Regioselectivity and Deuterium Isotope Effects in Geraniol Hydroxylation by the Cytochrome P-450 Monooxygenase fromCatharanthus roseus (L.) G. DON

Selective allylic hydroxylation of acyclic terpenoids by CYP154E1 from Thermobifida fusca YX

Allylic alcohols are valuable precursors in the synthesis of pharmaceutical intermediates, agrochemicals and natural products. Regioselective oxidation of parental alkenes is a challenging task for chemical catalysts and requires several steps including protection and deprotection. Many cytochrome P450 enzymes are known to catalyse selective allylic hydroxylation under mild conditions. Here, we describe CYP154E1 from Thermobifida fusca YX that enables this type of oxidation. Several acyclic terpenoids were tested as possible substrates for CYP154E1, and the regio- and chemoselectivity of their oxidation was investigated. Using a previously established bioinformatics approach we identified position 286 in the active site of CYP154E1 which is putatively involved in substrate binding and thereby might have an effect on enzyme selectivity. To tune regio- and chemoselectivity of the enzyme three mutants at position 286 were constructed and used for substrate oxidation. All formed products were analysed with GC-MS and identified using chemically synthesised authentic samples and known compounds as references. Best regioselectivity towards geraniol and nerol was observed with the wild type enzyme mainly leading to 8-hydroxy derivatives (8-hydroxygeraniol or 8-hydroxynerol) with high selectivity (100% and 96% respectively). Highest selectivities during the oxidation of geranylacetone and nerylacetone were observed with the following variants: V286F led mainly to 7-hydroxygeranylacetone (60% of the total product) and V286A produced predominantly 12-hydroxynerylacetone (75% of total product). Thus, CYP154E1 and its mutants expand the tool-box for allylic hydroxylation in synthetic chemistry.

What Factors Affect the Regioselectivity of Oxidation by Cytochrome P450? A DFT Study of Allylic Hydroxylation and Double Bond Epoxidation in a Model Reaction

Epoxidation (C=C) vis-à-vis allylic hydroxylation (C-H) reactions of propene with a model compound I (Cpd I) of the enzyme cytochrome P450 were studied using B3LYP density functional theory. Potential energy profiles and kinetic isotope effects (KIE) were calculated. The interactions in the protein pocket were mimicked by adding two external NH- - -S hydrogen bonds to the thiolate ligand and by introducing a nonpolar medium (with a dielectric constant, epsilon = 5.7) that can exert a polarization effect on the reacting species. A two-state reactivity (TSR) with high-spin (HS) and low-spin (LS) states were located for both processes (Ogliaro, F.; Harris, N.; Cohen, S.; Filatov, M.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 8977-8989. de Visser, S. P.; Ogilaro, F.; Harris, N.; Shaik, S. J. Am. Chem. Soc. 2001, 123, 3037-3047). The HS processes were found to be stepwise, whereas the LS processes were characterized as nonsynchronous but effectively concerted pathways. The computed KIE for C-H hydroxylation with and without tunneling corrections are large (>7), and they support the assignment of the corresponding transition states as hydrogen-abstraction species (Groves, J. T.; Han, Y.-Z. In Cytochrome P450: Structures, Mechanism and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995; Chapter 1; pp 3-48). In the gas phase, epoxidation is energetically favorable by 3.4 kcal mol(-1). Inclusion of zero-point energy reduces this difference but still predicts C=C/C-H > 1. Environmental effects were found to have major impact on the C=C/C-H ratio as well as on the stereoselectivity of the processes. Thus, two NH- - -S hydrogen bonds away from the reaction center reverse the regioselectivity and prefer hydroxylation, namely, C=C/C-H <1. The polarity of the medium further accentuates the trend and leads to a change by 2 orders of magnitude in the regioselectivity, C=C/C-H << 1. Furthermore, since the environmental interactions prefer the LS over the HS reactions, both hydroxylation and epoxidation processes are rendered more stereoselective, again by 2 orders of magnitude. It follows, therefore, that Cpd I is a chameleon oxidant (Ogliaro, F.; Cohen, S.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 12892-12893; Ogliaro, F.; de Visser, S. P.; Cohen, S.; Kaneti, J.; Shaik, S. Chembiochem. 2001, 2, 848-851; Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Angew. Chem., Int. Ed. 2001, 40, 2874-2878) that tunes its reactivity and selectivity patterns in response to the protein environment in which it is accommodated. A valence bond (VB) model, akin to "redox mesomerism" (Bernadou, J.; Fabiano, A.-S.; Robert, A.; Meunier, B. J. Am. Chem. Soc. 1994, 116, 9375-9376), is constructed and enables the description of a chameleon transition state. It shows that the good donor ability of the thiolate ligand and the acceptor ability of the iron porphyrin create mixed-valent situations that endow the transition state with a great sensitivity to external perturbations as in the protein pocket. The model is used to discuss the computed results and to relate them to experimental findings.

