Oxygen Activation Switch in the Copper Amine Oxidase of Escherichia coli.

Copper amine oxidases (CuAOs) are metalloenzymes that reduce molecular oxygen to hydrogen peroxide during catalytic turnover of primary amines. In addition to Cu2+ in the active site, two peripheral calcium sites, ∼32 Å from the active site, have roles in Escherichia coli amine oxidase (ECAO). The buried Ca2+ (Asp533, Leu534, Asp535, Asp678, and Ala679) is essential for full-length protein production, while the surface Ca2+ (Glu573, Tyr667, Asp670, and Glu672) modulates biogenesis of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor. The E573Q mutation at the surface site prevents calcium binding and TPQ biogenesis. However, TPQ biogenesis can be restored by a suppressor mutation (I342F) in the proposed oxygen delivery channel to the active site. While supporting TPQ biogenesis (∼60% WTECAO TPQ), I342F/E573Q has almost no amine oxidase activity (∼4.6% WTECAO activity). To understand how these long-range mutations have major effects on TPQ biogenesis and catalysis, we employed ultraviolet-visible spectroscopy, steady-state kinetics, inhibition assays, and X-ray crystallography. We show that the surface metal site controls the equilibrium (disproportionation) of the Cu2+-substrate reduced TPQ (TPQAMQ) Cu+-TPQ semiquinone (TPQSQ) couple. Removal of the calcium ion from this site by chelation or mutagenesis shifts the equilibrium to Cu2+-TPQAMQ or destabilizes Cu+-TPQSQ. Crystal structure analysis shows that TPQ biogenesis is stalled at deprotonation in the Cu2+-tyrosinate state. Our findings support WTECAO using the inner sphere electron transfer mechanism for oxygen reduction during catalysis, and while a Cu+-tyrosyl radical intermediate is not essential for TPQ biogenesis, it is required for efficient biogenesis.

Copper amine oxidases (CuAOs) have important roles in many organisms and have been implicated in a number of pathological conditions.[1−9] All CuAOs are homodimers that contain a strictly conserved tryrosine that, in the presence of Cu2+ and O2, undergoes autocatalytic posttranslational modification to generate the 2,4,5-trihydroxylphenylalanine quinone (TPQ) cofactor.[10−12] In addition to the active site Cu2+ binding site, some but not all CuAOs also contain one or two peripheral metal sites.[13] CuAO catalysis proceeds via a classical ping-pong mechanism with two half-reactions, reductive and oxidative (Scheme ). The reductive half-reaction (Scheme , steps 1–3) has been well characterized for a number of CuAOs. Cu2+-TPQOX (1) reacts with a primary amine to form a substrate Schiff base (2, TPQSSB). A conserved aspartate residue acts as a catalytic base (Asp383 in ECAO) to facilitate proton abstraction to form the product Schiff base (2 → 3, TPQPSB). Hydrolysis of TPQPSB (3) releases the aldehyde product, generating the aminoquinol (4, TPQAMQ).[14−18] Studies of the oxidative half-reaction are more challenging, but two mechanisms have been proposed, inner sphere and outer sphere electron transfer mechanisms.[19] It has been demonstrated that Cu2+-TPQAMQ (4) exists in equilibrium with Cu+-TPQSQ (semiquinone) (5).[20] The equilibrium (4 ↔ 5) for this interconversion (disproportionation reaction) depends on the enzyme source, pH, and temperature.[21,22] In the inner sphere electron transfer mechanism, O2 binds Cu+ in the Cu+-TPQSQ couple and is reduced to superoxide (O2•–),[23−26] consistent with the well-documented physiological redox active role for Cu2+/Cu+.[19,27,28] However, in the outer sphere electron transfer mechanism, O2 binds to a hydrophobic pocket proximate to TPQAMQ and is reduced by TPQAMQ to O2•–.[29,30] Although the first electron transfer to O2 is different in the two mechanisms, ultimately the Cu2+-hydroperoxide and iminoquinone (6, TPQIMQ) are generated. Hydrolysis of the TPQIMQ intermediate releases ammonium and H2O2 and regenerates state 1.

Scheme 1

Proposed Catalytic Mechanism of CuAO, a Reductive Half-Reaction (1 → 3) and an Oxidative Half-Reaction (inner sphere 5 → 6, outer sphere 4 → 6)

The highest turnover rates (kcat) are observed in plant CuAOs, which display considerable amounts of 5 (≈80%) in the oxidative half-reaction.[20,21,23,26] Non-plant eukaryotic CuAOs, such as Hansenula polymorpha amine oxidase (HPAO), exhibit the lowest kcat values and negligible amounts of 5.[29,30] Experiments performed by Shepard et al.[31] provided evidence of the redox role of copper in the form of 5 in the bacterial Arthrobacter globiformis amine oxidase (AGAO); however, this has not been established in Escherichia coli amine oxidase (ECAO), another bacterial CuAO.

