Probing the heme iron coordination structure of alkaline chloroperoxidase

Catalysis and Electron Transfer in De Novo Designed Metalloproteins

One of the hallmark advances in our understanding of metalloprotein function is showcased in our ability to design new, non-native, catalytically active protein scaffolds. This review highlights progress and milestone achievements in the field of de novo metalloprotein design focused on reports from the past decade with special emphasis on de novo designs couched within common subfields of bioinorganic study: heme binding proteins, monometal- and dimetal-containing catalytic sites, and metal-containing electron transfer sites. Within each subfield, we highlight several of what we have identified as significant and important contributions to either our understanding of that subfield or de novo metalloprotein design as a discipline. These reports are placed in context both historically and scientifically. General suggestions for future directions that we feel will be important to advance our understanding or accelerate discovery are discussed.

Homodimerization Counteracts the Detrimental Effect of Nitrogenous Heme Ligands on the Enzymatic Activity of Acanthamoeba castellanii CYP51

Acanthamoeba castellanii is a free-living amoeba that can cause severe eye and brain infections in humans. At present, there is no uniformly effective treatment for any of these infections. However, sterol 14α-demethylases (CYP51s), heme-containing cytochrome P450 enzymes, are known to be validated drug targets in pathogenic fungi and protozoa. The catalytically active P450 form of CYP51 from A. castellanii (AcCYP51) is stabilized against conversion to the inactive P420 form by dimerization. In contrast, Naegleria fowleri CYP51 (NfCYP51) is monomeric in its active P450 and inactive P420 forms. For these two CYP51 enzymes, we have investigated the interplay between the enzyme activity and oligomerization state using steady-state and time-resolved UV-visible absorption spectroscopy. In both enzymes, the P450 → P420 transition is favored under reducing conditions. The transition is accelerated at higher pH, which excludes a protonated thiol as the proximal ligand in P420. Displacement of the proximal thiolate ligand is also promoted by adding exogenous nitrogenous ligands (N-ligands) such as imidazole, isavuconazole, and clotrimazole that bind at the opposite, distal heme side. In AcCYP51, the P450 → P420 transition is faster in the monomer than in the dimer, indicating that the dimeric assembly is critical for stabilizing thiolate coordination to the heme and thus for sustaining AcCYP51 activity. The spectroscopic experiments were complemented with size-exclusion chromatography and X-ray crystallography studies. Collectively, our results indicate that effective inactivation of the AcCYP51 function by azole drugs is due to synergistic interference with AcCYP51 dimerization and promoting irreversible displacement of the proximal heme-thiolate ligand.

The pH-Induced Selectivity Between Cysteine or Histidine Coordinated Heme in an Artificial α-Helical Metalloprotein

De Novo metalloprotein design assesses the relationship between metal active site architecture and catalytic reactivity. Herein, we use an α-helical scaffold to control the iron coordination geometry when a heme cofactor is allowed to bind to either histidine or cysteine ligands, within a single artificial protein. Consequently, we uncovered a reversible pH-induced switch of the heme axial ligation within this simplified scaffold. Characterization of the specific heme coordination modes was done by using UV/Vis and Electron Paramagnetic Resonance spectroscopies. The penta- or hexa-coordinate thiolate heme (9≤pH≤11) and the penta-coordinate imidazole heme (6≤pH≤8.5) reproduces well the heme ligation in chloroperoxidases or cyt P450 monooxygenases and peroxidases, respectively. The stability of heme coordination upon ferric/ferrous redox cycling is a crucial property of the construct. At basic pHs, the thiolate mini-heme protein can catalyze O₂ reduction when adsorbed onto a pyrolytic graphite electrode.

