Evidence of Negative Cooperativity and Half-Site Reactivity within an

Jan 26, 2016 - Here, we report the very first example of half-site reactivity and negative cooperativity involving an important F420 cofactor-dependen...
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Evidence of Negative Cooperativity and Half-Site Reactivity within an F420-Dependent Enzyme: Kinetic Analysis of F420H2:NADP+ Oxidoreductase Ebenezer Joseph, Cuong Quang Le, Toan Nguyen, Mercy Oyugi, Mohammad Shawkat Hossain, Frank W. Foss, Jr., and Kayunta Johnson-Winters* Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, United States S Supporting Information *

ABSTRACT: Here, we report the very first example of half-site reactivity and negative cooperativity involving an important F420 cofactordependent enzyme. F420H2:NADP+ oxidoreductase (Fno) is an F420 cofactor-dependent enzyme that catalyzes the reversible reduction of NADP+ through the transfer of a hydride from the reduced F420 cofactor. These catalytic processes are of major significance in numerous biochemical processes. While the steady-state kinetic analysis showed classic Michaelis−Menten kinetics with varying concentrations of the F420 redox moiety, FO, such plots revealed non-Michaelis−Menten kinetic behavior when NADPH was varied. The double reciprocal plot of the varying concentrations of NADPH displays a downward concave shape, suggesting that negative cooperativity occurs between the two identical monomers. The transient state kinetic data show a burst prior to entering steady-state turnover. The burst suggests that product release is ratelimiting, and the amplitude of the burst phase corresponds to production of product in only one of the active sites of the functional dimer. These results suggest either half-site reactivity or an alternate sites model wherein the reduction of the cofactor, FO occurs at one active site at a time followed by reduction at the second active site. Thus, the data imply that Fno may be a functional regulatory enzyme.

F

+ 420 cofactor-dependent NADP oxidoreductase (Fno) is an enzyme that catalyzes the reversible reduction of NADP+. The reduced F420 cofactor (a 7,8-didemethyl-8-hydroxy-5deazariboflavin-5′-phosphoryllactyl(glutamyl)nglutamate) transfers the hydride to NADP+, producing NADPH (Figure 1). This hydride transfer occurs stereospecifically from the Si face of the F420 cofactor (C5 position) toward the Si face of NADP+ (C4 position)1 (Figure 1). Fno is found in methanogenic, halophilic, and sulfatereducing archaea.2,3 The recombinant expression and purification of Fno has been reported previously from Archeoglobus fulgidus and Methanobacterium thermoautotrophicum.1−3 Fno has also been purified from Methanococcus vannielii,4 Methanogenium organophilum,5 Halobacterium cutirubrum,6 and Streptomyces griseus.7 In some organisms, such as A. fulgidus, Fno has been shown to be homodimeric, whereas in others, such as M. thermoautotrophicum, it has been found to be a homotetrameric enzyme.3,5 In most of the methanogenic and sulfate-reducing archaea, the primary function of Fno is the production of NADPH. In these organisms, Fno is found only in low concentration.3,5 Previous experimental studies with Fno from M. thermoautotrophicum revealed that the reduction potential of FO (F420 cofactor precursor) is between −340 and −350 mV, as determined by equilibration methods.6 Jacobson and Walsh © XXXX American Chemical Society

found that the ionization of the 8-OH substituent (pKa of 5.7) in the oxidized FO, stabilized by cross-conjugation throughout the tricyclic deazaflavin, suppresses FO reactivity toward redox chemistry. However, in the reduced FO (FOH2), the phenolic group is electronically isolated to a single benzoic ring, resulting in a higher pKa of 9.7.6 The present experiments were conducted with the redox-active moiety of cofactor F420, the deazaflavin, FO, as a facsimile for the F420 cofactor, which is difficult to isolate in sufficient quantity for biophysical studies.8 Moreover, synthetic FO is fully catalytically active with Fno and has a higher purity than what can be produced from native sources. A detailed description of the improved synthesis of FO has been communicated elsewhere.8 As shown in Figure 2, the crystal structure of Fno from A. fulgidus was solved in 2001.1 The structure revealed that the enzyme is homodimeric with an α,β twisted fold. The structure was solved with one F420 cofactor molecule and one NADP+ molecule bound per monomer. These prosthetic groups are within 3.1 Å of one another, which is an acceptable donor− acceptor distance for a hydride transfer.1,6,9−12 Previous steadyReceived: July 8, 2015 Revised: October 30, 2015