Although the mode of O2 activation differs among CuAOs, it occurs in a deeply buried active site. The timely and specific delivery of O2 through the protein matrix to these buried active sites has been an active area of study.[32−37] Xenon complexes and in silico studies have suggested that molecular oxygen migration follows preferred routes and have identified a conserved β-sandwich as the likely major pathway for oxygen entry (Figure b, pathway 1).[32,38−40] Removing Ca2+ from the peripheral sites located at the surface of the protein (Figure a) near the opening of the β-sandwich results in a significant decrease in enzyme activity.[13] In the work presented here, we show that a single point mutation in ECAO at the surface site interferes with TPQ biogenesis, which can be restored by introducing a second mutation within a hydrophobic channel previously identified as a putative oxygen delivery pathway. We also demonstrate that the buried metal is required for full-length protein expression.

Figure 1

Overview of important structural features of ECAO (Protein Data Bank entry 1DYU). (a) The peripheral surface and buried metal binding sites (left) and the active site (right) are magnified. (b) ECAO monomer showing potential oxygen entry pathways (dashed lines) based on xenon binding sites and residues targeted for mutagenesis (blue sticks). TPQ, copper, and calcium ligands are shown as sticks colored by element. Copper and calcium ions are shown as bronze and magenta spheres, respectively. Water molecules are shown as red spheres, and xenons as green spheres.

Methods

Mutagenesis, Expression, and Purification of ECAO

Plasmid pKKecao[16] was used as the template for site-directed mutagenesis using a QuikChange protocol with KOD DNA polymerase[41] to generate the following single-mutation constructs where each plasmid identifies the mutation introduced: buried metal site, pKKD533N and pKKD678N; surface site, pKKE672K, pKKD670N, and pKKE573Q; and hydrophobic channel, pKKI342F. An additional site-directed mutagenesis step was performed using pKKE573Q as a template to generate the double mutation pKKE573Q/X (where X is a hydrophobic channel mutation V640F/L/A, I342F/W, V345L/A, or V536L or an active site mutation M699L/A/V or E490Q/I/A). Each variant was confirmed by DNA sequencing.

Wild-type ECAO (WTECAO) and mutational variant proteins were prepared from XL-1 blue cells transformed with the appropriate plasmid. Single colonies were used to inoculate 5 mL of 2YT medium with 100 μg/mL disodium carbenicillin and grown at 37 °C while being shaken at 220 rpm for 16 h. Large scale cultures of 500 mL of prewarmed 2YT medium with carbenicillin and 50 μM CuSO4 in 2 L baffled flasks were inoculated with 1 mL of the starter culture. Following incubation at 37 °C and 220 rpm, when the OD600 reached 0.6–0.8 isopropyl β-d-1-thiogalactopyranoside was added to a final concentration of 1 mM and incubation continued for 6 h. Cells were harvested and lysed, and ECAO was purified as previously described.[13] The gravimetrically derived correction factor of 0.76 was used to determine protein concentration based on its absorbance at 280 nm (ε280 = 2.1 × 105 M–1 cm–1).[42] The molecular masses of WTECAO and ECAO variants were determined by mass spectrometry in the Astbury Centre for Structural Molecular Biology of the University of Leeds.

Ultraviolet–Visible (UV–vis) Spectroscopy

A Shimadzu UV2401 PC spectrophotometer equipped with a temperature-controlled cell holder was used to record CuAO spectra. To detect TPQ, spectra were recorded over a range of 280–700 nm. Quantitative analysis of the TPQ content per dimer was achieved by titrating with 2-hydrazinopyridine (2-HP) as previously described.[13,43] Typically, 6–12 μM enzyme in 100 mM sodium phosphate (pH 7.0) was titrated by stepwise addition of 0.1–1 molar equivalent of 2-HP. At each titration step, the reaction was allowed to proceed until no further absorbance change was detectable.

Enzyme Activity Assays

Enzyme activity was determined as described previously.[13] Briefly, 20 μL of 0.2 μM purified enzyme was added to 980 μL of assay buffer [100 mM sodium phosphate (pH 7.0), 0.58 mM 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and 15 units mL–1 horseradish peroxidase (HRP)], and the reaction progress was recorded at 410 nm using a Cary 50 spectrophotometer (Varian). Kinetic parameters, KM(amine), and kcat for β-phenylethylamine (β-PEA) were calculated by fitting the initial rates (micromoles per minute per milligram) using Origin Pro 7.5.

To determine O2 kinetic parameters, nitrogen gas was used to displace O2 in buffers, solutions, and enzyme samples. N2 was passed over the headspace present in airtight glass vessels containing 2 mL of 0.2 μM protein. The assay buffer [100 mM sodium phosphate and 25 μM β-PEA (pH 7.0)] was degassed, and N2 was bubbled directly into the solution continuously while it was being stirred. A Hansatech Oxygraph system (Hansatech Instruments Ltd.) was assembled, and the electrode was calibrated according to the manufacturer’s instructions. Then, 980 μL of assay buffer was transferred to the Oxygraph glass reaction vessel from the N2-saturated buffer using an airtight Hamilton syringe and allowed to reach a baseline. Following this, 20 μL of O2-free enzyme was added and the rate of O2 consumption was recorded. Varying concentrations of O2 were obtained by mixing different ratios of oxygen-saturated and nitrogen-saturated assay buffer. The data were used to calculate the steady-state kinetic parameters, KM(oxygen), and kcat for O2 by fitting the data to the Michaelis–Menten equation using Origin Pro 7.5.

