Substrate modulates compound I formation in peroxide shunt pathway of Pseudomonas putida cytochrome P450(cam)

Novel Unspecific Peroxygenase from Truncatella angustata Catalyzes the Synthesis of Bioactive Lipid Mediators

Lipid mediators, such as epoxidized or hydroxylated eicosanoids (EETs, HETEs) of arachidonic acid (AA), are important signaling molecules and play diverse roles at different physiological and pathophysiological levels. The EETs and HETEs formed by the cytochrome P450 enzymes are still not fully explored, but show interesting anti-inflammatory properties, which make them attractive as potential therapeutic target or even as therapeutic agents. Conventional methods of chemical synthesis require several steps and complex separation techniques and lead only to low yields. Using the newly discovered unspecific peroxygenase TanUPO from the ascomycetous fungus Truncatella angustata, 90% regioselective conversion of AA to 14,15-EET could be achieved. Selective conversion of AA to 18-HETE, 19-HETE as well as to 11,12-EET and 14,15-EET was also demonstrated with known peroxygenases, i.e., AaeUPO, CraUPO, MroUPO, MweUPO and CglUPO. The metabolites were confirmed by HPLC-ELSD, MS1 and MS2 spectrometry as well as by comparing their analytical data with authentic standards. Protein structure simulations of TanUPO provided insights into its substrate access channel and give an explanation for the selective oxyfunctionalization of AA. The present study expands the scope of UPOs as they can now be used for selective syntheses of AA metabolites that serve as reference material for diagnostics, for structure-function elucidation as well as for therapeutic and pharmacological purposes.

Kinetic mechanism for the initial steps in MauG-dependent tryptophan tryptophylquinone biosynthesis

The diheme enzyme MauG catalyzes the biosynthesis of tryptophan tryptophylquinone (TTQ), the protein-derived cofactor of methylamine dehydrogenase (MADH). This process requires the six-electron oxidation of a 119 kDa MADH precursor protein with incompletely synthesized TTQ (PreMADH). The kinetic mechanism of the initial two-electron oxidation of this natural substrate by MauG was characterized. The relative reactivity of free MauG toward H(2)O(2) and the O(2) analogue CO was essentially the same as that of MauG in the preformed enzyme-substrate complex. The addition of H(2)O(2) to diferric MauG generated a diheme bis-Fe(IV) species [i.e., Fe(IV)=O/Fe(IV)] which formed at a rate of >300 s(-1) and spontaneously returned to the diferric state at a rate of 2 x 10(-4) s(-1) in the absence of substrate. The reaction of bis-Fe(IV) MauG with PreMADH exhibited saturation behavior with a limiting first-order rate constant of 0.8 s(-1) and a K(d) of < or = 1.5 microM for the MauG-PreMADH complex. The results were the same whether bis-Fe(IV) MauG was mixed with PreMADH or H(2)O(2) was added to the preformed enzyme-substrate complex to generate bis-Fe(IV) MauG followed by reaction with PreMADH. Stopped-flow kinetic studies of the reaction of diferrous MauG with CO yielded a faster major transition with a bimolecular rate constant of 5.4 x 10(5) M(-1) s(-1), and slower transition with a rate of 16 s(-1) which was independent of CO concentration. The same rates were obtained for binding of CO to diferrous MauG in complex with PreMADH. This demonstration of a random kinetic mechanism for the first two-electron oxidation reaction of MauG-dependent TTQ biosynthesis, in which the order of addition of oxidizing equivalent and substrate does not matter, is atypical of those of heme-dependent oxygenases that are not generally reactive toward oxygen in the absence of substrate. This kinetic mechanism is also distinct from that of the homologous diheme cytochrome c peroxidases that require a mixed valence state for activity.

Freeze-quenched iron-oxo intermediates in cytochromes P450

Since the discovery of cytochromes P450 and their assignment to heme proteins a reactive iron-oxo intermediate as the hydroxylating species has been discussed. It is believed that the electronic structure of this intermediate corresponds to an iron(IV)-porphyrin-pi-cation radical system (Compound I). To trap this intermediate the reaction of P450 with oxidants (shunt pathway) has been used. The common approaches are stopped-flow experiments with UV-visible spectroscopic detection or rapid-mixing/freeze-quench studies with EPR and Mössbauer spectroscopic characterization of the trapped intermediate. Surprisingly, the two approaches seem to give conflicting results. While the stopped-flow data indicate the formation of a porphyrin-pi-cation radical, no such species is seen by EPR spectroscopy, although the Mössbauer data indicate iron(IV) for P450cam (CYP101) and P450BMP (CYP102). Instead, radicals on tyrosine and tryptophan residues are observed. These findings are reviewed and discussed with respect to intramolecular electron transfer from aromatic amino acids to a presumably transiently formed porphyrin-pi-cation radical.