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Figure 1. Fno-catalyzed reaction. Fno catalyzes the reversible reduction of NADPH.1,4,5,8

concluded that LuxG displays half-site reactivity, and it was suggested that negative cooperativity could be involved.17 A second, more complex example was reported for mercuric reductase, a flavoprotein that catalyzes the reduction of Hg(SR)2 by NADPH in a variety of bacterial strains.27−30 Biphasic kinetics, along with redox titration experiments, suggested half-site reactivity within this enzyme.29 However, the data could not distinctly confirm negative cooperativity because three types of abnormal behavior had been observed.29 Miller et al. observed a sigmoidal curve, with binding or activity as a function of substrate concentration that was indicative of positive cooperativity. The negative derivations observed from hyperbolic plots were indicative of a sequential negative cooperative interaction between subunits.29 Miller et al. reported models for asymmetric dimers and negative cooperativity.29 In this article, we report the first example of an F420 cofactordependent enzyme that displays both half-site reactivity and negative cooperativity. Additionally, we report, for the first time, the dissociation constants of FO and NADPH from the ternary enzyme−cofactor complex. Overall, the results obtained provide valuable new insights into the kinetics and mechanism of Fno reactions. We believe the information obtained will prove to be of significant interest in helping to elucidate the reactivity details of the numerous other F420-dependent enzymes.

Figure 2. Crystal structure of Fno. (A) Homodimeric overall fold of Fno in the presence of the F420 cofactor. (B) Active site of Fno (PDB file 1jax).1

state kinetic analysis confirmed the existence of the ternary complex.1−5,7,13,14 Steady-state data indicated Km values of 20 and 40 μM for F420H2 and NADP+, respectively, at 65 °C and pH 5.5.1 While the order of substrate addition could not be deduced from steady-state kinetics, additional experiments were performed that suggest that the presence of NADP+ aids F420 binding.1 Here, we report the first pre-steady-state kinetic analysis of Fno hydride transfer activity. Our data suggest that this enzyme exhibits kinetic behavior consistent with negative cooperativity and half-site reactivity.15−17 Several enzymes display either negative cooperativity or half-site reactivity.17−27 Flavin reductases, related to bioluminescence (such as LuxG), have been studied by stopped-flow spectrophotometry.17 The data suggest that LuxG follows a sequential ordered mechanism in which NADH was the first cofactor to bind, followed by FMN. Initially, the LuxG·NADH·FMN complex forms, resulting in hydride transfer. The reactions with NADH in LuxG are biphasic. The two phases were both due to reduction of FMN, and each accounted for ∼50% of the total absorbance change. The fractions in the two phases remained constant over a wide range of FMN concentrations. From this information, it was

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MATERIALS AND METHODS

Reagents. FO was synthesized as reported elsewhere.8 NADPH was purchased from Akron Biotech. The plasmid pET24b used for Fno gene insertion was purchased from Novagen. LB broth was purchased from US Biologicals. Isopropyl β-D-1-thiogalactopyranoside (IPTG) was purchased from Gold Biotechnology. Potassium phosphate monobasic buffer was purchased from BDH. Potassium phosphate dibasic buffer was purchased from Amresco. MES buffer was purchased from Acros Organics. E. coli C41(DE3) cells were purchased from Lucigen. Kanamycin, Tris buffer, and ammonium sulfate were purchased from Fisher Scientific. Expression and Purification. The Fno gene from A. fulgidus was synthesized in vitro as described in ref 2 and cloned into pET24b plasmid by Genescript (see Supporting B