Anaerobic Reduction of ECAO

Solutions were made anaerobic by degassing and passing a steady stream of N2 into them while they were being stirred for 1 h. Protein solutions were not stirred, and anaerobiosis was achieved by passing N2 into the headspace, allowing gas exchange without bubbling directly into the solution. A 240 μL aliquot of protein with the active enzyme at a concentration of 100 μM (based on TPQ content) was added to a 1 mL quartz cuvette with a ground glass joint sealed with a rubber septum. The substrate-reduced enzyme was prepared by adding a 5-fold molar excess of anaerobic β-PEA. Spectra were recorded until no further changes were observed. Subsequently, O2 was allowed to enter the cuvette by diffusion, and spectra were recorded until no further change was observed. To determine the 5 content (Scheme ), a 16-fold molar excess of anaerobic KCN was added to anaerobically β-PEA-reduced enzyme and spectra were recorded until no further changes were observed.[44]

The level of 5 was calculated using eq

EDTA Treatment

All plasticware was soaked in 100 mM EDTA (pH 8.0) for 30 min and thoroughly rinsed in Chelex-treated water. Chelex-treated water and buffers were prepared by passing the solutions through a Chelex-100 (resin, mesh 100–200, Bio-Rad) column. EDTA (pH 7.0) was added to a final concentration 1000-fold higher than that of ECAO and incubated for 2 h. Excess EDTA was removed by three buffer exchange steps using a metal-free centrifugal filter device. Specific activity assays were performed at 25 °C in assay buffers supplemented with 3 mM CaCl2 and 0.05 mM CuSO4.

X-ray Crystallographic Structure Analysis

Crystals of WTECAO and its variants were grown as previously described.[16] In brief, the crystals were grown by the sitting drop vapor diffusion method in 100 mM HEPES (pH 6.9–7.2) and sodium citrate (1.10–1.35 M). A 3–5 μL aliquot of a protein sample (6–12 mg/mL) was mixed with the same volume of mother liquor. Prior to data collection, the drop solution was gradually exchanged with the well solution containing 5% (v/v) increments of glycerol to a final concentration of 30–45% (v/v) glycerol and subsequently flash-frozen in liquid nitrogen for X-ray data collection. Data were collected at Diamond Light Source (beamline I02). As observed with previous ECAO crystals, the proteins crystallized in space group P212121. The data were processed using XDS[45] and SCALA.[46] The CCP4 suite[47] was used for further processing, model building, and refinement. 1DYU,[16] an isomorphous WTECAO structure, was used as a starting model. Refinement and model building were performed using Refmac5[48] and COOT,[49] respectively. Rigid body refinement was performed to optimize the orientation of the starting model. COOT was used to build the missing side chains and add the metals and water molecules where appropriate. Successive rounds of positional refinement and rebuilding generated the final model. Details of crystal parameter, data collection, and refinement statistics are listed in Table S1.

Results

Mutations at the Buried Metal Site

Two residues coordinating the buried Ca2+ were mutated to asparagine (D553N and D678N). No full-length ECAO was detected in periplasmic extracts even when the growth conditions were supplemented with increasing concentrations of Ca2+ and no ECAO enzyme activity could be measured (Figures S1 and S2). These results were unexpected but suggest that Ca2+ in the buried binding site is required for the effective folding and/or stability of ECAO.

Mutations at the Surface Metal Site

In contrast to the buried metal site, which shows a preference for binding a divalent cation in all CuAOs, with the exception of H. polymorpha CuAO (HPAO),[6,41,50−56] the surface metal site is promiscuous and binds whatever mono- or divalent cation is available.[41] Site-directed mutagenesis of residues (E573, D670, and E672) comprising the surface Ca2+ binding site surprisingly resulted in significantly reduced CuAO activity, even though the expression levels of these mutants were similar to that of WTECAO and the site is some 32 Å from the active site (Figure ). The reduction in activity followed the trend D670N < E672K ≪ E573Q, with mutation E573Q showing the least activity (Figure ).

Figure 2

Activity profile of WTECAO and surface metal site mutants (D670N, E672K, and E573Q) where n = 3.

Effects of Surface Mutation E573Q on TPQ Biogenesis and Catalysis

The E573Q mutant lacked the distinct coloration afforded by the chromophore properties of TPQ. Although ESI-MS suggested the absence of TPQ, low levels were detected by UV–vis spectroscopy (Figure ) and confirmed by 2-HP titrations [∼16% of the WTECAO TPQ content (Figure , inset)]. Kinetic studies demonstrated a KM(amine) of 1.1 μM and a KM(O of 16.6 μM, consistent with that of the wild type; however, ≥88-fold reductions in both kcat(amine) and kcat(O were observed (Table ). Kinetic studies of E573Q demonstrated the observed activity was not due to contamination by wild-type ECAO expression in the host cells, as Y466F, carrying a mutation of the active site tyrosine that is incapable of undergoing processing to form TPQ, showed no activity (Figure ).[17]

Figure 3

UV–vis spectra of WTECAO, I342F, I342F/E573Q, and E573Q at 10 mg/mL measured in 20 mM Tris (pH 7.0). Inset 1 shows the TPQ content in each protein sample measured by titrating TPQ with 2-HP.

