Progesterone 5β-reductase of Erysimum crepidifolium: cDNA cloning, expression in Escherichia coli, and reduction of enones with the recombinant protein

Four enzymes control natural variation in the steroid core of Erysimum cardenolides

Plants commonly produce families of structurally related metabolites with similar defensive functions. This apparent redundancy raises the question of underlying molecular mechanisms and adaptive benefits of such chemical variation. Cardenolides, a class defensive compounds found in the wallflower genus Erysimum (L., Brassicaceae) and scattered across other plant families, show substantial structural variation, with glycosylation and hydroxylation being common modifications of a steroid core, which itself may vary in terms of stereochemistry and saturation. Through a combination of chemical mutagenesis and analysis of gene coexpression networks, we identified four enzymes involved in cardenolide biosynthesis in Erysimum that work together to determine stereochemistry at carbon 5 of the steroid core: Ec3βHSD, a 3β-hydroxysteroid dehydrogenase, Ec3KSI, a ketosteroid isomerase, EcP5βR2, a progesterone 5β-reductase, and EcDET2, a steroid 5α-reductase. We biochemically characterized the activity of these enzymes in vitro and generated CRISPR/Cas9 knockout lines to confirm activity in vivo. Cardenolide biosynthesis was not eliminated in any of the knockouts. Instead, mutant plants accumulated cardenolides with altered saturation and stereochemistry of the steroid core. Furthermore, we found variation in carbon 5 configuration among the cardenolides of 44 species of Erysimum, where the occurrence of some 5β-cardenolides is associated with the expression and sequence of P5βR2. This may have allowed Erysimum species to fine-tune their defensive profiles to target specific herbivore populations over the course of evolution.

SIGNIFICANCE STATEMENT

Plants use an array of toxic compounds to defend themselves from attack against insects and other herbivores. One mechanism through which plants may evolve more toxic compounds is through modifications to the structure of compounds they already produce. In this study, we show how plants in the wallflower genus Erysimum use four enzymes to fine-tune the structure of toxic metabolites called cardenolides. Natural variation in the sequence and expression of a single enzyme called progesterone 5β-reductase 2 partly explains the variation in cardenolides observed across the Erysimum genus. These alterations to cardenolide structure over the course of evolution suggests that there may be context-dependent benefits to Erysimum to invest in one cardenolide variant over another.

Citronellol biosynthesis in pelargonium is a multistep pathway involving progesterone 5β-reductase and/or iridoid synthase-like enzymes

Citronellol is a pleasant-smelling compound produced in rose (Rosa spp.) flowers and in the leaves of many aromatic plants, including pelargoniums (Pelargonium spp.). Although geraniol production has been well studied in several plants, citronellol biosynthesis has been documented only in crab-lipped spider orchid (Caladenia plicata) and its mechanism remains open to question in other species. We therefore profiled 10 pelargonium accessions using RNA sequencing and gas chromatography-MS analysis. Three enzymes from the progesterone 5β-reductase and/or iridoid synthase-like enzymes (PRISE) family were characterized in vitroand subsequently identified as citral reductases (named PhCIRs). Transgenic RNAi lines supported a role for PhCIRs in the biosynthesis of citronellol as well as in the production of mint-scented terpenes. Despite their high amino acid sequence identity, the 3 enzymes showed contrasting stereoselectivity, either producing mainly (S)-citronellal or a racemate of both (R)- and (S)-citronellal. Using site-directed mutagenesis, we identified a single amino acid substitution as being primarily responsible for the enzyme's enantioselectivity. Phylogenetic analysis of pelargonium PRISEs revealed 3 clades and 7 groups of orthologs. PRISEs from different groups exhibited differential affinities toward substrates (citral and progesterone) and cofactors (NADH/NADPH), but most were able to reduce both substrates, prompting hypotheses regarding the evolutionary history of PhCIRs. Our results demonstrate that pelargoniums evolved citronellol biosynthesis independently through a 3-step pathway involving PRISE homologs and both citral and citronellal as intermediates. In addition, these enzymes control the enantiomeric ratio of citronellol thanks to small alterations of the catalytic site.

Acropetal and basipetal cardenolide transport in Erysimum cheiranthoides (wormseed wallflower)

Plant specialized metabolites are often subject to within-plant transport and have tissue-specific distribution patterns. Among plants in the Brassicaceae, the genus Erysimum is unique in producing not only glucosinolates but also cardenolides. Ten cardenolides were detected with varying abundance in different tissues of Erysimum cheiranthoides L (Brassicaceae; wormseed wallflower). As is predicted by the optimal defense theory, cardenolides were most abundant in young leaves and reproductive tissues. The lowest concentrations were observed in senescing leaves and roots. Crosses between wildtype E. cheiranthoides and a mutant line with an altered cardenolide profile showed that the seed cardenolide phenotype is determined entirely by the maternal genotype. Prior to the development of the first true leaves, seedling cotyledons also had the maternal cardenolide profile. Hypocotyl grafting experiments showed that the root cardenolide profile is determined entirely by the aboveground plant genotype. In further grafting experiments, there was no evidence of cardenolide transport into the leaves, but a mixed cardenolide profile was observed in the stems and inflorescences of plants that had been grafted at vegetative and flowering growth stages, respectively. Together, these results indicate that E. cheiranthoides leaves are a site of cardenolide biosynthesis.