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normalized data point was divided by Fmax to obtain the fractional saturation point. Kd is the dissociation constant, and n is the Hill coefficient. Ligand concentration was corrected for dilution during addition to the assay. Steady-State Kinetics. The steady-state kinetic measurements were carried out using a Hitech Scientific DX2 stoppedflow spectrophotometer at 22 °C. To determine the FO kinetic parameters, Fno (0.2 μM; 50 mM MES/NaOH, pH 6.5) was mixed with varying FO concentrations (1.3−30 μM) while the NADPH concentration was held constant at 600 μM in 50 mM MES/NaOH, pH 6.5. To determine the NADPH kinetic parameters, Fno (0.2 μM; 50 mM MES/NaOH, pH 6.5) was mixed with a constant concentration FO (25 μM) while varying the NADPH concentration (∼2 μM to 1 mM in 50 mM MES/NaOH, pH 6.5). The time trace measurements were collected in diodearray mode between 350 and 800 nm following FO reduction at 420 nm. The concentrations of NADPH and FO were determined using extinction coefficients of 6.22 mM−1 cm−1 at 340 nm (in 50 mM Tris-HCl, pH 7.4) and 41.4 mM−1 cm−1 at 420 nm (in 50 mM potassium phosphate buffer, pH 7.0), respectively.3 The initial rate of the reaction was calculated using an extinction coefficient of 34.7 mM−1 cm−1 for FO at pH 6.5 at 420 nm. The initial rate was converted to the macroscopic rate constant by dividing by the concentration of enzyme. A plot of substrate concentration vs rate constant was used to determine the steady-state kinetic parameters. The kinetic data for NADPH were fitted to the double reciprocal plot using eq 2.

Information, Figure S1). The plasmid was transformed into C41(DE3) E. coli according to Agilent Technologies protocol. The method reported by Warkentin et al.1 was modified for the purification of Fno. Briefly, the frozen E. coli C41(DE3) cell stock containing the Fno gene in pET24b plasmid was inoculated in a 10 L LB culture in the presence of 50 μg/mL of kanamycin until an OD600 of 0.6 was reached. The culture was induced with 1 mM IPTG at 37 °C overnight followed by harvesting. The cell pellet was lysed by sonication at 4 °C after resuspending the cells in 1.5 M potassium phosphate buffer, pH 8.0. The supernatant after centrifugation (16 000 g) was heated in a water bath at 90 °C for 30 min and then centrifuged (16 000 g). Ammonium sulfate was added to 40%, and the mixture was centrifuged at 16 000 g for 30 min. Ammonium sulfate (70%) was added to the supernatant, and the mixture was centrifuged at 16 000 g for 30 min. The ammonium sulfate precipitate was resuspended in 50 mM Tris-HCl, pH 7.5, and treated with 0.05% polyethylenimine. The supernatant was then dialyzed in 50 mM Tris-HCl buffer, pH 7.5, overnight and loaded onto a DEAE-Cellulose from Sigma (2.5 × 6 cm), which was equilibrated with 50 mM Tris-HCl (pH 7.5). The active fractions were combined and concentrated by filtration (30 kDa cutoff) (Millipore) to about 1 mL. The 1 mL sample was loaded onto a 1 × 40 cm size-exclusion column (Sephacryl S200 HR from GE Healthcare). The column was previously equilibrated with two column volumes of 50 mM MES/NaOH (pH 6.5). The purified protein was stored at −80 °C in 50 mM MES/NaOH (pH 6.5) buffer in the presence of 20% glycerol (see Supporting Information for SDS-PAGE gel, Figure S2). All column chromatography was operated through a Bio-Rad FPLC system (see Supporting Information, Figure S3). The concentration of the pure protein was determined using Bradford reagent at 595 nm. A detailed report of the purification protocol will be reported elsewhere.31 Binding of FO and NADPH to Fno. FO binding studies were performed using fluorescence spectroscopy (Horiba FluoroMax spectrofluorometer) at 22 °C. To obtain the dissociation constant (Kd) of FO, 0.2 μM Fno was titrated with varying concentrations of FO in 50 mM MES/NaOH at pH 6.5. The binding assay was monitored in a Spectrosil Quartz submicro cell (160 μL nominal volume) from Starna Cells. The sample was excited at 290 nm, and the emission scan was performed between 300 and 800 nm. To obtain the Kd of NADPH, 0.2 μM Fno was titrated with varying concentrations of NADPH in 50 mM MES/NaOH at pH 6.5. The binding assay was monitored in a similar fashion as described above for the FO binding studies. The excitation and emission slit widths were 4 and 8 nm, respectively. Either 1 or 2 μL aliquots of FO (0−300 nM) or NADPH (0−1780 nM) were titrated into a solution containing Fno. A decrease in tryptophan emission at 340 nm was observed for both FO and NADPH titration and was used for calculation of the dissociation constants.33 A plot of substrate concentration vs normalized fluorescence was used to determine the dissociation constants. The Hill coefficient and the dissociation constant for FO and NADPH binding were determined by fitting the plots to a sigmoidal function (eq 1) using Sigma Plot, version 13.0. ΔF = [Fmax[L]n /(Kd + [L]n ]