Oxidation of Flame Retardant Tetrabromobisphenol A by a Biocatalytic Nanofiber of Chloroperoxidase

Background: Tetrabromobisphenol (TBBPA), a flame retardant compound, is considered a ubiquitous pollutant, with potential impact on the environment and human health. Several technologies have been applied to accelerate its degradation and minimize environmental impacts. Due to its aromaticity character, peroxidase enzymes may be employed to carry out its transformation in mild conditions. Therefore, the purpose of this work was to determine the capacity of the enzyme chloroperoxidase (CPO) to oxidize TBBPA in several water samples. Methods: The oxidation capacity of CPO was evaluated in catalytic conditions using water samples from surface and groundwater, as well as effluents from wastewater treatment plants. The biocatalytic performance of CPO was improved due to its immobilization on nanofibers composed of polyvinyl alcohol and chitosan (PVA/chitosan). Results: Free and immobilized CPO were able to transform more than 80% in short reaction times (60 min); producing more biodegradable and less toxic products. Particularly, the immobilized enzyme was catalytically active in a wider range of pH than the free enzyme with the possibility of reusing it up to five times. Conclusions: The biocatalytic oxidation of TBBPA under environmental conditions is highly efficient, even in complex media such as treated effluents of wastewater treatment plants.

Transcriptional Regulation of Gene Expression by Metalloproteins

FNR and SoxR are transcriptional regulators containing an iron–sulfur cluster. The iron–sulfur cluster in FNR acts as an oxygen sensor by reacting with oxygen. The structural change of the iron–sulfur cluster takes place when FNR senses oxygen, which regulates the transcriptional regulator activity of FNR through the change of the quaternary structure. SoxR contains the [2Fe–2S] cluster that regulates the transcriptional activator activity of SoxR. Only the oxidized SoxR containing the [2Fe–2S]2+ cluster is active as the transcriptional activator. CooA is a transcriptional activator containing a protoheme that acts as a CO sensor. CO is a physiological effector of CooA and regulates the transcriptional activator activity of CooA. In this review, the biochemical and biophysical properties of FNR, SoxR, and CooA are described.

Self-repairing metal–organic hybrid complexes for reinforcing immobilized chloroperoxidase reusability

A self-repairing metal–chloroperoxidase (CPO) hybrid nanocatalyst with a sodium alginate (SA) coating displayed robust reusability under acidic conditions.

Chloroperoxidase-Catalyzed Epoxidation of Cis-β-Methylstyrene: NH-S Hydrogen Bonds and Proximal Helix Dipole Change the Catalytic Mechanism and Significantly Lower the Reaction Barrier

Proximal hydrogen bonding of the axial sulfur with the backbone amides (NH-S) is a conserved feature of heme-thiolate enzymes such as chloroperoxidase (CPO) and cytochrome P450 (P450). In CPO, the effect of NH-S bonds is amplified by the dipole moment of the proximal helix. Our gas-phase DFT studies show that the proximal pocket effect significantly enhances CPO's reactivity toward the epoxidation of olefinic substrates. Comparison of models with and without proximal pocket residues shows that with them, the barrier for Cβ-O bond formation is lowered by about ∼4.6 kcal/mol, while Cα-O-Cβ ring closure becomes barrierless. The dipole moment of the proximal helix was estimated to contribute 1/3 of the decrease, while the rest is attributed to the effect of NH-S bonds. The decrease of the reaction barrier correlates with increased electron density transfer to residues of the proximal pocket. The effect is most pronounced on the doublet spin surface and involves a change in the electron-transfer mechanism. A full enzyme QMMM study on the doublet spin surface gives about the same barrier as the gas-phase DFT study. The free-energy barrier was estimated to be in agreement with the experimental results for the CPO-catalyzed epoxidation of styrene.

A Compact Structure of Cytochrome c Trapped in a Lysine-Ligated State: Loop Refolding and Functional Implications of a Conformational Switch