Table 1

Kinetic Parameters for the Reaction with β-PEA and O2, Measured in 100 mM Sodium Phosphate (pH 7.0)

		WTECAO	I342F	E573Q/I342F	E573Q
β-PEA	kcat (s–1)	14.6 ± 0.5	16.5 ± 0.6	0.67 ± 0.004	0.15 ± 0.0007
	KM (μM)	1.3 ± 0.2	1.4 ± 0.2	1.4 ± 0.03	1.1 ± 0.21
	kcat/KM (μM–1 s–1)	11 ± 1.8	12 ± 1.7	0.48 ± 0.021	0.14 ± 0.026
	relative activity (%)	100	113	4.6	1
O2	kcat (s–1)	43.2 ± 1	59.6 ± 5.4	1.67 ± 0.09	0.49 ± 0.02
	KM (μM)	25.2 ± 3.8	24.1 ± 2.7	20.7 ± 2.2	16.6 ± 2
	kcat/KM (μM–1 s–1)	1.7 ± 0.26	2.8 ± 0.37	0.81 ± 0.1	0.3 ± 0.04
	relative activity (%)	100	138	3.9	1.1

Rescue of E573Q by Suppressor Mutations

Attempts to reconstitute activity in the E573Q mutant by saturating with oxygen, incubating at 37 °C (24 h), adding Ca2+ (1 mM), prolonging the incubation time (>1 year at 4 °C), or combinations of these had no effect on TPQ formation or enzyme activity. We therefore attempted to rescue the E573Q phenotype by introducing second site mutations into the active site (M699L/A/V and E490Q/I/A) (Figures S3 and S4) and in the hydrophobic channel proposed to be the route of oxygen entry (V640F/L/A, I342F/W/D, V345L/A/F, V536L/A/F, and Y468A) (Figure S5).[41] The only mutant that showed any level of rescue of the E573Q phenotype was the I342F mutant. Unfortunately, it did not prove to be possible to produce protein from the I342W or I342D variant.

Effects of a Suppressor Mutation, I342F, on TPQ Biogenesis and Catalysis in E573Q

The I342F mutation resulted in partial suppression of the effects of the E573Q mutation. Unlike E573Q, the double mutant, I342F/E573Q, was pink in color, indicating the presence of substantial TPQ. The UV–vis spectroscopy data (TPQ content and 2HP titrations) showed that the I342F mutation partially suppressed the effects of the E573Q mutation, by increasing the TPQ content to ∼60% of the wild-type (WT) TPQ content (Figure ). Interestingly, however, the kinetic data showed that the turnover rate of I342F/E573Q with respect to both substrates was only ∼4% of that of the WT (Table ). The measured KM(amine) (1.4 μM) and KM(O (20.7 μM) were consistent with those of WTECAO (Table ). The I342F single mutant was similar to the wild type with a KM(amine) of 1.4 μM, a KM(O of 24.1 μM, a kcat(amine) of 16.5 s–1, and a kcat(O of 59.6 s–1 (Table ). The UV–vis spectrum of I342F showed increased absorbance in the 350 nm region relative to that of the WT and a shift from 480 to 470 nm; together, these changes suggest that there are microenvironmental effects that perturb the electronic structure of the TPQ (Figure ).

Detecting Differences in the Active Sites of I342F, WT, I342F/E573Q, and E573Q

2HP titrations determined 1.5 TPQs per dimer in WTECAO, 1.6 TPQs per dimer in I342F, 1 TPQ per dimer in I342F/E573Q, and 0.2 TPQ per dimer in E573Q (Figure , inset); 1.5 TPQ molecules instead of 2 were detected in WTECAO most probably because of the inefficient processing of the highly conserved tyrosine residue to form TPQ.[43,57] It is possible that the shoulder at 350 nm is a result of inefficient processing of the precursor Y466 such that TPQ biogenesis is stalled at the Cu(II)-tyrosinate intermediate that has a λmax of 350 nm.[58] Upon reaction of E573Q and I342F/E573Q with molar equivalents of 2HP, the azo form of adduct I and adduct II[70] were immediately observed as the UV–vis spectra show two peaks at 416 and 520 nm (Figure ).[59] A full conversion from the azo form to adduct II was observed with prolonged incubation at room temperature in I342F/E573Q.[59] By contrast, for WTECAO and I342F, only the hydrazone form of adduct I was detected. No spectral change was observed for Y466F as expected (Figure ).

Figure 4

Effects of reacting 10 mg/mL WTECAO, I342F, I342F/E573Q, or E573Q with a 10-fold molar excess of 2-HP in 100 mM sodium phosphate (pH 7.0) at 25 °C.

