the first report of the direct expression and purification of snake venom cardiotoxins.

Catalytically active forms of the individual subunits of Vibrio harveyi luciferase and their kinetic and binding properties

Contradictory findings have recently been reported regarding the (in)abilities of individual subunits of the Vibrio harveyi alpha beta dimeric luciferase to catalyze bioluminescence. We have produced individual alpha and beta subunits separately in Escherichia coli JM109 cells by recombinant DNA techniques. Both subunits were purified to more than 90% homogeneity and found to be catalytically active, with their general catalytic properties and the specific activities similar to those reported earlier (Sinclair, J. F., Waddle, J. J., Waddill, E. F., and Baldwin, T. O. (1993) Biochemistry 32, 5036-5044). Individual subunits were significantly distinct from the native luciferase with respect to inactivations by trypsin and N-ethylmaleimide, and the stability of the flavin 4a-hydroperoxide intermediate. The active species in isolated alpha and beta samples were each the predominant protein species, corresponding to a 42,000 M(r) alpha monomer and a 67,000 M(r) beta dimer, respectively. These findings clearly indicate that the activities of the individual subunits are not due to trace contaminations of the respective counter subunits. The much reduced specific activities of the individual subunits are, in part, a consequence of diminished abilities to oxidize the aldehyde substrate. Kinetic and equilibrium measurements indicate that alpha and beta 2 each contained a reduced flavin site, an aldehyde substrate site, and an aldehyde inhibitor site. The on and off rates of the decanal inhibitor binding were substantially slower than the bindings of decanal and reduced riboflavin 5'-phosphate substrates. These findings are consistent with a scheme that the aldehyde inhibitor blocks the binding of the reduced flavin substrate.

Probing the Vibrio harveyi luciferase beta subunit functionality and the intersubunit domain by site-directed mutagenesis

While the critical role of the bacterial luciferase alpha subunit in catalysis has been amply documented, the beta subunit was only known to be involved in thermal stability and substrate binding. Two conserved histidyl residues at position 81 and 82 of the beta subunit of Vibrio harveyi luciferase were each mutated to an alanine, aspartate, or lysine to probe further the beta functionality. These mutations resulted in higher Km values for reduced riboflavin 5'-phosphate, less efficient oxidations of the aldehyde substrate, and decreased light-emitting activities. beta His82 appears to be significantly more critical than beta His81. For the beta His82-mutated luciferases, the maximal light intensities and total light outputs were reduced to 19-4% of that for the wild-type enzyme, and the values of Vmax/Km,flavin were decreased by 2-3 orders of magnitude. The reduced light emission activities for these mutated luciferases can be correlated to lower yields of the flavin 4a-hydroperoxide intermediate, reduced productions of the excited flavin emitter, and/or enhanced quenching of the emitter. The beta subunit and the conserved beta His82 in particular have thus been shown to be critical not only to flavin binding but also to catalytic characteristics of luciferase. The dimeric structure of luciferase is essential to its high catalytic efficiency. To characterize the intersubunit domain, three sets of single/double mutants were constructed, and the additivities of mutational effects were tested to screen for residues that could interact across the subunit interface.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization and preliminary crystallographic analysis of NADPH:FMN oxidoreductase from Vibrio harveyi

Crystals of NADPH:FMN oxidoreductase from Vibrio harveyi have been obtained and characterized by X-ray diffraction. This enzyme plays a role in the generation of light in luminescent bacteria by providing reduced FMN to luciferase. Large, high quality crystals were grown using polyethylene glycol 6000 at pH 7.0. They crystallize in the monoclinic space group P2(1) with cell dimensions a = 51.2 A, b = 85.9 A, c = 58.1 A, beta = 109.3 degrees, and diffract to 1.8 A. We expect two molecules per asymmetric unit. High resolution data sets have been recorded and a search is under way for heavy-atom derivatives.

