Osami Shoji

Highly Selective Hydroxylation of Benzene to Phenol by Wild-type Cytochrome P450BM3 Assisted by Decoy Molecules†

Phenol is a key intermediate in industry for the synthesis of drugs, dyes, and functional polymers. Because phenol is currently produced by the cumene process, which involves high energy consumption and significant formation of side products such as acetone and methylstyrene, direct hydroxylation of benzene has attracted much attention as an alternative method of production. The direct hydroxylation of benzene to phenol using a variety of catalysts, electrochemical oxidation systems, and photochemical systems has thus been extensively investigated. In contrast to many oxidation reactions at high temperature, monooxygenases in nature, such as cytochrome P450, efficiently catalyze the oxidation of inert alkanes and aromatic compounds under mild conditions. Thus, various engineered enzymes have been constructed by site-directed mutagenesis, random mutagenesis, and chemical modification. Biocatalysts for exclusive hydroxylation of the benzene ring using natural enzymes, if they could be developed, would be ideal systems for the production of phenol. Herein we report an efficient and selective hydroxylation of benzene to phenol catalyzed by wild-type cytochrome P450BM3 (P450BM3) with the assistance of decoy molecules. We and Zilly et al. have recently developed a simple and unique system for the hydroxylation of gaseous alkanes, such as propane and butane, catalyzed by wild-type P450BM3. Wild-type P450BM3 exclusively catalyzes the hydroxylation of long-alkyl-chain fatty acids and never hydroxylates small alkanes, because the active site of P450BM3 is optimized for the hydroxylation of fatty acids (Figure 1a, upper) and the first step of the catalytic cycle of P450BM3 starts only when a fatty acid binds to the substrate binding site of P450BM3, which is accompanied by the removal of the water molecule coordinated to the heme iron (Figure 1b, left). However, in the presence of perfluorinated carboxylic acids (PFCs) as inert dummy substrates (decoy molecules), gaseous alkanes can be hydroxylated by wild-type P450BM3. The decoy molecules initiate the activation of molecular oxygen in the same manner as long-alkyl-chain fatty acids and induce the generation of compound I (Figure 1b, right). Because the C F bonds of PFCs are not oxidizable, compound I exclusively hydroxylates gaseous alkanes. Moreover, shortalkyl-chain PFCs partially occupy the substrate binding site of P450BM3 to afford a space for small alkanes, which leads to efficient hydroxylation of the small alkanes (Figure 1a, lower). This attractive advantage encouraged us to perform benzene hydroxylation for the selective production of phenol. The hydroxylation of benzene by P450BM3 was examined by employing a series of PFCs (PFC8–PFC12; Table 1). The catalytic turnover rates of phenol formation were very much

Peroxygenase reactions catalyzed by cytochromes P450

Cytochromes P450 (P450s) catalyze monooxygenation of a wide range of less reactive organic molecules under mild conditions. By contrast with the general reductive oxygen activation pathway of P450s, an H2O2-shunt pathway does not require any supply of electrons and protons for the generation of a highly reactive intermediate (compound I). Because the low cost of H2O2 allows us to use it in industrial-scale synthesis, the H2O2-shunt pathway is an attractive process for monooxygenation reactions. This review focuses on the P450-catalyzed monooxygenation of organic molecules using H2O2 as the oxidant.

Crystal structure of H2O2-dependent cytochrome P450SPalpha with its bound fatty acid substrate: insight into the regioselective hydroxylation of fatty acids at the alpha position.

Cytochrome P450(SPα) (CYP152B1) isolated from Sphingomonas paucimobilis is the first P450 to be classified as a H(2)O(2)-dependent P450. P450(SPα) hydroxylates fatty acids with high α-regioselectivity. Herein we report the crystal structure of P450(SPα) with palmitic acid as a substrate at a resolution of 1.65 Å. The structure revealed that the C(α) of the bound palmitic acid in one of the alternative conformations is 4.5 Å from the heme iron. This conformation explains the highly selective α-hydroxylation of fatty acid observed in P450(SPα). Mutations at the active site and the F-G loop of P450(SPα) did not impair its regioselectivity. The crystal structures of mutants (L78F and F288G) revealed that the location of the bound palmitic acid was essentially the same as that in the WT, although amino acids at the active site were replaced with the corresponding amino acids of cytochrome P450(BSβ) (CYP152A1), which shows β-regioselectivity. This implies that the high regioselectivity of P450(SPα) is caused by the orientation of the hydrophobic channel, which is more perpendicular to the heme plane than that of P450(BSβ).Cytochrome P450SPα (CYP152B1) isolated from Sphingomonas paucimobilis is the first P450 to be classified as a H2O2-dependent P450. P450SPα hydroxylates fatty acids with high α-regioselectivity. Herein we report the crystal structure of P450SPα with palmitic acid as a substrate at a resolution of 1.65 Å. The structure revealed that the Cα of the bound palmitic acid in one of the alternative conformations is 4.5 Å from the heme iron. This conformation explains the highly selective α-hydroxylation of fatty acid observed in P450SPα. Mutations at the active site and the F–G loop of P450SPα did not impair its regioselectivity. The crystal structures of mutants (L78F and F288G) revealed that the location of the bound palmitic acid was essentially the same as that in the WT, although amino acids at the active site were replaced with the corresponding amino acids of cytochrome P450BSβ (CYP152A1), which shows β-regioselectivity. This implies that the high regioselectivity of P450SPα is caused by the orientation of the hydrophobic channel, which is more perpendicular to the heme plane than that of P450BSβ.