A shift from the hydrazone to azo form of adduct I was observed only when WT–2HP and I342F–2HP complexes were heated (Figure ). At 50 °C, the λmax shifts from 432 to 416 nm with a further increase in absorbance (Figure ). A significant increase in adduct II was also observed at 50 °C (Figure ). The increase in absorbance associated with the conversion of hydrazone adduct I (432 nm) to azo adduct I (416 nm) in WT and I342F suggests the molar extinction coefficient for the azo form is higher than that of the hydrazone form, consistent with previous reports (Figure ).[59]

Figure 5

Effect of temperature on the hydrazone form of adduct I in (a) WTECAO and (b) I342F.

The formation of adduct II has been associated with active site mutations that alter the hydrogen-bonding network within the active site, resulting in greater flexibility of the TPQ–2HP complex. An increase in temperature may also induce large movements that enable the TPQ–2HP complex to coordinate the active site Cu2+.[59] In this case, the data provide an indication that mutations E573Q and E573Q/I342F may influence the active site to promote formation of adduct II in a proportion of the molecules at low temperatures (Figure ), but further characterization would be required to confirm this.

Effects of the Long-Range Mutations on the Catalytic Intermediates

The levels of 5 (Scheme ), in the absence of oxygen, were measured in the presence and absence of CN– at 25 °C. Upon reaction of WT with β-PEA under anaerobic conditions, features that are characteristic of 5,[21,26,60] i.e., three peaks at 350, 438, and 467 nm, were observed (Figure ). The apparent level of 5 obtained using eq is 34.3% as reported in ref (61). In the absence of CN–, the levels of 5 (∼46.7 μM) were similar to those detected in the presence of CN– (∼50 μM), suggesting that 5 is the major intermediate in the ECAO catalytic pathway.

Figure 6

Anaerobic reduction of WTECAO with β-PEA at 25 °C in the absence (TPQred) and presence of cyanide (TPQred+KCN): (a) UV–vis spectra and (b) difference spectra.

Removal of Ca2+ from the surface metal site resulted in a 37-fold decrease in the specific activity of the protein, consistent with previous work[13] (Table ). Incubating the EDTA-treated WT enzyme with 3 mM CaCl2 (1000-fold excess) resulted in an increase in the specific activity to half that of the WT (Table ). Analysis of metal content by Smith et al.[13] showed that WTECAO contained 1.05 ± 0.01 coppers per subunit and that following incubation with EDTA it retained 0.95 ± 0.05 copper per subunit, demonstrating that the deeply buried active site copper is tightly bound and not accessible to EDTA, which suggests that a lack of recovery of enzyme activity is not due to the loss of the active site copper.[13]

Table 2

Specific Activities (micromoles per minute per milligram) of WTECAO with and without EDTA Treatment or Ca2+ and Cu2+ Supplementation

WTECAO	8.6 ± 0.4
EDTA-treated ECAO	0.23 ± 0.04
EDTA-treated ECAO with 3 mM CaCl2	4.4 ± 0.32
EDTA-treated ECAO with 3 mM CaCl2 and 0.05 μM CuSO4	1.7 ± 0.4
WTECAO with 3 mM CaCl2 and 0.05 μM CuSO4	4.6 ± 0.7

EDTA-treated WT, EDTA-treated WT with 3 mM CaCl2, and WT were reacted with β-PEA under anaerobic conditions. At equal concentrations of 1 (Figure b), a significant difference in the levels of 5 was observed in these samples (Figure ). The difference spectra were obtained by subtracting the initial state of the enzyme (1) (Figure c). On the basis of the ε468 for TPQSQ (4500 M–1 cm–1) reported for the A. globiformis enzyme,[25] 1.3 and 2.7 μM 6 were detected in EDTA-treated WT with 3 mM CaCl2 and WT, respectively. The levels of 5 formed in these samples correlate with the observed specific activities. EDTA-treated WT with 3 mM CaCl2 displayed 50% of the WT level of 5 and exhibited ∼50% of WT activity (Figure ). The amount of 5 in EDTA-treated WT was significantly smaller.

Figure 7

Effect of EDTA and Ca2+ treatment on the Cu+-TPQSQ and Cu2+-TPQAMQ equilibrium in WTECAO at 25 °C: (a) TPQ content, (b) Cu+-TPQSQ content, and (c) difference spectra of Cu+-TPQSQ.

To further investigate the differences in the turnover rate and active site environments, the reductive half-reaction for each enzyme was monitored under anaerobic conditions. Before reaction, the UV–vis spectra of WT, I342F, and E573Q/I342F displayed a λmax at 480 nm confirming the presence of TPQox (1) (Figure ). Characteristic features of 5 were observed for WT and I342F (Figure ). The spectra for both proteins displayed peaks at 438 and 467 nm and a small shoulder at 357 nm, features that have been previously assigned to 5.[21,26] Difference spectra for WT and I342F obtained by subtracting the initial state of TPQ from the reduced state showed the presence of both 4 (310 nm) and 5 (310, 438, and 467 nm) as previously observed for PSAO and AGAO.[21,26,60]

Figure 8

Spectroscopic reactivity of anaerobically substrate-reduced (a, c, and e) WTECAO and (b, d, and f) I342F at 25 °C. (a and b) Reductive half-reactions, (c and d) difference spectra for the reductive half-reactions, and (e and f) oxidative half-reactions. Spectral changes were recorded every 0.1 s; the red spectrum was measured at 0.3 s, the purple spectrum at 4.1 s, the green spectrum at 6.7 s, and the blue spectrum at 14.3 s.