Plasma protein C activity is enhanced by arsenic but inhibited by fluorescent humic acid associated with blackfoot disease

Blackfoot disease is a peripheral vascular disease causally related to the fluorescent humic acid found in the drinking water of endemic areas in Taiwan. We compared the effects of humic acid (HA) purified from the well water of Blackfoot disease endemic areas with the effects of commercial humic acid (Aldrich) as well as trivalent arsenic (As2O3) on protein C activity, which plays an important role in regulation of blood coagulation and fibrinolysis. Humic acid, either purified from drinking water or obtained commercially, dose-dependently inhibited both activated protein C activity and the activation of protein C induced by Protac, a snake venom-derived protein C activator. In contrast to humic acid, arsenic oxide dose-dependently enhanced both activated protein C activity and the Protac-stimulated activation of protein C. In the presence of humic acid the enhancement effect of arsenic oxide was completely abolished, resulting in concentration-dependent inhibition of protein C activity. Therefore, the results of this study indicate that humic acid is a potent protein C inhibitor even in the presence of arsenic, which enhances the protein C activity. Since protein C is a potent anticoagulant and profibrinolytic agent, acquired defects of protein C induced by humic acid might cause a thrombophilic or hypercoagulable state. Whether this is one of the possible mechanisms of humic acid-induced thrombotic disorders in Blackfoot disease needs to be further characterized.

Vibrio harveyi NADPH-flavin oxidoreductase: cloning, sequencing and overexpression of the gene and purification and characterization of the cloned enzyme

NAD(P)H-flavin oxidoreductases (flavin reductases) from luminous bacteria catalyze the reduction of flavin by NAD(P)H and are believed to provide the reduced form of flavin mononucleotide (FMN) for luciferase in the bioluminescence reaction. By using an oligonucleotide probe based on the partial N-terminal amino acid sequence of the Vibrio harveyi NADPH-FMN oxidoreductase (flavin reductase P), a recombinant plasmid, pFRP1, was obtained which contained the frp gene encoding this enzyme. The DNA sequence of the frp gene was determined; the deduced amino acid sequence for flavin reductase P consists of 240 amino acid residues with a molecular weight of 26,312. The frp gene was overexpressed, apparently through induction, in Escherichia coli JM109 cells harboring pFRP1. The cloned flavin reductase P was purified to homogeneity by following a new and simple procedure involving FMN-agarose chromatography as a key step. The same chromatography material was also highly effective in concentrating diluted flavin reductase P. The purified enzyme is a monomer and is unusual in having a tightly bound FMN cofactor. Distinct from the free FMN, the bound FMN cofactor showed a diminished A375 peak and a slightly increased 8-nm red-shifted A453 peak and was completely or nearly nonfluorescent. The Kms for FMN and NADPH and the turnover number of this flavin reductase were determined. In comparison with other flavin reductases and homologous proteins, this flavin reductase P shows a number of distinct features with respect to primary sequence, redox center, and/or kinetic mechanism.

Mechanism of aldehyde inhibition of Vibrio harveyi luciferase. Identification of two aldehyde sites and relationship between aldehyde and flavin binding

Vibrio harveyi luciferase is sensitive to aldehyde substrate inhibition, and two kinetic schemes have been previously postulated to account for such an inhibition. One scheme depicts a sequential binding of 2 aldehyde molecules, yielding an active enzyme-aldehyde binary complex and subsequently an inactive enzyme-(aldehyde)2 ternary complex (Holzman, T. F., and Baldwin, T. O. (1983) Biochemistry 22, 2838-2846). This two-aldehyde model was later withdrawn, and recently, a different scheme was proposed, following which the prior binding of one aldehyde to the native luciferase forms an inactive dead-end complex (Abu-Soud, H. M., Clark, A. C., Francisco, W. A., Baldwin, T. O., and Raushel, F. M. (1993) J. Biol. Chem. 268, 7699-7706). In this work, kinetic and equilibrium studies were carried out to elucidate further the mechanism of aldehyde inhibition. Two, presumably independent, aldehyde-binding sites were detected, with a higher affinity site for the aldehyde substrate and a weaker affinity site for the aldehyde inhibitor. Binding to and dissociation from the inhibitor site by decanal were revealed by chemical relaxation analysis to be slow processes. Furthermore, whereas the binding of the decanal substrate enhances the affinity of the reduced riboflavin 5'-phosphate (FMNH2) site, the binding of decanal to the inhibitor site competes against FMNH2 binding, thus resulting in inhibition of luciferase activity. These findings are not compatible with either of the two earlier schemes mentioned above. A new kinetic model is formulated for the mechanism of aldehyde inhibition. Theoretical kinetic behaviors predicted on the basis of this model are in excellent agreement with experimental observations. A particularly reactive cysteine (residue 106) on the alpha subunit has been previously demonstrated to be at or near an aldehyde site (Fried, A., and Tu, S.-C. (1984) J. Biol. Chem. 259, 10754-10759). Evidence is presented to indicate that this residue is at or near the aldehyde inhibitor site. Relative locations of this residue and binding sites for FMNH2, the aldehyde substrate, and the aldehyde inhibitor are proposed.