Aromatic C–H bond hydroxylation by P450 peroxygenases: a facile colorimetric assay for monooxygenation activities of enzymes based on Russig’s blue formation

Aromatic C–H bond hydroxylation of 1-methoxynaphthalene was efficiently catalyzed by the substrate misrecognition system of the hydrogen peroxide dependent cytochrome P450BSβ (CYP152A1), which usually catalyzes hydroxylation of long-alkyl-chain fatty acids. Very importantly, the hydroxylation of 1-methoxynaphthalene can be monitored by a color change since the formation of 4-methoxy-1-naphthol was immediately followed by its further oxidation to yield Russig’s blue. Russig’s blue formation allows us to estimate the peroxygenation activity of enzymes without the use of high performance liquid chromatography, gas chromatography, and nuclear magnetic resonance measurements.

Direct hydroxylation of primary carbons in small alkanes by wild-type cytochrome P450BM3 containing perfluorocarboxylic acids as decoy molecules

Cytochrome P450BM3 (P450BM3) is a long-alkyl-chain fatty acid hydroxylase that shows an extremely high catalytic turnover rate and high coupling efficiency of NADPH for product formation (product formation rate per NADPH consumption rate). Although P450BM3 exclusively hydroxylates long-alkyl-chain fatty acids, we have found that simple addition of perfluorocarboxylic acids (PFs) as inert dummy substrates (decoy molecules) turns P450BM3 into a small alkane hydroxylase. For example, PF-bound P450BM3 oxidizes propane and butane to 2-propanol and 2-butanol, respectively. The coupling efficiency of small alkane hydroxylation, however, is very low compared with that of long-alkyl-chain fatty acid hydroxylation. In this study, we examined the experimental conditions for small alkane hydroxylation in an effort to improve the coupling efficiency and to realize the hydroxylation of their primary carbons. To increase the concentration of gaseous substrates in the reaction mixture, we performed reactions under the high-pressure condition of 0.5 MPa small alkanes. Propane hydroxylation under high-pressure conditions significantly improved the coupling efficiency to 48%. Furthermore, 1-propanol, which has never been observed under lower-pressure conditions, was produced. It is noteworthy that a detectable amount of ethanol was observed in the ethane hydroxylation under the pressure condition of 0.5 MPa, whereas methane was not hydroxylated. These results indicate that by increasing the concentration of small alkanes, the “P450BM3–decoy molecule system” can catalyze hydroxylation reactions of the primary carbons in small alkanes.

Understanding substrate misrecognition of hydrogen peroxide dependent cytochrome P450 from Bacillus subtilis

Cytochrome P450BSβ, a H2O2-dependent cytochrome P450 catalyzing the hydroxylation of long-alkyl-chain fatty acids, lacks the general acid–base residue around the heme, which is indispensable for the efficient generation of the active species using H2O2. On the basis of the crystal structure of the palmitic acid bound form of cytochrome P450BSβ, it was suggested that the role of the general acid–base function was provided by the carboxylate group of fatty acids. The participation of the carboxylate group of the substrate was supported by the fact that cytochrome P450BSβ can catalyze oxidations of nonnatural substrates such as styrene and ethylbenzene in the presence of a series of short-alkyl-chain carboxylic acids as a dummy molecule of fatty acid. We refer to a series of short-alkyl-chain carboxylic acids as a “decoy molecule”. As shown here, we have clarified the crystal structure of the decoy-molecule-bound form and elucidated that the location of its carboxylate group is virtually the same as that of palmitic acid in the heme cavity, indicating that the carboxylate group of the decoy molecule serves as the general acid–base catalyst. This result further confirms that the role of the acid–base function is satisfied by the carboxylate group of the substrates. In addition, the structure analysis of the substrate-free form has clarified that no remarkable structural change is induced by the binding of the decoy molecule as well as fatty acid. Consequently, whether the carboxylate group is positioned in the active site provides the switching mechanism of the catalytic cycle of cytochrome P450BSβ.