It has been suggested that the alkaline form of cytochrome c (cyt c) regulates function of this protein as an electron carrier in oxidative phosphorylation and as a peroxidase that reacts with cardiolipin (CL) during apoptosis. In this form, Met80, the native ligand to the heme iron, is replaced by a Lys. While it has become clear that the structure of cyt c changes, the extent and sequence of conformational rearrangements associated with this ligand replacement remain a subject of debate. Herein we report a high-resolution crystal structure of a Lys73-ligated cyt c conformation that reveals intricate change in the heme environment upon this switch in the heme iron ligation. The structure is surprisingly compact, and the heme coordination loop refolds into a β-hairpin with a turn formed by the highly conserved residues Pro76 and Gly77. Repositioning of residue 78 modifies the intraprotein hydrogen-bonding network and, together with adjustments of residues 52 and 74, increases the volume of the heme pocket to allow for insertion of one of the CL acyl moieties next to Asn52. Derivatization of Cys78 with maleimide creates a solution mimic of the Lys-ligated cyt c that has enhanced peroxidase activity, adding support for a role of the Lys-ligated cyt c in the apoptotic mechanism. Experiments with the heme peptide microperoxidase-8 and engineered model proteins provide a thermodynamic rationale for the switch to Lys ligation upon perturbations in the protein scaffold.

Silica Sol-Gel Entrapment of the Enzyme Chloroperoxidase

The enzyme chloroperoxidase (CPO) was immobilized in silica sol-gel beads prepared from tetramethoxysilane. The average pore diameter of the silica host structure (~3 nm) was smaller than the globular CPO diameter (~6 nm) and the enzyme remained entrapped after sol-gel maturation. The catalytic performance of the entrapped enzyme was assessed via the pyrogallol peroxidation reaction. Sol-gel beads loaded with 4 μg CPO per mL sol solution reached 9–12% relative activity compared to free CPO in solution. Enzyme kinetic analysis revealed a decrease inkcatbut no changes inKMorKI. Product release or enzyme damage might thus limit catalytic performance. Yet circular dichroism and visible absorption spectra of transparent CPO sol-gel sheets did not indicate enzyme damage. Activity decline due to methanol exposure was shown to be reversible in solution. To improve catalytic performance the sol-gel protocol was modified. The incorporation of 5, 20, or 40% methyltrimethoxysilane resulted in more brittle sol-gel beads but the catalytic performance increased to 14% relative to free CPO in solution. The use of more acidic casting buffers (pH 4.5 or 5.5 instead of 6.5) resulted in a more porous silica host reaching up to 18% relative activity.

Redox-dependent stability, protonation, and reactivity of cysteine-bound heme proteins

Cysteine-bound hemes are key components of many enzymes and biological sensors. Protonation (deprotonation) of the Cys ligand often accompanies redox transformations of these centers. To characterize these phenomena, we have engineered a series of Thr78Cys/Lys79Gly/Met80X mutants of yeast cytochrome c (cyt c) in which Cys78 becomes one of the axial ligands to the heme. At neutral pH, the protonation state of the coordinated Cys differs for the ferric and ferrous heme species, with Cys binding as a thiolate and a thiol, respectively. Analysis of redox-dependent stability and alkaline transitions of these model proteins, as well as comparisons to Cys binding studies with the minimalist heme peptide microperoxidase-8, demonstrate that the protein scaffold and solvent interactions play important roles in stabilizing a particular Cys-heme coordination. The increased stability of ferric thiolate compared with ferrous thiol arises mainly from entropic factors. This robust cyt c model system provides access to all four forms of Cys-bound heme, including the ferric thiol. Protein motions control the rates of heme redox reactions, and these effects are amplified at low pH, where the proteins are less stable. Thermodynamic signatures and redox reactivity of the model Cys-bound hemes highlight the critical role of the protein scaffold and its dynamics in modulating redox-linked transitions between thiols and thiolates.

Identification of proximal and distal axial ligands in Leishmania major pseudoperoxidase