Construction and characterization of hybrid luciferases coded by lux genes from Xenorhabdus luminescens and Vibrio fischeri

Molecular cloning techniques were employed to obtain hybrid luciferases with their alpha and beta subunits encoded by luxA and luxB genes, respectively, from Xenorhabdus luminescens strain HW or Vibrio fischeri. Although the two wild-type luminous bacteria are phylogenetically diverged, the hybrid luciferase Xf comprising an alpha from X. luminescens HW and a beta from V. fischeri and the hybrid luciferase VI comprising an alpha from V. fischeri and a beta from X. luminescens HW were both functional in bioluminescence. Their general kinetic properties were close to the wild-type enzymes from which the alpha subunit was derived. The X. luminescens HW enzyme is distinct in having a high optimal temperature for in vitro bioluminescence, a high thermal stability and a sensitivity to aldehyde substrate inhibition. Comparisons of the Xf and VI hybrid luciferases with the two wild-type enzymes indicated that these unusual properties of the X. luminescens HW luciferase originated primarily from the alpha subunit.

Fluorescent polyene aliphatics as spectroscopic and mechanistic probes for bacterial luciferase: evidence against carbonyl product from aldehyde as the primary excited species

The fluorescent alpha-parinaric acid (alpha-PAC) and beta-parinaric acid (beta-PAC) were converted to the corresponding aldehydes and alcohols all of which exhibited absorption and fluorescence properties closely resembling those of the parent acids. alpha-PAC and beta-PAC each binds to luciferase in competition with aldehyde. The hydrophobic nature of the aldehyde site was indicated by the enhanced fluorescence quantum yields of the bound alpha-PAC and beta-PAC. These two polyene acids and the beta-parinaryl alcohol were shown to stabilize the luciferase flavin-peroxide intermediate. alpha-Parinaraldehyde (alpha-PAD) and beta-parainaraldehyde (beta-PAD) were active substrates for Vibrio harveyi and Vibrio fischeri luciferases and, for the former enzyme, exhibited Km values similar to and quantum yields about 20-30% as those for decanal and dodecanal. For the V. harveyi luciferase with reduced FMN as a co-substrate, the alpha-PAD- or beta-PAD-initiated luminescence was indistinguishable from the normal emission obtained with octanal (lambda max 495 nm) showing no additional 430-nm component correlatable with emission from excited alpha-PAC or beta-PAC. In reactions using reduced 2-thioFMN for V. harveyi luciferase or reduced FMN for V. fischeri luciferase plus yellow fluorescent protein, the replacement of octanal by beta-PAD again resulted in no additional 430-nm emission. The lack of any emission correlatable with excited alpha-PAC, beta-PAC, or equivalent carbonyl product was not due to the quenching of the polyene moiety by chemical transformation, binding to luciferase, or a 100% energy transfer to the flavin 4a-hydroxide emitter.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional consequences of site-directed mutation of conserved histidyl residues of the bacterial luciferase alpha subunit