Chiral‐Substrate‐Assisted Stereoselective Epoxidation Catalyzed by H2O2‐Dependent Cytochrome P450SPα

The stereoselective epoxidation of styrene was catalyzed by H(2) O(2) -dependent cytochrome P450(SPα) in the presence of carboxylic acids as decoy molecules. The stereoselectivity of styrene oxide could be altered by the nature of the decoy molecules. In particular, the chirality at the α-positions of the decoy molecules induced a clear difference in the chirality of the product: (R)-ibuprofen enhanced the formation of (S)-styrene oxide, whereas (S)-ibuprofen preferentially afforded (R)-styrene oxide. The crystal structure of an (R)-ibuprofen-bound cytochrome P450(SPα) (resolution 1.9 Å) revealed that the carboxylate group of (R)-ibuprofen served as an acid-base catalyst to initiate the epoxidation. A docking simulation of the binding of styrene in the active site of the (R)-ibuprofen-bound form suggested that the orientation of the vinyl group of styrene in the active site agreed with the formation of (S)-styrene oxide.

Single-Step Reconstitution of Apo-Hemoproteins at the Disruption Stage of Escherichia coli Cells

Hemoproteins on their metal: We report a novel strategy for the reconstitution of hemoproteins with non-natural metal complexes; simple addition of manganese and ruthenium porphyrin to E. coli cells immediately prior to homogenization yields the reconstituted proteins. We believe that this simple approach could become a standard reconstitution method for hemoproteins.

GluN2B‐Selective N‐Methyl‐d‐aspartate (NMDA) Receptor Antagonists Derived from 3‐Benzazepines: Synthesis and Pharmacological Evaluation of Benzo[7]annulen‐7‐amines

Given their high neuroprotective potential, ligands that block GluN2B‐containing N‐methyl‐D‐aspartate (NMDA) receptors by interacting with the ifenprodil binding site located on the GluN2B subunit are of great interest for the treatment of various neuronal disorders. In this study, a novel class of GluN2B‐selective NMDA receptor antagonists with the benzo[7]annulene scaffold was prepared and pharmacologically evaluated. The key intermediate, N‐(2‐methoxy‐5‐oxo‐6,7,8,9‐tetrahydro‐5H‐benzo[7]annulen‐7‐yl)acetamide (11), was obtained by cyclization of 3‐acetamido‐5‐(3‐methoxyphenyl)pentanoic acid (10 b). The final reaction steps comprise hydrolysis of the amide, reduction of the ketone, and reductive alkylation, leading to cis‐ and trans‐configured 7‐(ω‐phenylalkylamino)benzo[7]annulen‐5‐ols. High GluN2B affinity was observed with cis‐configured γ‐amino alcohols substituted with a 3‐phenylpropyl moiety at the amino group. Removal of the benzylic hydroxy moiety led to the most potent GluN2B antagonists of this series: 2‐methoxy‐N‐(3‐phenylpropyl)‐6,7,8,9‐tetrahydro‐5H‐benzo[7]annulen‐7‐amine (20 a, Ki=10 nM) and 2‐methoxy‐N‐methyl‐N‐(3‐phenylpropyl)‐6,7,8,9‐tetrahydro‐5H‐benzo[7]annulen‐7‐amine (23 a, Ki=7.9 nM). The selectivity over related receptors (phencyclidine binding site of the NMDA receptor, σ1 and σ2 receptors) was recorded. In a functional assay measuring the cytoprotective activity of the benzo[7]annulenamines, all tested compounds showed potent NMDA receptor antagonistic activity. Cytotoxicity induced via GluN2A subunit‐containing NMDA receptors was not inhibited by the new ligands.

Inhibition of Heme Uptake in Pseudomonas aeruginosa by its Hemophore (HasAp ) Bound to Synthetic Metal Complexes

The heme acquisition system A protein secreted by Pseudomonas aeruginosa (HasA(p)) can capture several synthetic metal complexes other than heme. The crystal structures of HasA(p) harboring synthetic metal complexes revealed only small perturbation of the overall HasA(p) structure. An inhibitory effect upon heme acquisition by HasA(p) bearing synthetic metal complexes was examined by monitoring the growth of Pseudomonas aeruginosa PAO1. HasA(p) bound to iron-phthalocyanine inhibits heme acquisition in the presence of heme-bound HasA(p) as an iron source.