1/k = (K m/kcat)(1/[S] + (1/kcat))

(2)

where k is the first-order macroscopic rate constant (or turnover number) obtained by dividing the initial rate by the enzyme concentration, Km is the Michaelis constant, and [S] is concentration of NADPH. The individual rate constants and Km for the two phases were obtained from linear fits of the two phases. The data was also fitted using nonlinear regression using eq 3, and the two phases were also fitted individually using the Michaelis−Menten eq 4. k = kcat1[S]/(K m1 + [S]) + kcat2[S]/(K m2 + [S])

(3)

k = kcat[S]/(K m + [S])

(4)

Pre-Steady-State Experiment. The rapid kinetic experiments were performed in the Hitech Scientific DX2 stoppedflow spectrophotometer at 22 °C. Fno (1.0, 1.5, and 2.0 μM; 50 mM MES/NaOH, pH 6.5) was mixed with 10 μM FO, forming the Fno−FO complex. NADPH (25 μM) in 50 mM MES/ NaOH, pH 6.5, was then mixed with the Fno−FO complex. The experiment was repeated using 10 μM NADPH and Fno, forming the Fno−NADPH complex. FO (25 μM) was mixed against the Fno−NADPH complex. Time trace measurements were made in either single wavelength log mode at 420 nm or diode-array mode between 350 and 800 nm following FO reduction at 420 nm. The data were plotted to determine the pre-steady-state parameters. SigmaPlot, version 13.0, was used to fit the data to the exponential decay function using eq 5.

(1)

absorbance = A 0*e(−k * t ) − (v*t ) + c

where ΔF is the change in fluorescence emission at 340 nm caused upon addition of either FO or NADPH ligand (L). Fmax is the maximum normalized fluorescence (Fmax = 1). Then, each

(5)

where A0 is the amplitude of the burst phase, k is the observed burst rate constant, v is the observed steady-state rate, and c C

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Figure 3. Binding of FO and NADPH to Fno was carried out in 50 mM MES/NaOH (pH 6.50) buffer at room temperature in the fluorometer. FO or NADPH was titrated into 0.2 μM Fno, and the fluorescence emission was monitored at 340 nm after excitation at 290 nm. Actual fluorescence values were inverted and normalized before plotting. (A) Emission spectra of a wtFno solution containing 0, 2.46, 4.90, 7.32, 9.71, 12.1, 14.4, 19.2, 24.0, 33.4, 57.0, 104, 151, 197, or 313 nM FO (upper to lower spectra respectively) when excited at 290 nm. (B) Plot of FO concentration vs normalized fluorescence upon FO addition. The solid circles represent fluorescence at various concentration of FO, and the solid line represents the fit to eq 1. (C) Emission spectra of wtFno solution containing 0, 1.24, 2.48, 3.70, 4.90, 6.10, 7.28, 8.45, 10.8, 11.9, 14.2, 16.5, 21.1, 30.4, 53.5, 99.5, 191, 420, 874, or 1780 nM NADPH (upper to lower spectra, respectively) when excited at 290 nm. (D) Plot of NADPH concentration vs change in fluorescence upon NADPH addition. The solid circles represent the difference in fluorescence at various concentrations of NADPH, and the solid line represents the fit to eq 1.