The available sequences for the different bacterial luciferases reveal five conserved histidyl residues at positions 44, 45, 82, 224, and 285 of the alpha subunit. Ten variants of Vibrio harveyi luciferase were obtained by selective site-directed mutations of these five histidines. The essentiality of alpha His44 and alpha His45 was indicated by 4-7 orders of magnitude of bioluminescence activity reductions resulting from the substitution of either histidine by alanine (alpha H44A or alpha H45A), aspartate (alpha H44D or alpha H45D), or lysine (alpha H45K). Moreover, alpha H44A and alpha H45A were distinct from the native luciferase in thermal stabilities. Mutations at the other three positions also resulted in activity reductions ranging from a fewfold to 3 orders of magnitude. Despite these widely different bioluminescence light outputs, mutated luciferases exhibited, in nonturnover in vitro assays, light emission decay rates mostly similar to that of the native luciferase using octanal, decanal, or dodecanal as a substrate. This is attributed to a similarity in the catalytic rate constants of the light-emitting pathway for the native and mutated luciferases, but various mutated luciferases suffer in different degrees from competing dark reaction(s). In accord with this interpretation, the bioluminescence activities of mutated luciferases showed a general parallel with the relative stabilities of their 4a-hydroperoxyflavin intermediate species. Furthermore, the drastically reduced bioluminescence activities for luciferases with the alpha His44 or alpha His45 substituted by aspartate, alanine, or lysine were accompanied by little or no activities for consuming the aldehyde substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

EEG spectral analysis and topographic mapping in Wilson's disease

Computerized EEG spectral analysis and topographic mapping were performed on 14 patients with Wilson's disease (WD) and 10 normal subjects of comparable ages. The predominant EEG changes in WD were diffuse but uneven topographic abnormalities with a decrease in alpha activity, an increase in theta and delta activities, and a low voltage background mainly in the alpha frequency band. Eleven patients (80%) had at least one of the above EEG changes. Furthermore, topographic mapping provided more clearly defined foci of slowing and epileptiform activity. Patients with cerebral white matter involvement, akinetic-rigid syndrome, dystonia, or psychiatric symptoms tended to have more abnormal EEGs. It is concluded that EEG changes in WD are common and the quantitative EEG analysis can increase the likelihood of detecting mild or even subtle EEG abnormalities in individual patients as well as in the patient group.

Electrochemical superoxidation of flavins: generation of active precursors in luminescent model systems

Using 3-methyllumiflavin and tetraacetyliriboflavin as examples, we have shown that the socalled "fully oxidized" flavins can be "superoxidized" at an anodic potential of 1.8 to 1.9 V giving flavin radical cation transients which are rapidly transformed in subsequent chemical reactions. An attack by H2O subsequent to the superoxidation of 3-methyllumiflavin provides a route for the formation of 4a-hydroxy-3-methyllumiflavin radical cation, as evident from the subsequent decomposition to the protonated form of the starting flavin. When 3-methyllumiflavin is superoxidized in the presence of a base, a recycling process occurs, allowing superoxidized flavin to be trapped in a slower, competitive conversion. The relatively more stable trapped product is active in reacting with H2O2 to emit chemiluminescence. Electrochemical oxidation of H2O2 in acetonitrile at 1.30 V in the presence of an oxidized flavin results in a direct protonation of the flavin by H+ generated from the electrolysis of H2O2. Minor reactions presumably provide alternative formations of the 4a-hydroperoxy- and 4a-hydroxy-flavin radical cation transients by the direct addition of HOO. and HO. radicals, which also arise in the oxidation of H2O2, to protonated flavin. Under such conditions the superoxidized flavin radical cation is apparently also formed, either directly or by process(es) such as decomposition of the flavin 4a-adduct radical cations. Subsequent reductions of either the superoxidized flavin or the flavin 4a-adduct radical cations lead to an almost steady level of luminescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Elicitation of an oxidase activity in bacterial luciferase by site-directed mutation of a noncatalytic residue