At TPQ concentrations similar to that of WT, the difference spectrum of I342F/E573Q on reduction with β-PEA under N2 saturating conditions showed only the characteristic features of 4, a peak, at 310 nm (Figure ). Similar treatment of E573Q also resulted in only 4, the intermediate observed in substrate-reduced HPAO, BSAO, Co2+-AGAO, and Ni2+-AGAO (Figure ).[22,30,62,63] Although different catalytic species were observed, TPQ was successfully regenerated in all four proteins: WT, I342F, I342F/E573Q, and E573Q (Figures and 9).

Figure 9

Spectroscopic reactivity of anaerobically substrate-reduced (a, c, and e) I342F/E573Q and (b, d, and f) E573Q at 25 °C. (a and b) Reductive half-reactions, (c and d) difference spectra for the reductive half-reactions, and (e and f) oxidative half-reactions. Spectral changes were recorded every 0.1 s; the red spectrum was measured at 3.3 s, the purple spectrum at 9.3 s, the green spectrum at 10.7 s, and the blue spectrum at 15.8 s.

Crystal Structure Analysis

To understand the effects of the mutations and in particular those of I342F in the background of mutation E573Q, crystal structures of E573Q/I342F [Protein Data Bank (PDB) entry 6GRR] and E573Q (PDB entry 6EZZ) were determined by X-ray crystallography and refined to 1.7 and 1.8 Å, respectively. The data collection and processing statistics are shown in Table S1. The mutant structures showed a high level of structural similarity to WTECAO, with the most significant differences occurring at the active site and mutation sites.

No TPQ was observed in the structure of either mutant; rather, the 2Fo – Fc maps showed well-defined electron density corresponding to a tyrosine residue directly coordinated to the copper as observed in the early stages of TPQ biogenesis (Figure ).[11] There is some extra electron density at the C2 and O4 positions of Y466 in both structures (Figure a,b), and this is more evident in I342F/E573Q in the Fo – Fc map (green mesh) in Figure b. This was expected for E573Q as the biochemical and mass spectrometry studies did not detect significant levels of TPQ. However, this was not the case with I342F/E573Q where 60% of the WTECAO TPQ content was observed. This may indicate substantial flexibility or preferential crystallization of the precursor form. Collecting data at room temperature may provide more information with regard to TPQ flexibility.[64]

Figure 10

Electron density maps of the (a) E573Q (PDB entry 6EZZ) and (b) I342F/E573Q (PDB entry 6GRR) active sites. The 2Fo – Fc (gray mesh) and Fo – Fc (green mesh) maps are contoured at 1 and 3 rmsd, respectively.

In addition to the lack of the TPQ, subtle changes were observed in the active sites of both I342F/E573Q and E573Q. Comparison between these two variants and the 1DYU (off-copper)[16] WTECAO active site (Figure a,b) showed that H526, D383, and M699 each displayed positional flexibility. In particular, C3 of M699 is rotated ∼45° toward the C3 position of Y466 (Figure a,b), where oxygen is proposed to attack the aromatic ring during the first step of TPQ biogenesis to yield dopaquinone.[11] Superimposing the E573Q and I342F/E573Q structures onto the 1OAC (on-copper) WTECAO[55] structure showed that for both variants M699 still displays a different conformation and D383 adopts a conformation similar to that observed when TPQ is present in its “on-copper” conformation[55] (Figure c,d).

Figure 11

Superposition of the E573Q (yellow) and I342F/E573Q (magenta) active sites onto (a and b) WTECAO, where TPQ is in the “off-copper” conformation (PDB entry 1DYU) (cyan), and (c and d) WTECAO, where TPQ is in the “on-copper” conformation (PDB entry 1OAC) (green).

Exterior to the active site, while the 1DYU WTECAO structure shows differences in the surface metal (calcium ion) site for the two crystallographically independent subunits of ECAO, when residue 573 is mutated from glutamate to glutamine the ECAO loses its ability to bind a metal ion at this site (Figure ). In subunit A of both E573Q and I342F/E573Q, the calcium ion is replaced by a water molecule; however, in subunit B, no water molecule is observed (Figure a,b). The lack of a surface metal site in E573Q and I342F/E573Q results in a significant increase in flexibility as indicated by the B factor values (Figure d) and poorly defined side-chain electron density in loops 569–574 and 667–673. In contrast to E573Q, mutation I342F has no obvious effects on the overall structure of the enzyme. F342 has fewer rotational degrees of freedom than isoleucine does and fits perfectly in the hydrophobic channel partially stacking with W472 (Figure ).

Figure 12

Structures of peripheral surface metal sites of (a) E573Q (yellow), (b) I342F/E573Q (magenta), and (c) WTECAO (PDB entry 1DYU) (cyan). (d) Structures of subunit A with B factors for individual atoms represented by a ribbon structure in which the ribbon thickness indicates the magnitude of the B factor; thin indicates low B factors, and thick indicates high B factors. Bound calcium ions are represented as green spheres, whereas in panel d, the surface and buried calcium sites are represented by the top and bottom green spheres, respectively.