accounts for the nonzero baseline. wtFno (20 μM), FO (4 μM), and NADPH (600 μM) were used for single-turnover kinetic studies. The exponential decay progress curve was fit with eq 6 using SigmaPlot, version 13.0. absorbance = A 0*e(−k * t ) + c

To confirm that the binding events observed were real, we conducted a control binding experiment under the abovementioned conditions using tryptophan rather than Fno. Little change in fluorescence was observed for tryptophan addition in comparison to that with Fno. No clear binding events with FO could be observed for the control experiments (Supporting Information, Figure S4). Binding of tryptophan with NADPH for the control experiment did not display a good fit (see Supporting Information, Figures S5 and S6). Steady-State Kinetics of wtFno. The steady-state reduction of FO, as catalyzed by Fno, yields plots of k versus substrate concentration that display typical saturation kinetics (Figure 4).8 The assay was conducted at pH 6.5 to shift the absorbance peak of FO from 400 to 420 nm in order to avoid interference from NADPH. Above pH 6.5, the reaction slows considerably. The kinetic parameters Km and kcat were determined by fitting the plot of varying FO concentration (μM) versus rate constant, k (s−1), to the Michaelis−Menten hyperbolic equation The Km and kcat determined from the plot were 4.00 ± 0.39 μM and 5.3 ± 0.1 s−1, respectively.8 The steady-state kinetic parameters for NADPH were determined under the same conditions as those described for FO by varying NADPH at a constant FO concentration of 25 μM (final concentration). When NADPH was varied, a plot of

(6)

where A0 is the amplitude, k is the observed rate constant, t is time in seconds, and c accounts for the nonzero baseline.

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RESULTS Binding Assay of wtFno. Upon addition of FO or NADPH, the tryptophan fluorescence of Fno was quenched, indicating a change in the tryptophan environment (Figure 3A,C, respectively). The strong change in fluorescence intensity, as shown in Figure 3, with added FO or NADPH provides an excellent method of measuring both dissociation constants (Figure 3B,D).32 From these fluorescence data, Kd for FO binding was determined to be 3.6 ± 0.7 nM. The binding data for NADPH were fit using the Hill equation (eq 1), as shown in Figure 3. The plot gave a Hill coefficient of 0.6 and a Kd of 2.0 ± 0.3 nM. The data was also fit to the Adair equation (not shown). This fit gave a similar Kd as the Hill equation. However, the fit was better (better R value, with less error) with the Hill equation. D

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affect the magnitude of Km. However, the Kd are affected only by kon and koff. We determined the Kd of NADP+ and used this number to determine the Kd of oxidized FO in the presence of NADP+ (see Supporting Information, Table S1). The dissociation constants for NADP+ (see Supporting Information, Table S2) and FO in the presence of NADP+ were all in the low nanomolar range. The difference between Km and Kd could be due to the motion of Fno during catalysis, affecting kcat and therefore Km. This could have an effect on the Km of both cofactors. However, no catalysis occurs with the binding experiments, which limits the motion of Fno. Pre-Steady-State Kinetics of wtFno. A solution containing wtFno−FO was mixed with equal volumes of solutions containing NADPH in the stopped-flow spectrophotometer. The reduction of the FO peak was monitored by absorbance changes at 420 nm. Spectra collected at different time intervals within the wavelength range of 350−800 nm did not show any formation of new peaks during the reaction, suggesting the absence of any observable intermediate peaks for the reduction of FO by NADPH. However, the reaction progress curve exhibited initial burst kinetics, followed by a slow phase rate, as shown in Figure 5A. These data were then fit using eq 5. The