Flavin-dependent external monooxygenases and oxidases could catalyze the same flavin oxidation reaction involving distinct mechanisms. To gain insights into enzyme structure-function relationship, site-directed mutagenesis was carried out for Vibrio harveyi luciferase, a monooxygenase. The substitution of the alpha subunit cysteine 106 by alanine shows unambiguously that the alphaCys106 is not essential to catalysis. The corresponding substitution by valine resulted in a substantial reduction of the bioluminescence activity correlatable with the induction of a new flavin oxidation activity typical for oxidases. These findings indicate that mutation of a single noncatalytic residue at the active center of a flavoenzyme could transform one enzyme type to another, thus highlighting the subtlety of enzyme active site structure in relation to catalysis and the versatility of enzyme evolution.

Dithionite treatment of flavins: spectral evidence for covalent adduct formation and effect on in vitro bacterial bioluminescence

Intrigued by the apparent requirement of dithionite for FMN reduction (as opposed to photoreduction or catalytic hydrogenation) in the H2O2-initiated bacterial bioluminescence reaction, we chose 5-ethyl-3-methyllumiflavinium cation I as a model to investigate possible flavin adduct formation by treatment with dithionite or (bi)sulfite. In the range of pH 5-8, the reaction of dithionite with 5-ethyl-3-methyllumiflavinium cation, which is in equilibrium with the 5-ethyl-4a-hydroxy-3-methyl-4a, 5-dihydrolumiflavin pseudobase II (X = OH), is not limited to the formation of flavosemiquinone and dihydroflavin following two one-electron steps. Several parallel and sequential reactions may take place involving the intermediacy of covalent flavin adducts. Addition of (bi)sulfite gave a 4a-sulfiteflavin adduct II (X = SO3-). Consistent with the S2O4(2-) in equilibrium with 2 SO2-. equilibrium, the reaction of dithionite and II (X = OH; SO3-) gave rise to two flavin adducts in competitive nucleophilic displacements: a 4a-sulfoxylate-flavin radical (II, X = SO2.) and a 4a-dithioniteflavin adduct (II, X = S2O4-), respectively. On increasing the (S2O4(2-), SO2.-)/flavin ratio under N2, the formation of the 4a-sulfoxylate-flavin radical became predominant. The II (X = SO2.) so formed was in equilibrium with the flavosemiquinone and bisulfate and can be trapped by reacting with hydroxylamine. In the initial presence of oxygen, II (X = SO2.) was highly reactive toward O2, giving a fast oxidation to II (X = SO3-) and effectively suppressing the formation of the flavosemiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical modification and characterization of the alpha cysteine 106 at the Vibrio harveyi luciferase active center

Vibrio harveyi luciferase, an alpha beta dimer, was effectively inactivated by treatment with the methylation agent methyl p-nitrobenzene sulfonate. However, inactivation of luciferase in the presence of excess amounts of this reagent did not follow pseudo-first-order kinetics. After taking the autodecay of this reagent into consideration in kinetic analysis, the pseudo-first-order constants and subsequently the second-order rate constant (83 min-1 M-1 at pH 7 and 23 degrees C) were determined. The inactivation rate can be retarded by the addition of the decanal or the reduced FMN substrate but not by the reaction product FMN. The binding of decanal specifically protected one target residue against modification with a concomitant protection of luciferase against inactivation. A pentapeptide containing this specific target residue was isolated and identified to be Phe-Gly-Ile-X-Arg with X corresponding to the S-methylated form of the cysteinyl residue at position 106 of the luciferase alpha subunit. It is concluded that this reactive alpha Cys-106 is at the aldehyde site and is also near the reduced flavin site of luciferase. The modified enzyme exhibited no gross conformational changes detectable by protein fluorescence measurements, which may be due to the small size change of the target cysteinyl residue after methylation. The methylated enzyme still retained the ability to bind one decanal and one reduced FMN without any substantial changes in binding affinities. The cause of luciferase inactivation by the methylation of alpha Cys-106 has been shown to be the impaired ability to form the 4a-hydroperoxy-flavin intermediate from the bound flavin substrate or to stabilize this intermediate.