Hydroxylation of specifically deuterated limonene enantiomers by cytochrome p450 limonene-6-hydroxylase reveals the mechanism of multiple product formation

The regiochemistry and facial stereochemistry of the limonene-6-hydroxylase- (CYP71D18-) mediated hydroxylation of the monoterpene olefin limonene are determined by the absolute configuration of the substrate. (-)-(4S)-Limonene is hydroxylated at the C6 allylic position to give (-)-trans-carveol as the only product, whereas (+)-(4R)-limonene yields multiple hydroxylation products with (+)-cis-carveol predominating. Specifically deuterated limonene enantiomers were prepared to investigate the net stereospecificity of hydroxylation at C6 and the mechanism of multiple product formation. The results of isotopically sensitive branching experiments of competitive and noncompetitive design were consistent with a nondissociative kinetic mechanism, indicating that (4R)-limonene has sufficient freedom of motion within the active site of CYP71D18 to allow formation of either the trans-3- or cis-6-hydroxylated product. However, the kinetic isotope effects resulting from deuterium abstraction were significantly smaller than expected for an allylic hydroxylation, and they did not approach the intrinsic isotope effect. (4S)-Limonene is oxygenated with almost complete stereospecificity for hydrogen abstraction from the trans-6-position, demonstrating rigid orientation during hydrogen abstraction and hydroxyl delivery. The oxygenation of (4R)-limonene leading to the formation of (+/-)-trans-carveol is accompanied by considerable allylic rearrangement and stereochemical scrambling, whereas the formation of (+)-cis-carveol proceeds without allylic rearrangement and with nearly complete stereospecificity for hydrogen abstraction from the cis-6-position. These results demonstrate that a single cytochrome P450 enzyme catalyzes the hydroxylation of small antipodal substrates with distinct stereochemistries and reveal that substrate-dependent positional motion of the intermediate carbon radical (and, therefore, hydroxylation stereospecificity) is determined by active-site binding complementarity. Thus, epimerization and allylic rearrangement are not inherent features of these reactions but occur when loss of active-site complementarity allows increased substrate mobility.

In vitro metabolism of cannabigerol in several mammalian species

Microsomal incubations were prepared from the livers of male mice, rats, cats, guinea-pigs, hamsters and gerbils and both male and female rabbits and were incubated with cannabigerol (CBG), a constituent of marihuana. Metabolites were extracted with ethyl acetate, concentrated by chromatography on Sephadex LH-20 and examined as trimethylsilyl (TMS) and (2H9)TMS derivatives by gas chromatography/mass spectrometry. Structural elucidation was aided by hydrogenation of the metabolites to tetrahydro derivatives. Similar metabolites were produced by each of the species but the ratios of the individual compounds differed considerably. Twelve metabolites were identified. The major metabolites were monohydroxy compounds with the hydroxyl group at C-8', C-9', C-4' or at one of any position of the pentyl chain. Reduction of the delta-6' double bond was prominent in the cat to give 8'-hydroxy-6',7'-dihydro-CBG. The other major metabolic route was epoxidation of this double bond and hydrolysis to give 6',7'-dihydroxy-6',7'-dihydro-CBG. Although epoxidation of the other double bond was detected, the resulting metabolite was present in low concentration and hydrolysis was not observed. The mass spectral fragmentation of CBG and its metabolites was dominated by formation of the tropylium ion by cleavage of the C-1'--C-2' bond and by ions formed by cleavage of the C-3'--C-4' and C-4'--C-5' bonds. In addition, compounds containing hydroxylation at C-1"--C-4" (pentyl chain) gave rise to the same abundant diagnostic ions that have been observed for corresponding metabolites of other cannabinoids.