An artificial electron donor supported catalytic cycle of Pseudomonas putida cytochrome P450cam

Putidaredoxin (PdX), the physiological effector of cytochrome P450cam (P450cam), serves to gate electron transfer into oxy-P450cam during the catalytic cycle of the enzyme. Redox-linked structural changes in PdX are necessary for the effective P450cam turnover reaction. PdX is believed to be difficult to be replaced by an artificial electron donor in the reaction pathway of P450cam. We demonstrate that the catalytic cycle of wild-type P450cam can be supported in the presence of an artificial reductant, potassium ferrocyanide. Upon rapid mixing of ferrocyanide ion with P450cam, we observed an intermediate with spectral features characteristic of compound I. The rate constant for the formation of compound I in the presence of ferrocyanide supported reaction cycle was found to be comparable to the ones observed for H2O2 supported compound I formation in wild-type P450cam, but was much lower than those observed for classical peroxidases. The results presented in this paper form the first kinetic analysis of this intermediate for an artificial electron-driven P450cam catalytic pathway in solution.

Reaction of Ferric Cytochrome P450cam with Peracids

Reactions of substrate-free ferric cytochrome P450cam with peracids to generate Fe=O intermediates have previously been investigated with contradictory results. Using stopped-flow spectrophotometry, the reaction with m-chloroperoxybenzoic acid demonstrated an Fe(IV)=O + porphyrin pi-cation radical (Cpd I) (Egawa, T., Shimada, H., and Ishimura, Y. (1994) Biochem. Biophys. Res. Commun. 201, 1464-1469). By contrast, with peracetic acid, Fe(IV)=O plus a tyrosyl radical were observed by freeze-quench Mossbauer and EPR spectroscopy (Schunemann, V., Jung, C., Trautwein, A. X., Mandon, D., and Weiss, R. (2000) FEBS Lett. 479, 149-154). Our detailed kinetic studies have resolved these contradictory results. At pH >7, a significant fraction of Cpd I is formed transiently, whereas at low pH only a species with a Soret band at 406 nm, presumably Fe(IV)=O + tyrosyl radical, is observed. Evidence for formation of an acylperoxo complex en route to Cpd I was obtained. Because of rapid heme destruction, steps subsequent to formation of the highly oxidized forms could not be fully characterized. Heme destruction was avoided by including peroxidase substrates (e.g. guaiacol), which were oxidized to characteristic peroxidase products as the Fe(III)-P450 was regenerated. Addition of ascorbate to either of the high valent species also reforms the Fe(III) state with only a small loss of heme absorbance. These results indicate that typical peroxidase chemistry occurs with P450cam and offer an explanation for the contrasting results reported earlier. The delineation of improved conditions (pH, temperature, choice of peracid) for generating highly oxidized species with P450cam should be valuable for their further characterization.

Structural and functional characterization of “laboratory evolved” cytochrome P450cam mutants showing enhanced naphthalene oxygenation activity

To elucidate molecular mechanisms for the enhanced oxygenation activity in the three mutants of cytochrome P450cam screened by 'laboratory evolution' [Nature 399 (1999) 670], we purified the mutants and characterized their functional and structural properties. The electronic absorption and resonance Raman spectra revealed that the structures of heme binding site of all purified mutants were quite similar to that of the wild-type enzyme, although the fraction of the inactivated form, called "P420," was increased. In the reaction with H(2)O(2), only trace amounts of the naphthalene hydroxylation product were detected by gas chromatography. We, therefore, conclude that the three mutants do not exhibit significant changes in the structural and functional properties from those of wild-type P450cam except for the stability of the axial ligand in the reduced form. The enhanced fluorescence in the whole-cell assay would reflect enhancement in the oxygenation activity below the detectable limit of the gas chromatography and/or contributions of other reactions catalyzed by the heme iron.