Previous optical and electron paramagnetic resonance (EPR) spectroscopic studies of the newly discovered peroxynitrite scavenging pseudoperoxidase from Leishmania major (LmPP) suggested that ferric LmPP contained a six-coordinate low-spin (6cLS) heme with a thiolate ligand, presumably a cysteine, bound to its heme iron. To identify the axial ligands of LmPP, we exploit a systematic mutational analysis of potential heme ligands. On the basis of UV-visible and EPR spectroscopy, we report that the substitution of the proximal His206 with alanine in LmPP alters the 6cLS to a five-coordinate high spin (5cHS) form at pH 4.0 that has a spectrum characteristic of a Cys-ligated 5cHS derivative. The electronic absorption and EPR analysis of all alanine-substituted Cys and Met single mutants establish that when Cys107 is replaced with alanine, a new species appears that has a spectrum characteristic of a histidine-ligated 5cHS derivative at pH 4.0. Together, these results suggest that His206 and Cys107 act as the proximal and distal axial ligands in ferric LmPP, respectively. However, the electronic properties of reduced wild-type LmPP are similar to those of known 5cHS His-ligated heme proteins at pH 8.8, indicating that the thiolate bond was broken upon reduction. Furthermore, the wild-type protein was only partially reduced at pH 4.0, but the E105L mutant was completely reduced to form a 5cHS ferrous heme. These results imply that the presence of an acidic residue near the distal site may prevent reduction of the heme iron at acidic pH.

Weak coordination of neutral S- and O-donor proximal ligands to a ferrous porphyrin nitrosyl. Characterization of 6-coordinate complexes at low T

The interaction of the S- and O-donor ligands tetrahydrothiophen (THT) and tetrahydrofuran (THF) with the ferrous nitrosyl complex Fe(TTP)(NO) (TTP(2-) is meso-tetra-p-tolyl-porphyrinatodianion) was studied at various temperatures both in solid state and solution using electronic and infrared absorption spectroscopy. Upon addition of these ligands to a cryostat containing sublimed layers of Fe(TTP)(NO), no complex formation was detected at room temperature. However, upon lowering the temperature, spectral changes were observed that are consistent with ligand binding in axial position trans to the NO (the proximal site) and formation of the six-coordinate adducts. Analogous behavior was observed in solution. In both media, the six-coordinate adducts are stable only at low temperature and dissociate to the 5-coordinate nitrosyl complexes upon warming. The NO stretching frequencies of the six-coordinate thioether and ether complexes were recorded and binding constants for the weak bonding of proximal THF and THT ligands were determined from the spectral changes. These parameters are compared with those obtained for the N-donor ligand pyrrolidine.

Modeling heme protein active sites with the his93gly cavity mutant of sperm whale myoglobin: complexes with nitrogen-, oxygen- and sulfur-donor proximal ligands

Recent investigations of the His93Gly (H93G) "cavity" mutant of myoglobin as a versatile scaffold for modeling heme states are described. The difference in accessibility of the two sides of the heme in H93G myoglobin makes it possible to generate mixed ligand adducts in the ferric state that are difficult to prepare with heme models in organic solvents. In addition, the protection provided to the heme by the protein environment allows for the preparation of stable oxyferrous and oxo-iron(IV) complexes at near-ambient temperatures with variable ligands trans to the normally reactive dioxygen and oxo substituents. The extensive range of possible complexes that can be generated using the H93G system is illustrated with examples involving imidazole, phenolate, benzoate, thiolate and thiol ligands bound to the proximal side of the heme iron.

The H93G Myoglobin Cavity Mutant as a Versatile Scaffold for Modeling Heme Iron Coordination Structures in Protein Active Sites and Their Characterization with Magnetic Circular Dichroism Spectroscopy

Preparation of heme model complexes is a challenging subject of long-standing interest for inorganic chemists. His93Gly sperm whale myoglobin (H93G Mb) has the proximal His replaced with the much smaller non-coordinating Gly. This leaves a cavity on the proximal side of the heme into which a wide variety of exogenous ligands can be delivered. The end result is a remarkably versatile scaffold for the preparation of model heme adducts to mimic the heme iron coordination structure of native heme proteins. In this review, we first summarize the quantitative evidence for differential ligand binding affinities of the proximal and distal pockets of the H93G Mb cavity mutant that facilitates the preparation of mixed-ligand derivatives. Then we review our use of magnetic circular dichroism and electronic absorption spectroscopy to characterize nitrogen-, oxygen-, and sulfur-donor-ligated H93G Mb adducts with an emphasis on species not easily prepared by other heme model system approaches and those that serve as spectroscopic models for native heme proteins.