Molecular cloning of salicylate hydroxylase genes from Pseudomonas cepacia and Pseudomonas putida

The sal gene encoding Pseudomonas cepacia salicylate hydroxylase was cloned and the sal encoding Pseudomonas putida salicylate hydroxylase was subcloned into plasmid vector pRO2317 to generate recombinant plasmids pTK3 and pTK1, respectively. Both cloned genes were expressed in the host Pseudomonas aeruginosa PAO1. The parental strain can utilize catechol, a product of the salicylate hydroxylase-catalyzed reaction, but not salicylate as the sole carbon source for growth due to a natural deficiency of salicylate hydroxylase. The pTK1- or pTK3-transformed P. aeruginosa PAO1, however, can be grown on salicylate as the sole carbon source and exhibited activities for the cloned salicylate hydroxylase in crude cell lysates. In wild-type P. cepacia as well as in pTK1- or pTK3-transformed P. aeruginosa PAO1, the presence of glucose in addition to salicylate in media resulted in lower efficiencies of sal expression P. cepacia apparently can degrade salicylate via the meta cleavage pathway which, unlike the plasmid-encoded pathway in P. putida, appears to be encoded on chromosome. As revealed by DNA cross hybridizations, the P. cepacia hsd and ht genes showed significant homology with the corresponding plasmid-borne genes of P. putida but the P. cepacia sal was not homologous to the P. putida sal. Furthermore, polyclonal antibodies developed against purified P. cepacia salicylate hydroxylase inactivated the cloned P. cepacia salicylate hydroxylase but not the cloned P. putida salicylate hydroxylase in P. aeruginosa PAO1. It appears that P. cepacia and P. putida salicylate hydroxylases, being structurally distinct, were probably derived through convergent evolution.

Delineation of bacterial luciferase aldehyde site by bifunctional labeling reagents

Previously we have established that a highly reactive cysteinyl group on the alpha subunit is at the aldehyde site of the (alpha beta) dimeric Vibrio harveyi luciferase. Three isomeric bifunctional reagents have been synthesized and used to further delineate the luciferase aldehyde site. These probes differ in their relative positions of and distances between the two functional groups active in chemical and photochemical labelings, respectively. Each of the probes can effectively and reversibly inactivate luciferase by forming a disulfide linkage primarily to the reactive cysteinyl residue. Upon subsequent photolysis, a diazoacetate arm in each probe was activated for photochemical labeling of amino acid residues within reach. After reductive regeneration of the reactive cysteinyl residue, 0.35-0.40 probe per dimeric luciferase was found to have been photochemically incorporated, correlating well with the degree of irreversible enzyme inactivation. Low but significant amounts of the three isomeric probes initially attached to the alpha reactive cysteine through a disulfide have been found to photochemically tag certain residues on beta. The latter residues are estimated to be no more than 8-11 A away from the alpha reactive cysteine. Thus the reactive cysteinyl residue, and hence the aldehyde site, must be at or near the alpha beta subunit interface. Furthermore, the structural integrity of the microenvironment surrounding this reactive cysteinyl residue is crucial to luciferase activity. An HPLC method for the isolation of luciferase alpha and beta subunits has also been developed.

Studies of electron-transfer properties of salicylate hydroxylase from Pseudomonas cepacia and effects of salicylate and benzoate binding