Figure 13

Structure of the putative oxygen entry channel. (a) The 2Fo – Fc map is contoured at 1 rmsd showing the electron density of residues in the hydrophobic channel as a gray mesh: E573Q (yellow) and I342F/E573Q (magenta). (b) View down the channel highlighting residues mutated in ref (41) and in this study: WTECAO (green) and I342F/E573Q (magenta).

Discussion

The E573Q mutation, some 32 Å from the active site, has a major long-range conformational effect on the formation of TPQ and catalytic turnover. Residue 573 is located near the opening of the hydrophobic channel identified as the putative O2 delivery pathway. Although low levels of TPQ were detected for this variant (16% WT TPQ), attempts to drive TPQ biogenesis under O2 saturating conditions failed, demonstrating the inability of the mutant to activate O2 for TPQ formation. A comparison of the structure of WTECAO with E573Q shows only minor changes at the active site and surface mutation site. The lack of efficient TPQ processing in E573Q may be due to (a) a change in the geometry and/or bond lengths in the copper primary coordination sphere, (b) an inability to form the ligand to metal charge transfer (LMCT) complex required for O2 activation, and/or (c) the inability of O2 to enter the active site or adopt a productive position due to alterations in ECAO dynamics associated with the preferred O2 entry channel.

The first two suggestions can be dismissed as the E573Q copper tetrahedral geometry and the distances between copper and tyrosine are identical to those reported for other CuAO precursor states,[65,66] and the distance between copper and histidine is similar to that of WTECAO (Figure S6).[16] However, fine-tuning an optimized structure of the copper coordination sphere(s) is known to be important for both cofactor biogenesis and catalysis.[13,66,67] Potentially subtle changes, beyond the detection limit of crystallography, in the positioning of copper, the three histidines, and key side chains[60] may have conceivably stalled biogenesis. Currently, during TPQ biogenesis in ECAO, there are no reported data to suggest the formation of an LMCT intermediate, a species successfully observed and characterized in HPAO.[58] Furthermore, attempts to detect a 350 nm Cu(II)–tyrosinate complex spectrophotometrically, even in the presence of a saturating level of O2, were unsuccessful, and as no such complex has been observed in AGAO,[11] it may not occur in any bacterial CuAO.

The proposed rate-limiting step in biogenesis is the formation of a peroxy intermediate that occurs as a result of direct electron transfer from the metal-coordinating tyrosinate complex to the bound O2.[68] It is possible that changes in active site flexibility or dynamic motions, demonstrated by the preferred formation of a copper-coordinated TPQ–2HP species (adduct II) in E573Q, inhibit the accessibility of O2 proximal to precursor Y466 where some effect of a steric/electronic clash, on O2 activation, could disfavor formation of a peroxy-Cu(II)-bound intermediate. TPQ biogenesis has been shown to be a dynamic process requiring rotation of the precursor tyrosine and surrounding side chains during the reaction.[11] In the crystal structures of both E573Q and I342F/E573Q, the hydrophobic channel remains open; however, because of the bulky nature of phenyalanine at position 342 the channel is narrower in I342F/E573Q (Figure b) than in E573Q, although O2 delivery is apparently more efficient in the former based on TPQ formation. Therefore, protein dynamics are likely to play a key role in the improved delivery of O2 to the active site for TPQ biogenesis.

To probe oxygen entry and/or delivery, multiple residues were mutated in the β-sandwich funnel (hydrophobic channel).[41] In the E573Q background, the addition of the I342F mutation led to a partial rescue of TPQ biogenesis, reaching 60% of that of WT. Associating the reactivation of TPQ biogenesis in the double mutant with an improvement in the delivery of O2 to the active site is tempting as I342 corresponds to a “hinge” region connecting two critical regions of the putative hydrophobic oxygen entry channel. However, there is no direct link between residues E573 and I342 and the active site residues required for TPQ biogenesis. Possible reasons for the recovery of TPQ biogenesis in I342F/E573Q are that the I342F mutation has physically levered open the hydrophobic channel by increasing the steric bulk at the hinge region to promote access of O2 to the active site or it counteracts the dynamic effects imposed by the E573Q mutation allowing activation of O2 by the LMCT complex. Exposing E573Q that had been held under N2 saturating conditions to air led to the reoxidation of the small amounts of TPQ present, indicating that access of O2 to the active site is not entirely blocked (Figure d).

While xenon complex crystal structures and in silico experiments identified a major putative O2 entry pathway, mutagenesis experiments designed to block individual pathways have had little effect on the overall kcat/KM, implying that O2 uses multiple dynamic routes to reach the active site.[69] One possibility is that two different O2 pathways are employed for TPQ biogenesis and the catalysis of O2 reduction.