Low temperature photo-oxidation of chloroperoxidase Compound II

Oxidation of the heme-thiolate enzyme chloroperoxidase (CPO) from Caldariomyces fumago with peroxynitrite (PN) gave the Compound II intermediate, which was photo-oxidized with 365 nm light to give a reactive oxidizing species. Cryo-solvents at pH ≈ 6 were employed, and reactions were conducted at temperatures as low as -50° C. The activity of CPO as evaluated by the chlorodimedone assay was unaltered by treatment with PN or by production of the oxidizing transient and subsequent reaction with styrene. EPR spectra at 77K gave the amount of ferric protein at each stage in the reaction sequence. The PN oxidation step gave a 6:1 mixture of Compound II and ferric CPO, the photolysis step gave an approximate 1:1 mixture of active oxidant and ferric CPO, and the final mixture after reaction with excess styrene contained ferric CPO in 80% yield. In single turnover reactions at -50°C, styrene was oxidized to styrene oxide in high yield. Kinetic studies of styrene oxidation at -50°C displayed saturation kinetics with an equilibrium constant for formation of the complex of K(bind)=3.8 x 10(4)M(-1) and an oxidation rate constant of k(ox)=0.30s(-1). UV-Visible spectra of mixtures formed in the photo-oxidation sequence at ca. -50° C did not contain the signature Q-band absorbance at 690 nm ascribed to CPO Compound I prepared by chemical oxidation of the enzyme, indicating that different species were formed in the chemical oxidation and the photo-oxidation sequence.

Low-frequency dynamics of Caldariomyces fumago chloroperoxidase probed by femtosecond coherence spectroscopy

Ultrafast laser spectroscopy techniques are used to measure the low-frequency vibrational coherence spectra and nitric oxide rebinding kinetics of Caldariomyces fumago chloroperoxidase (CPO). Comparisons of the CPO coherence spectra with those of other heme species are made to gauge the protein-specific nature of the low-frequency spectra. The coherence spectrum of native CPO is dominated by a mode that appears near 32-33 cm(-1) at all excitation wavelengths, with a phase that is consistent with a ground-state Raman-excited vibrational wavepacket. On the basis of a normal coordinate structural decomposition (NSD) analysis, we assign this feature to the thiolate-bound heme doming mode. Spectral resolution of the probe pulse ("detuned" detection) reveals a mode at 349 cm(-1), which has been previously assigned using Raman spectroscopy to the Fe-S stretching mode of native CPO. The ferrous species displays a larger degree of spectral inhomogeneity than the ferric species, as reflected by multiple shoulders in the optical absorption spectra. The inhomogeneities are revealed by changes in the coherence spectra at different excitation wavelengths. The appearance of a mode close to 220 cm(-1) in the coherence spectrum of reduced CPO excited at 440 nm suggests that a subpopulation of five coordinated histidine-ligated hemes is present in the ferrous state at a physiologically relevant pH. A significant increase in the amplitude of the coherence signal is observed for the resonance with the 440 nm subpopulation. Kinetics measurements reveal that nitric oxide binding to ferric and ferrous CPO can be described as a single-exponential process, with rebinding time constants of 29.4 +/- 1 and 9.3 +/- 1 ps, respectively. This is very similar to results previously reported for nitric oxide binding to horseradish peroxidase.

Chlorinations catalyzed by chloroperoxidase occur via diffusible intermediate(s) and the reaction components play multiple roles in the overall process

The chlorination mechanism of the fungal enzyme chloroperoxidase (CPO) has been debated for (1) active site chlorination and (2) diffusible species mediated chlorination. Based upon the conversion of approximately 35 different substrates belonging to different reactive groups, it was found that substrate dimensions and topography had no pronounced effect on rates of CPO chlorination reaction. Epoxidation of indene was dependent on its concentration where as chlorination was not. Also, effective conversion was seen in the chlorination mixture for substrates that could not be epoxidized or sulfoxidized. Some insoluble substrates and certain molecules that exceeded the active site dimensions were chlorinated at rates comparable to the rates required for CPO's more natural substrate, monochlorodimedone. By terminating the enzymatic reaction with an active site ligand (azide), the amount of diffusible species was correlated to CPO in the reaction mixture. The preferential utilization of a substrate, earlier attributed to the active site, is found to be due to the specificity afforded by the reaction environment. It was found that the reaction medium components of peroxide, chloride and hydronium ions affected the reaction rates through varying roles in the enzymatic and non-enzymatic process. Besides these experimental evidences, key mechanistic and kinetic arguments are presented to infer that the final chlorine transfer occurs outside the active site via a diffusible species.