Figure 4. (A) Steady-state kinetics plot of varying NADPH concentrations versus k. The reaction is carried out in 50 mM MES/NaOH (pH 6.5) buffer at 22 °C in a stopped-flow spectrophotometer. A constant FO concentration of 25 μM was used. Left inset is phase 1 (0−100 μM) of the experiment, and the right inset is phase 2 (150−800 μM), both of which are fitted to the Michaelis−Menten equation. (B) Double reciprocal plot of steadystate kinetics of varying NADPH concentrations is plotted using eq 2. The kinetic parameters are shown in Table 1. The plot shows a concave downward curve typical of negative cooperativity.

NADPH concentration vs k (s−1) did not display the typical hyperbola at NADPH concentrations greater than 100 μM (Figure 4A). The plot displayed non-Michaelis−Menten behavior with increasing concentrations of NADPH. The two phases included a steeper phase at low concentrations of NADPH (Figure 4A insets). The steady-state parameters were determined using eq 2, as shown in Figure 4B using a double reciprocal plot. The macroscopic rate constants for the first and second phases are 4.16 ± 0.07 and 5.41 ± 0.04 s−1, respectively. The rate constant for the second phase matches with the kcat obtained for steady-state kinetics with varying FO. The values for Km1 and Km2 were 2.33 ± 0.19 and 61.6 ± 5.9 μM, respectively. The Km is vastly different from the determined Kd. The dissociation constant determination was conducted with just Fno and FO or Fno and NADPH, but it was not determined in the presence of both cofactors simultaneously. The Km is determined in the presence of both FO and NADPH with the catalytic term kcat. If Fno undergoes conformational changes during turnover, then this could affect the enzyme’s affinity for the cofactors (kon and koff) as well as kcat. These three terms

Figure 5. Pre-steady-state kinetics. (A) Pre-steady-state kinetics of hydride transfer to FO from NADPH by Fno is shown for varying Fno concentrations: 1.0 μM (solid circle), 1.5 μM (open circle), and 2.0 μM (solid triangle). The solid lines represent the fit using eq 5. The reaction was carried out in 50 mM MES/NaOH (pH 6.5) buffer at 22 °C. Fno was mixed with 10 μM FO, forming the Fno−FO complex. NADPH (25 μM) in 50 mM MES/NaOH, pH 6.5, was then mixed with the Fno−FO complex. (B) Plot of the ratio of FO reduced per Fno subunit with varying FO concentration showing half-site reactivity.

burst kinetics suggests that the rate-limiting step is after the hydride transfer step. The rate-limiting step is either product release or a conformational change. The hydride transfer reflected by the fast burst phase is 87.1 ± 2.4 s−1 and the slow phase rate constant of 1.99 ± 0.02 s−1. The magnitude of the burst was directly proportional to the concentration of wtFno. The magnitude of the burst for 1.0, E

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Biochemistry Scheme 1. Steady-State, Pre-Steady-State, and Binding Data Revealed the Allosteric Nature of Fnoa

a

On the basis of this model, the hydride transfer reaction from NADPH to FO occurs at one subunit. Once the product(s) is released from one subunit, the hydride transfer reaction occurs at the second subunit. Followed by the release of the products from the second subunit, the first subunit is re-engaged for the reaction.

1.5, and 2.0 μM wtFno corresponds to 0.46, 0.46, and 0.53 μM FO reduced by NADPH, respectively. This averages out to be 46−54% of product formation (hydride transfer) during the burst phase. This suggests that it is the half-site reactivity model given in Scheme 1 for wtFno. To make sure that the half-site reactivity is not an artifact, we verified the purity of the enzyme. The wtFno sample is of high purity, as indicated by SDS-PAGE (Supporting Information, Figure S2) and gel filtration chromatography (Supporting Information, Figure S3).8 Further investigation for half-site reactivity was performed as discussed below by studying the effect of burst amplitude on increasing FO concentration. A plot of FO concentration versus burst amplitude was observed to be ∼50% FO reduction (Figure 5b). This supports half-site reactivity in Fno, as indicated earlier in this section. The kobs under single-turnover conditions (Figure 6) was determined to be 47.9 ± 0.5 s−1 using eq 6. This value is different than the observed burst rate constant because kburst is a more complex expression than kobs because it includes multiple steps such as hydride transfer, conformational changes, and product release, which are reflected in the magnitude of kburst. Preincubation of wtFno with NADPH under single-turnover conditions also gave a similar kobs of 53.8 ± 0.6 s−1.