There is no structural evidence of the effects afforded by the I342F mutation; however, inhibition studies using 2HP highlighted differences in the active sites of I342F/E573Q (adduct II) and WT (adduct I), suggesting increased active site flexibility in the double mutant (Figure ). Adduct II is formed when the TPQ–2HP complex rearranges to coordinate the active site Cu2+, suggesting increased flexibility in the active site.[59] The formation of adduct II has previously been observed with active site mutations Y381F and Y369F.[59,70] This implies that TPQ in I342F/E573Q preferentially adopts an “on-copper” conformation unlike that in WTECAO and I342F.

Although TPQ biogenesis is reactivated in E573Q/I342F, introducing the I342F mutation does not lead to a corresponding enhancement of the catalytic activity of E573Q. To understand the effect of E573Q on catalysis, the catalytic reaction was followed under anaerobic conditions. Reducing WTECAO, I342F, I342F/E573Q, and E573Q with β-PEA under anaerobic conditions revealed an interesting and distinct difference in catalytic intermediates formed over the course of the reaction.

In WTECAO and I342F, the characteristic features of 5 were observed (Figure ) and the difference spectra showed both 4 (310 nm) and 5 (λmax, 357, 438, and 468 nm) as observed for LSAO, PSAO, AGAO, and APAO, which are all CuAOs that use the inner sphere electron transfer mechanism to activate O2.[21,23,25,26,71,72] In such CuAOs, the intermediates exist in an equilibrium that favors 5 and exhibit fast turnover rates as the electron transfer rate between Cu+ and O2 is greater than or equal to the overall rate of turnover and thus is not the rate-limiting step.[26,31] The ability to form 34% of the Cu+-TPQSQ (5), as previously reported, suggests that ECAO uses the inner sphere electron transfer mechanism as reported for AGAO, another bacterial CuAO.[25,31]

For E573Q and I342F/E573Q, the difference spectra showed only the characteristic feature for 5 as observed with BSAO and HPAO, CuAOs that use the outer sphere electron transfer mechanism.[29,30,63] The ability of a second sphere residue to influence the equilibrium between 4 and 5 has been shown with HPAO.[22] Detecting only 4 in E573Q and I342F/E573Q suggests a shift in the equilibrium in favor of 5. The ∼100-fold decrease in activity of E573Q can be explained by a change from the inner to outer sphere electron transfer mechanism. If this is the case, the data presented here represent the first report in CuAOs of a residue distant from the active site affecting the properties of the redox active metalloenzyme.

In support of this, removal of Ca2+ by EDTA chelation reduced activity significantly as well as the levels of intermediate 5. Addition of Ca2+ to the same sample partially restored activity and increased 5 levels, confirming that the peripheral metal sites play a role in regulating the equilibrium that exists between 4 and 5. These data also support the hypothesis that ECAO uses the inner sphere electron transfer mechanism in the oxidative half-reaction and demonstrates an allosteric role for Ca2+ sites. Mammalian CuAOs, BSAO, VAP-1, and hDAO have been shown to have a surface metal binding site similar to that of ECAO. It is possible that in mammalian systems, control of Ca2+ availability may represent a method for regulating the CuAO activity and thereby controlling the production of H2O2 that can be important in cell signaling but that, together with product aldehydes, can in excess be detrimental to cells.

E573Q affects the enzyme’s ability to form 5 interfering with the redox potential of the Cu2+/Cu+ couple such that reduction to Cu+ is disfavored. This would suggest that the E573Q mutation results in a substantially weakened ability to form the Cu+-tyrosyl radical intermediate thus stalling TPQ biogenesis at the O2 activation stage (D in Scheme ). The E573Q crystal structure supports this hypothesis as Y466 interacts with the copper ion at a bond distance of 2.03 Å, resembling the structures observed in the early stages of TPQ biogenesis and suggesting that biogenesis is stalled after the deprotonation of Y466. The improvement in TPQ biogenesis associated with the I342F mutation may result from changes in the dynamic behavior of the enzyme. These changes may be a result of improved structural stability associated with the stacking interaction between W472 and F342 in I342F/E573Q. It is still reasonable to associate this improvement in TPQ biogenesis with improved delivery of O2 to the active site.

Scheme 2

Proposed Mechanism for TPQ Biogenesis in Copper Amine Oxidases[73]

Conclusion

Through a combination of biochemical and site-directed mutagenesis studies, we have demonstrated a likely regulatory role for the surface metal site in ECAO. Removing the surface site Ca2+ by chelation or mutagenesis (E573Q) affects TPQ biogenesis and catalytic efficiency in amine oxidation at the active site, some 32 Å away. This interferes with the Cu2+/Cu+ couple and supports the proposal of an inner sphere electron transfer mechanism in this enzyme. EPR spectroscopy studies will provide more insight into the effects of the E573Q mutation on the redox state of copper. The lack of obvious structural differences in the crystal structures of the mutants and WTECAO suggests a role for protein dynamics in transmitting the effects of mutations that are 32 Å (E573Q) and 22 Å (I342F) from the active site. Reactivation of TPQ biogenesis in I342F/E573Q could be associated with improved delivery of O2 to the active site. Further studies to understand the role of the surface site in ECAO dynamics and delivery of O2 to the active site remain an important goal.