Spectroscopic characterization of the ferric states of Amphitrite ornata dehaloperoxidase and Notomastus lobatus chloroperoxidase: His-ligated peroxidases with globin-like proximal and distal properties

Amphitrite ornata dehaloperoxidase (DHP) and Notomastus lobatus chloroperoxidase (NCPO) catalyze the peroxide-dependent dehalogenation of halophenols and halogenation of phenols, respectively. Both enzymes have histidine (His) as their proximal heme iron ligand. Crystallographic examination of DHP revealed that it has a globin fold [M.W. LaCount, E. Zhang, Y.-P. Chen, K. Han, M.M. Whitton, D.E. Lincoln, S.A. Woodin, L. Lebioda, J. Biol. Chem. 275 (2000) 18712-18716] and kinetics studies established that ferric DHP is the active state [R.L. Osborne, L.O. Taylor, K. Han, B. Ely, J.H. Dawson, Biochem. Biophys. Res. Commun. 324 (2004) 1194-1198]. NCPO likely has these same properties. Previous work with His-ligated heme proteins has revealed characteristic spectral distinctions between dioxygen binding globins and peroxide-activating peroxidases. Since DHP, and likely NCPO, is a peroxide-activating globin, we have sought to determine in the present investigation whether the ferric resting states of these two novel heme-containing enzymes are myoglobin-like or peroxidase-like. To do so, we have examined their exogenous ligand-free ferric states as well as their azide, imidazole and NO bound ferric adducts (and ferrous-NO complexes) with UV-Visible absorption and magnetic circular dichroism spectroscopy. We have also compared each derivative to the analogous states of horse heart myoglobin (Mb) and horseradish peroxidase (HRP). The spectra observed for parallel forms of DHP and NCPO are virtually identical to each other as well as to the spectra of the same Mb states, while being less similar to the spectra of corresponding HRP derivatives. From these data, we conclude that exogenous ligand-free ferric DHP and NCPO are six-coordinate with water and neutral His as ligands. This coordination structure is distinctly different from the ferric resting state of His-ligated peroxidases and indicates that DHP and NCPO do not activate bound peroxide through a mechanism dependent on a push effect imparted by a partially ionized proximal His as proposed for typical heme peroxidases.

Chloroperoxidase from Caldariomyces fumago is active in the presence of an ionic liquid as co-solvent

Chloroperoxidase from Caldariomyces fumago catalyses the oxidation of 1,2-dihydronaphthalene to (1R,2R)-(+)-dihydroxytetrahydronaphthalene in homogenous citrate buffer/ionic liquid mixtures, using t-butyl hydroperoxide as O2 donor. It tolerates up to 30 (v/v) 1,3-dimethylimidazolium methylsulfate or 1-butyl-3-methylimidazolium methylsulfate. The enzyme activity in these ionic liquid co-solvent systems is retained for 24 h, but it falls to 3 h using non-ionic organic solvents such as t-BuOH or acetone.

Neutral thiol as a proximal ligand to ferrous heme iron: implications for heme proteins that lose cysteine thiolate ligation on reduction