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DISCUSSION Negative cooperativity is a phenomenon that has been observed in a variety of protein systems.20,21,34−40 For multisubunit proteins in particular (such as Fno), the specific phenomenon in which the affinity is higher for the first ligand than for the second ligand is known as negative cooperativity.14,15 Negative cooperativity can also be described as a decrease in ligand affinity with increasing ligand concentration. In our study, the steady-state kinetic plot for NADPH exhibits a nonhyperbolic curve at concentrations greater than 100 μM (Figure 4A). The first phase (NADPH concentrations between 0 and 100 μM (Figure 4A, left inset)) was determined to be steeper with a lower Km compared to that of the second phase (NADPH concentrations between 100 and 800 μM; (Figure 4A, right inset) (Table 1). The double reciprocal plot with NADPH showed downward curvature at high substrate concentrations, suggesting negative cooperativity (Figure 4B).

Figure 6. Single-turnover kinetics. Kinetic trace of FO reduction under single-turnover conditions. The solid line represents the curve fit using the single-exponential decay equation (eq 6). The upper (A) and lower (B) panels represent single-turnover experiments without and with preincubation of NADPH with wtFno, respectively.

Table 1. Steady-State Kinetic Parameters for Fno-Catalyzed Hydride Transfer from NADPH to FO at 22 °C and pH 6.5 Based on a Decrease of FO Absorbance at 420 nm substrate

Km (μM)

k1 (s−1)a

kcat (s−1)

FO NADPH phase 1 NADPH phase 2

4.00 ± 0.39 2.33 ± 0.19 61.6 ± 5.9

NA 4.16 ± 0.07 NA

5.27 ± 0.14 NA 5.41 ± 0.04

k1 is the rate constant for the fit of phase 1 with varying NADPH concentrations. a

F

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Scheme 2. Active Site Connectivity in the Fno Homodimera

This indication of negative cooperativity is explained in detail by the Koshland−Nemethy−Filmer (KNF) model.34 On the basis of this model, the protein is assumed to exist in one conformation in the absence of ligand. Once a ligand binds to the protein, it causes a conformational change that is transmitted to the other subunit (Scheme 1). Such observations of negative cooperativity in enzyme kinetics were made in one of the initial studies reported by Levitzki and Koshland for cytidine triphosphate (CTP) synthetase.14,16 Out of several molecular events suggested by them, negative homotropic kinetics for Fno could be due to ligand-induced conformational changes that affect subunit interactions, as suggested by our studies. The crystal structure of Fno reveals that the binding of NADP shows a considerable number of interactions within the Fno active site as compared to that with the deazaflavin cofactor. Once a NADPH molecule is bound to Fno and the hydride is transferred to FO, it would be easy for FOH2 to be released from the Fno active site. However, the release of NADPH could cause considerable conformation changes that are transmitted to the active site of the other subunit (also accounting for differences between the Km and Kd). This could also account for the strong negative cooperativity observed in the kinetic behavior of NADPH with Fno. This mechanism of action would also suggest the possibility of half-site reactivity. In fact, our pre-steady-state kinetics data through half-site reactivity indicate that only one subunit of Fno is catalytically active at a time. Such half-site reactivity is referred to as an extreme case of negative cooperativity.34 There are few flavindependent oxidoreductases that are reported to exhibit half-site reactivity. Similar half-site reactivity is observed in the bacterial 7-acetylflavo-GADPH, wherein about 42% of the enzymebound flavin cofactor is reduced in a very rapid step followed by a slow step.16 A recent finding for another oxidoreductase, flavin oxidoreductase LuxG, also shows half-site reactivity.17 Our findings for Fno are the first reports in F420H2:NADP+ oxidoreductases and suggest that Fno is a regulatory enzyme within archaea that is essential for controlling NADPH concentrations and related cellular function. The reaction of Fno is an equilibrium reaction. Metabolites within the cell can change the direction of the equilibrium, depending upon the needs of the cell for NADPH. On the basis of the steady-state and pre-steady-state kinetics presented here (half-site reactivity), we propose a mechanism for Fno (Scheme 1). Initially, FO and NADPH bind the active site of both subunits of the homodimeric Fno. This causes a conformational change where the symmetric subunits yield an intermediate asymmetric dimer, such as described by Koshland32 and Miller.29 As a result of the conformational change, the second subunit becomes more difficult to reduce (negative cooperativity). The hydride transfer occurs in one subunit (half-site reactivity), which is the first (and fast) phase described in Figure 5A. Given that the pre-steady-state kinetic results reveal burst kinetics, it is plausible that the slow step could be product release, protonation of the nitrogen at position 1, or conformational changes within Fno. To get an in depth understanding of the slow step, additional experiments need to be conducted to rule out some possibilities. To understand the possible mechanism of negative cooperativity and half-site reactivity, we took a closer look at the Fno crystal structure, which revealed active site connectivity, as shown in Scheme 2. His133 and Thr192 are located close to the C5 atom of the F420 cofactor where the