The pH dependence of the redox behavior of salicylate hydroxylase from Pseudomonas cepacia as well as the effects of salicylate, benzoate, and chloride binding is described. At pH 7.6 in 0.02 M potassium phosphate buffer E1(0')(EFl ox/EFl.-) is -0.150 V and E2(0')(EFl.-/EFl red H-) is -0.040 V versus the standard hydrogen electrode (SHE). A maximum of 5% of FAD anion semiquinone is thermodynamically stabilized under these conditions. However, in coulometric and dithionite titrations more semiquinone is kinetically formed, indicating slow transfer of the second electron. The potential/pH dependence is consistent with a two-electron, one-proton transfer. Upon salicylate binding the midpoint potential is shifted 0.020 V negative from -0.094 to -0.114 V vs SHE at pH 7.6. A maximum of 7% of the neutral semiquinone is stabilized both in potentiometric and coulometric titrations. This small potential shift indicates that the substrate is bound nearly to the same extent to all three oxidation states of the enzyme. It is clear that the substrate binding does not make the reduction of the flavin thermodynamically more favorable. In contrast to salicylate, the potential shift caused by the effector, benzoate, is much more significant. (A maximum potential shift of -0.07 V is calculated.) Benzoate binds most tightly to the oxidized form and is least tightly bound to the two-electron-reduced form of the enzyme. For the reduction of the free enzyme the transfer of the second electron or the transfer of the proton is rate limiting, as is shown by the kinetic formation of the anionic semiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudomonas cepacia 3-hydroxybenzoate 6-hydroxylase: stereochemistry, isotope effects, and kinetic mechanism

A neutral flavin semiquinone species was formed upon photoreduction of Pseudomonas cepacia 3-hydroxybenzoate 6-hydroxylase whereas no flavin radical was detected by anaerobic reduction with NADH in the presence of m-hydroxybenzoate. In the latter case, the formation of flavin semiquinone is apparently thermodynamically unfavorable. A stereospecificity for the abstraction of the 4R-position hydrogen of NADH has been demonstrated for this hydroxylase. Deuterium and tritium isotope effects were observed with (4R)-[4-2H]NADH and (4R)-[4-3H]NADH as substrates. The DV effect indicates the existence of at least one slow step after the isotope-sensitive enzyme reduction by dihydropyridine nucleotide. A minimal kinetic mechanism has been deduced on the basis of initial velocity measurements and studies on deuterium and tritium isotope effects. Following this scheme, m-hydroxybenzoate and NADH bind to the hydroxylase in a random sequence. The flavohydroxylase is reduced by NADH, and NAD+ is released. Oxygen subsequently binds to and reacts with the reduced flavohydroxylase-m-hydroxybenzoate complex. Following the formation and release of water and gentisate, the oxidized holoenzyme is regenerated. The enzyme has a small (approximately 2-fold) preference for the release of NADH over m-hydroxybenzoate from the enzyme-substrates ternary complex.

Pseudomonas cepacia 3-hydroxybenzoate 6-hydroxylase: induction, purification, and characterization

A single strain of Pseudomonas cepacia cells was differentially induced to synthesize salicylate hydroxylase, 3-hydroxybenzoate 6-hydroxylase, or 4-hydroxybenzoate 3-hydroxylase. A procedure was developed for the purification of 3-hydroxybenzoate 6-hydroxylase to apparent homogeneity. The purified hydroxylase appears to be a monomer with a molecular weight of about 44,000 and exhibits optimal activity near pH 8. The hydroxylase contains one FAD per enzyme molecule and utilizes NADH and NADPH with similar efficiencies. The reaction stoichiometry for this enzyme has been determined. In comparison with other aromatic flavohydroxylases, this enzyme is unique in inserting a new hydroxyl group to the substrate at a position para to an existing one.

Kinetic analysis of enzyme inactivation by an autodecaying reagent

A simple method is described for the determination of both the pseudo-first-order rate constant and the second-order rate constant for enzyme inactivation by a chemical reagent which itself undergoes exponential decay. The validity of this method has been demonstrated in two test cases in which the labile diethyl pyrocarbonate was used to inactivate salicylate hydroxylase and bacterial luciferase.