Cysteine plays a key role as a metal ligand in metalloproteins. In all well-recognized cases, however, it is the anionic cysteinate that coordinates. Several cysteinate-ligated heme proteins are known, but some fail to retain thiolate ligation in the ferrous state, possibly following protonation to form neutral cysteine. Ligation by cysteine thiol in ferrous heme proteins has not been documented. To establish spectroscopic signatures for such systems, we have prepared five-coordinate adducts of the ferrous myoglobin H94G cavity mutant with neutral thiol and thioether sulfur donors as well as six-coordinate derivatives such as with CO and, when possible, with NO and O(2). A thiol-ligated oxyferrous complex is reported, to our knowledge for the first time. Further, a bis-thioether ferrous H93G model for bis-methionine ligation, as found in Pseudomonas aeruginosa bacterioferritin heme protein, is described. Magnetic CD spectroscopy has been used due to its established ability in axial ligand identification. The magnetic CD spectra of the H93G complexes have been compared with those of ferrous H175CD235L cytochrome c peroxidase to show that its proximal ligand is neutral cysteine. We had previously reported this cytochrome c peroxidase mutant to be cysteinate-ligated in the ferric state, but the ferrous ligand was undetermined. The spectral properties of ferrous liver microsomal cytochrome P420 (inactive P450) are also consistent with thiol ligation. This study establishes that neutral cysteine can serve as a ligand in ferrous heme iron proteins, and that ferric cysteinate-ligated heme proteins that fail to retain such ligation on reduction may simply be ligated by neutral cysteine.

Utilization of peroxide and its relevance in oxygen insertion reactions catalyzed by chloroperoxidase

Chloroperoxidase (CPO) catalyzed oxygen insertions are highly enantioselective and hence of immense biotechnological potential. A peroxide activation step is required to give rise to the compound I species that catalyzes this chiral reaction. A side reaction, a catalase type peroxide dismutation, is another feature of CPO's versatility. This work systematically investigates the utilization of different peroxides for the two reactions, i.e. the catalase type reaction and the oxygen insertion reaction. For the oxygen insertion reaction, indene and phenylethyl sulfide were chosen as substrate models for epoxidation and sulfoxidation respectively. The results clearly show that CPO is stable towards hydrogen peroxide and has a total number of turnovers near one million prior to deactivation. The epoxidation reactions terminate before completion because the enzyme functioning in its catalatic mode quickly removes all of the hydrogen peroxide from the reaction mixture. Sulfoxidation reactions are much faster than epoxidation reactions and thus are better able to compete with the catalase reaction for hydrogen peroxide utilization. A preliminary study towards optimizing the reaction system components for a laboratory scale synthetic epoxidation is reported.

Reversible pressure deformation of a thermophilic cytochrome P450 enzyme (CYP119) and its active-site mutants

The pressure stability of the thermophilic CYP119 from Sulfolobus solfataricus and its active-site Thr213 and Thr214 mutants was investigated. At 20 degrees C and pH 6.5, the protein undergoes a reversible P450-to-P420 inactivation with a midpoint at 380 MPa and a reaction volume change of -28 mL/mol. The volume of activation of the process was -9.5 mL/mol. The inactivation transition was retarded, and the absolute reaction volume was decreased by increasing temperature or by mutations that decrease the size of the active-site cavity. High pressure affected the tryptophan fluorescence yield, which decreased by about 37% at 480 MPa. The effect was reversible and suggested considerable contraction of the protein. Aerobic decomposition of iron-aryl complexes of the CYP119 T213A mutant under increasing hydrostatic pressure resulted in variation of the N-arylprotoporphyrin-IX regioisomer (N(B):N(A):N(C):N(D)) adduct pattern from 39:47:07:07 at 0.1 MPa to 23:36:14:27 at 400 MPa. Preincubation of the protein at 400 MPa followed by complex formation and decomposition gave the same regioisomer distribution as untreated protein. The results indicate that the protein is reversibly inactivated by pressure, in contrast to the irreversible inactivation of P450(cam) and other P450 enzymes, and that this inactivation process is modulated by changes in the active-site cavity dimensions.