a

Proximities and H-bonds are shown in Fno crystal structure from PDB file 1JAY. Both active sites are connected through a series of Hbonds. As shown for subunit A, similar interactions are observed in subunit B. At the interface of the subunit interactions are Ser190, Arg186, and three water molecules (1274, 254, and 1254). Thr192 forms a H-bond with Ser190, which is at the subunit interface. Ser190 also forms a H-bond with another subunit interface residue, Arg186. Both Arg186 and Ser190 on either subunit form H-bonds.

hydride transfer occurs, but they do not appear to form any Hbonds with any oxygen or ring nitrogen. The main chain nitrogen of Thr192 forms a H-bond with the main chain oxygen of Ser190, which is at the subunit−subunit interface. The side chain oxygen of Ser190 forms a H-bond with another residue at the interface, Arg186. The side chain amino groups of Arg186 of both the subunits could form H-bonds with each other, whereas the side chain oxygens of Ser190 on both subunits are connected through two water molecules at the interface. We propose that these interactions at the subunit− subunit interface provide the molecular basis for intersubunit communication during catalysis and play a regulatory role in the negative cooperativity observed in Fno kinetic studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biochem.5b00762. wtFno sequence; SDS-PAGE gel of the Fno purification protocol; S-200 chromatogram; FO control binding experiment in the presence of tryptophan; NADPH control binding experiment in the presence of tryptophan; emission change at 340 nm of NADPH control binding experiment with tryptophan; steady-state kinetics of reduction of FO by Fno at varying NADPH concentrations; binding data of Fno at varying concentrations of FO in the presence of 200 nM NADP+; and binding data of Fno in the presence of varying concentrations of NADP+ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 817-272-3802. Fax: 817272-3808. Funding

This work was supported by the National Science Foundation, grant numbers 1120837 (to K.J.-W.) and 1303803 (to F.W.F.). G

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Biochemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We also thank Drs. John H. Enemark and Graham R. Moran for helpful discussions. ABBREVIATIONS Fno, F420H2:NADP+ oxidoreductase; NADP+, nicotinamide adenine dinucleotide phosphate; E. coli, Escherichia coli; IPTG, isopropyl β-D-1-thiogalactopyranoside; FO, F420 cofactor precursor; LB, Luria−Bertani broth; Kd, dissociation constant; kcat, Michaelis−Menten catalytic rate constant (turnover number)

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DOI: 10.1021/acs.biochem.5b00762 Biochemistry XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.biochem.5b00762 Biochemistry XXXX, XXX, XXX−XXX