In vivo and in vitro hydroxylation of cineole and camphor by cytochromes P450CYP101A1, CYP101B1 and N242A CYP176A1

Exploring the substrate specificity of Cytochrome P450cin

Cytochromes P450 are enzymes that catalyse the oxidation of a wide variety of compounds that range from small volatile compounds, such as monoterpenes to larger compounds like steroids. These enzymes can be modified to selectively oxidise substrates of interest, thereby making them attractive for applications in the biotechnology industry. In this study, we screened a small library of terpenes and terpenoid compounds against P450_cin and two P450_cin mutants, N242A and N242T, that have previously been shown to affect selectivity. Initial screening indicated that P450_cin could catalyse the oxidation of most of the monoterpenes tested; however, sesquiterpenes were not substrates for this enzyme or the N242A mutant. Additionally, both P450_cin mutants were found to be able to oxidise other bicyclic monoterpenes. For example, the oxidation of (R)- and (S)-camphor by N242T favoured the production of 5-endo-hydroxycamphor (65-77% of the total products, dependent on the enantiomer), which was similar to that previously observed for (R)-camphor with N242A (73%). Selectivity was also observed for both (R)- and (S)-limonene where N242A predominantly produced the cis-limonene 1,2-epoxide (80% of the products following (R)-limonene oxidation) as compared to P450_cin (23% of the total products with (R)-limonene). Of the three enzymes screened, only P450_cin was observed to catalyse the oxidation of the aromatic terpene p-cymene. All six possible hydroxylation products were generated from an in vivo expression system catalysing the oxidation of p-cymene and were assigned based on ¹H NMR and GC-MS fragmentation patterns. Overall, these results have provided the foundation for pursuing new P450_cin mutants that can selectively oxidise various monoterpenes for biocatalytic applications.

A Toolbox for Diverse Oxyfunctionalisation of Monoterpenes

The successful implementation of synthetic biology for chemicals biosynthesis relies on the availability of large libraries of well-characterized enzymatic building blocks. Here we present a scalable pipeline that applies the methodology of synthetic biology itself to bootstrap the creation of such a library. By designing and building a cytochrome P450 enzyme collection and testing it in a custom-made untargeted GC/MS-metabolomics-based approach, we were able to rapidly create and characterize a comprehensive enzyme library for the controlled oxyfunctionalisation of terpene scaffolds with a wide range of activities and selectivities towards several monoterpenes. This novel resource can now be used to access the extensive chemical diversity of terpenoids by pathway engineering and the assembly of biocatalytic cascades to subsequently produce libraries of oxygenated terpenoids and their derivatives for diverse applications, including drug discovery.

Stereoselective hydroxylation of isophorone by variants of the cytochromes P450 CYP102A1 and CYP101A1

The stereoselective oxidation of hydrocarbons is an area of research where enzyme biocatalysis can make a substantial impact. The cyclic ketone isophorone was stereoselectively hydroxylated (≥95%) by wild-type CYP102A1 to form (R)-4-hydroxyisophorone, an important chiral synthon and flavour and fragrance compound. CYP102A1 variants were also selective for 4-hydroxyisophorone formation and the product formation rate increased over the wild-type enzyme by up to 285-fold, with the best mutants being R47L/Y51F/I401P and A74G/F87V/L188Q. The latter variant, which contained mutations in the distal substrate binding pocket, was marginally less selective. Combining perfluorodecanoic acid decoy molecules with the rate accelerating variant R47L/Y51F/I401P engendered further improvement with the purified enzymes. However when the decoy molecules were used with A74G/F87V/L188Q the amount of product generated by the enzyme was reduced. Addition of decoy molecules to whole-cell turnovers did not improve the productivity of these CYP102A1 systems. WT CYP101A1 formed significant levels of 7-hydroxyisophorone as a minor product alongside 4-hydroxyisophorone. However the F87W/Y96F/L244A/V247L CYP101A1 mutant was ≥98% selective for (R)-4-hydroxyisophorone. A comparison of the two enzyme systems using whole-cell oxidation reactions showed that the best CYP101A1 variant was able to generate more product. We also characterised that the further oxidation metabolite 4-ketoisophorone was produced and then subsequently reduced to levodione by an endogenous Escherichia coli ene reductase.

The selective oxidation of substituted aromatic hydrocarbons and the observation of uncoupling via redox cycling during naphthalene oxidation by the CYP101B1 system

Oxidation of polyaromatic hydrocarbons by P450s can be lowered by redox cycling but CYP101B1 regioselectively hydroxylated substituted naphthalenes and biphenyls.

CYP101J2, CYP101J3, and CYP101J4, 1,8-Cineole-Hydroxylating Cytochrome P450 Monooxygenases from Sphingobium yanoikuyae Strain B2

We report the isolation and characterization of three new cytochrome P450 monooxygenases: CYP101J2, CYP101J3, and CYP101J4. These P450s were derived from Sphingobium yanoikuyae B2, a strain that was isolated from activated sludge based on its ability to fully mineralize 1,8-cineole. Genome sequencing of this strain in combination with purification of native 1,8-cineole-binding proteins enabled identification of 1,8-cineole-binding P450s. The P450 enzymes were cloned, heterologously expressed (N-terminally His₆ tagged) in Escherichia coli BL21(DE3), purified, and spectroscopically characterized. Recombinant whole-cell biotransformation in E. coli demonstrated that all three P450s hydroxylate 1,8-cineole using electron transport partners from E. coli to yield a product putatively identified as (1S)-2α-hydroxy-1,8-cineole or (1R)-6α-hydroxy-1,8-cineole. The new P450s belong to the CYP101 family and share 47% and 44% identity with other 1,8-cineole-hydroxylating members found in Novosphingobium aromaticivorans and Pseudomonas putida Compared to P450_cin (CYP176A1), a 1,8-cineole-hydroxylating P450 from Citrobacter braakii, these enzymes share less than 30% amino acid sequence identity and hydroxylate 1,8-cineole in a different orientation. Expansion of the enzyme toolbox for modification of 1,8-cineole creates a starting point for use of hydroxylated derivatives in a range of industrial applications.

IMPORTANCE

CYP101J2, CYP101J3, and CYP101J4 are cytochrome P450 monooxygenases from S. yanoikuyae B2 that hydroxylate the monoterpenoid 1,8-cineole. These enzymes not only play an important role in microbial degradation of this plant-based chemical but also provide an interesting route to synthesize oxygenated 1,8-cineole derivatives for applications as natural flavor and fragrance precursors or incorporation into polymers. The P450 cytochromes also provide an interesting basis from which to compare other enzymes with a similar function and expand the CYP101 family. This could eventually provide enough bacterial parental enzymes with similar amino acid sequences to enable in vitro evolution via DNA shuffling.