Proximal cysteine residue is essential for the enzymatic activities of cytochrome P450cam

To investigate the functional and structural roles of the proximal thiolate ligand in cytochrome P450cam, we prepared the C357H mutant of the enzyme in which the axial cysteine residue (Cys357) was replaced with a histidine residue. We obtained the unstable C357H mutant by developing a new preparation procedure involving in vitro folding of P450cam from the inclusion bodies. The C357H mutant in the ferrous-CO form exhibited the Soret peak at 420 nm and the Fe-CO stretching line at 498 cm-1, indicating a neutral histidine residue as the axial ligand. However, another internal ligand is coordinated to the heme iron as the sixth ligand in the ferric and ferrous forms of the C357H mutant, suggesting the collapse of the substrate-binding site. The C357H mutant showed no catalytic activity for camphor hydroxylation and the reduced heterolytic/homolytic ratio of the O-O bond scission in the reaction with cumene hydroperoxide. The present observations indicate that the thiolate coordination in P450cam is important for the construction of the heme pocket and the heterolysis of the O-O bond.

Nitration and inactivation of cytochrome P450BM-3 by peroxynitrite. Stopped-flow measurements prove ferryl intermediates

Peroxynitrite (PN) is likely to be generated in vivo from nitric oxide and superoxide. We have previously shown that prostacyclin synthase, a heme-thiolate enzyme essential for regulation of vascular tone, is nitrated and inactivated by submicromolar concentrations of PN [Zou, M.-H. & Ullrich, V. (1996) FEBS Lett. 382, 101-104] and we have studied the effect of heme proteins on the PN-mediated nitration of phenolic compounds in model systems [Mehl, M., Daiber, A. & Ullrich, V. (1999) Nitric Oxide: Biol. Chem. 2, 259-269]. In the present work we show that bolus additions of PN or PN-generating systems, such as SIN-1, can induce the nitration of P450BM-3 (wild-type and F87Y variant), for which we suggest an autocatalytic mechanism. HPLC and MS-analysis revealed that the wild-type protein is selectively nitrated at Y334, which was found at the entrance of a water channel connected to the active site iron center. In the F87Y variant, Y87, which is directly located at the active site, was nitrated in addition to Y334. According to Western blots stained with a nitrotyrosine antibody, this nitration started at 0.5 microM of PN and was half-maximal between 100 and 150 microM of PN. Furthermore, PN caused inactivation of the P450BM-3 monooxygenase as well as the reductase activity with an IC50 value of 2-3 microM. As two thiol residues/protein molecule were oxidized by PN and the inactivation was prevented by GSH or dithiothreitol, but not by uric acid (a powerful inhibitor of the nitration), our data strongly indicate that the inactivation is due to thiol oxidation at the reductase domain rather then to nitration of Y residues. Stopped-flow data presented here support our previous hypothesis that ferryl-species are involved as intermediates during the reactions of P450 enzymes with PN.

Replacement of the proximal heme thiolate ligand in chloroperoxidase with a histidine residue

Chloroperoxidase is a versatile heme enzyme which can cross over the catalytic boundaries of other oxidative hemoproteins and perform multiple functions. Chloroperoxidase, in addition to catalyzing classical peroxidative reactions, also acts as a P450 cytochrome and a potent catalase. The multiple functions of chloroperoxidase must be derived from its unique active site structure. Chloroperoxidase possesses a proximal cysteine thiolate heme iron ligand analogous to the P450 cytochromes; however, unlike the P450 enzymes, chloroperoxidase possesses a very polar environment distal to its heme prosthetic group and contains a glutamic acid residue in close proximity to the heme iron. The presence of a thiolate ligand in chloroperoxidase has long been thought to play an essential role in its chlorination and epoxidation activities; however, the research reported in this paper proves that hypothesis to be invalid. To explore the role of Cys-29, the amino acid residue supplying the thiolate ligand in chloroperoxidase, Cys-29 has been replaced with a histidine residue. Mutant clones of the chloroperoxidase genome have been expressed in a Caldariomyces fumago expression system by using gene replacement rather than gene insertion technology. C. fumago produces wild-type chloroperoxidase, thus requiring gene replacement of the wild type by the mutant gene. To the best of our knowledge, this is the first time that gene replacement has been reported for this type of fungus. The recombinant histidine mutants retain most of their chlorination, peroxidation, epoxidation, and catalase activities. These results downplay the importance of a thiolate ligand in chloroperoxidase and suggest that the distal environment of the heme active site plays the major role in maintaining the diverse activities of this enzyme.