Isolation and characterization of a cyclic AMP receptor protein from luminous Vibrio harveyi cells

A cAMP receptor protein (CRP) species was purified from the luminous Vibrio harveyi cells to apparent homogeneity. This protein had a dimeric structure with a molecular weight of 23,000 per subunit. Among all eight nucleotides tested, only cAMP (Kd = 3 to 4 microM at 0 degrees C and 52 microM at 23 degrees C) and cGMP (Kd = 6 to 10 microM at 0 degrees C and 67 microM at 23 degrees C) bound to this protein. Its binding to poly(dI-dC), poly(dA-dT), and DNA fragments isolated from V. harveyi cells were all significantly enhanced by the addition of cAMP. Based on patterns of limited proteolysis by trypsin, this CRP assumes different conformations in the absence and presence of cAMP. Also consistent with this conclusion is the finding that the binding of cAMP to CRP induced about 50% quenching of the CRP fluorescence with a concomitant 3-nm blue shift from the original 336-nm emission peak. The binding of cGMP resulted in similar fluorescence changes but had no apparent effect on the pattern of proteolysis by trypsin. Using an in vitro transcription system known to be dependent on cAMP and Escherichia coli CRP, the synthesis of a run-off transcript product was also significantly enhanced by cAMP and this V. harveyi CRP.

Affinity labeling of the aldehyde site of bacterial luciferase

2-Bromo[1-14C]1-decanal was synthesized as an affinity labeling probe for the aliphatic aldehyde site of Vibrio harveyi luciferase. In the presence of excess amounts of this probe, the inactivation of bacterial luciferase occurred following apparent first order kinetics. This inactivation was markedly retarded in the presence of decanal but neither butanal (a very poor aldehyde substrate) nor FMN (a reaction product derived from reduced FMN) showed any significant protective effect. Upon mixing luciferase with the affinity labeling probe, a noncovalent complex was formed prior to the covalent attachment. At pH 6 and 23 degrees C, the dissociation constant for the binding step and the rate constant for the covalent modification step were determined to be 23 microM and 1 min-1, respectively. The displacement of a bound aldehyde substrate by this probe added secondarily was also demonstrated. The inactivation of luciferase was correlated with both the incorporation of about 1.2 molecules of the probe and the loss of 0.8 to 1.1 cysteinyl residues/luciferase alpha beta dimer. The presence of an essential sulfhydryl group at the aldehyde site of luciferase has thus been demonstrated. This sulfhydryl group was a constituent residue of the alpha subunit and was near the alpha beta subunit interface. This residue appears to be the same essential cysteinyl group previously identified by chemical modification (Nicoli, M.Z., Meighen, E.A., and Hastings, J.W. (1974) J. Biol. Chem. 249, 2385-2392). The labeled luciferase did not exhibit any significant binding for the reduced FMN substrate.

The kinetic mechanism of salicylate hydroxylase as studied by initial rate measurement, rapid reaction kinetics, and isotope effects

The kinetic mechanism of Pseudomonas cepacia salicylate hydroxylase has been examined by steady state initial rate measurements, and stopped flow and equilibrium studies. Results indicate that salicylate and NADH bind to the hydroxylase randomly. The enzyme is reduced and NAD+ is released. Oxygen subsequently binds to the reduced enzyme . substrate complex, leading to the production of hydroxylated product, CO2, and water. Based on results of anaerobic rapid mixing experiments, the rate of enzyme reduction by NADH is enhanced 290- and 240-fold when the hydroxylase is complexed with salicylate and benzoate (a nonsubstrate effector), respectively. Salicylate enhances, whereas benzoate slightly weakens, the NADH binding to the enzyme. Primary isotope effects were observed with (4R)-[4-2H]- and (4R)-[4-3H]NADH but not with the (4S)-[4-2H]NADH. Using varying concentrations of benzoate, the pattern of tritium isotope effect on Vm/Km, T(V/K), also indicates that benzoate and NADH bind to the enzyme randomly. The intrinsic isotope effects, Dk, of (4R)-[4-2H]NADH on the reduction of enzyme . salicylate and enzyme . benzoate complexes were found to be 5.57 and 5.96, respectively. The former is much repressed but the latter is only slightly so in the expression of their corresponding deuterium isotope effects on Vm, DV. Furthermore, values of DV (1.69 to 5.07) show a rough correlation with the extents of uncoupling of substrate hydroxylation and H2O2 formation activities for a series of benzenoid effectors. These results indicate that relative to the step of enzyme reduction, the subsequent reaction(s) leading to H2O2 formation must be fast whereas that for substrate hydroxylation contains at least one slow step.