PRISEs (progesterone 5β-reductase and/or iridoid synthase-like 1,4-enone reductases): Catalytic and substrate promiscuity allows for realization of multiple pathways in plant metabolism

PRISEs (progesterone 5β-reductase and/or iridoid synthase-like 1,4-enone reductases) are involved in cardenolide and iridoid biosynthesis. We here investigated a PRISE (rAtSt5βR) from Arabidopsis thaliana, a plant producing neither cardenolides nor iridoids. The structure of rAtSt5βR was elucidated with X-ray crystallography and compared to the known structures of PRISEs from Catharanthus roseus (rCrISY) and Digitalis lanata (rDlP5βR). The three enzymes show a high degree of sequence and structure conservation in the active site. Amino acids previously considered to allow discrimination between progesterone 5β-reductase and iridoid synthase were interchanged among rAtSt5βR, rCrISY and rDlP5βR applying site-directed mutagenesis. Structural homologous substitutions had different effects, and changes in progesterone 5β-reductase and iridoid synthase activity were not correlated in all cases. Our results help to explain fortuitous emergence of metabolic pathways and product accumulation. The fact that PRISEs are found ubiquitously in spermatophytes insinuates that PRISEs might have a more general function in plant metabolism such as, for example, the detoxification of reactive carbonyl species.

A multisubstrate reductase from Plantago major: structure-function in the short chain reductase superfamily

The short chain dehydrogenase/reductase superfamily (SDR) is a large family of NAD(P)H-dependent enzymes found in all kingdoms of life. SDRs are particularly well-represented in plants, playing diverse roles in both primary and secondary metabolism. In addition, some plant SDRs are also able to catalyse a reductive cyclisation reaction critical for the biosynthesis of the iridoid backbone that contains a fused 5 and 6-membered ring scaffold. Mining the EST database of Plantago major, a medicinal plant that makes iridoids, we identified a putative 5β-progesterone reductase gene, PmMOR (P. major multisubstrate oxido-reductase), that is 60% identical to the iridoid synthase gene from Catharanthus roseus. The PmMOR protein was recombinantly expressed and its enzymatic activity assayed against three putative substrates, 8-oxogeranial, citral and progesterone. The enzyme demonstrated promiscuous enzymatic activity and was able to not only reduce progesterone and citral, but also to catalyse the reductive cyclisation of 8-oxogeranial. The crystal structures of PmMOR wild type and PmMOR mutants in complex with NADP⁺ or NAD⁺ and either 8-oxogeranial, citral or progesterone help to reveal the substrate specificity determinants and catalytic machinery of the protein. Site-directed mutagenesis studies were performed and provide a foundation for understanding the promiscuous activity of the enzyme.

Steroid 5β-Reductase from Leaves of Vitis vinifera: Molecular Cloning, Expression, and Modeling

A steroid 5β-reductase gene corresponding to the hypothetical protein LOC100247199 from leaves of Vitis vinifera (var. 'Chardonnay') was cloned and overexpressed in Escherichia coli. The recombinant protein showed 5β-reductase activity when progesterone was used as a substrate. The reaction was stereoselective, producing only 5β-products such as 5β-pregnane-3,20-dione. Other small substrates (terpenoids and enones) were also accepted as substrates, indicating the highly promiscuous character of the enzyme class. Our results show that the steroid 5β-reductase gene, encoding an orthologous enzyme described as a key enzyme in cardenolide biosynthesis, is also expressed in leaves of the cardenolide-free plant V. vinifera. We emphasize the fact that, on some occasions, different reductases (e.g., progesterone 5β-reductase and monoterpenoid reductase) can also use molecules that are similar to the final products as a substrate. Therefore, in planta, the different reductases may contribute to the immense number of diverse small natural products finally leading to the flavor of wine.

Progesterone 5β-reductase genes of the Brassicaceae family as function-associated molecular markers

This study aimed to define progesterone 5β-reductases (P5βR, EC 1.3.99.6, enone 1,4-reductases) as function-associated molecular markers at the plant family level. Therefore cDNAs were isolated from 25 Brassicaceae species, including two species, Erysimum crepidifolium and Draba aizoides, known to produce cardiac glycosides. The sequences were used in a molecular phylogeny study. The cladogram created is congruent to the existing molecular analyses. Recombinant His-tagged forms of the P5βR cDNAs from Aethionema grandiflorum, Draba aizoides, Nasturtium officinale, Raphanus sativus and Sisymbrium officinale were expressed in E. coli. Enone 1,4-reductase activity was demonstrated in vitro using progesterone and 2-cyclohexen-1-one as substrates. Evidence is provided that functional P5βRs are ubiquitous in the Brassicaceae. The recombinant P5βR enzymes showed different substrate preferences towards progesterone and 2-cyclohexen-1-one. Sequence comparison of the catalytic pocket of the P5βR enzymes and homology modelling using Digitalis lanata P5βR (PDB ID: 2V6G) as template highlighted the importance of the hydrophobicity of the binding pocket for substrate discrimination. It is concluded that P5βR genes or P5βR proteins can be used as valuable function-associated molecular markers to infer taxonomic relationship and evolutionary diversification from a metabolic/catalytic perspective.

Iridoid synthase activity is common among the plant progesterone 5β-reductase family

Catharanthus roseus, the Madagascar periwinkle, synthesizes bioactive monoterpenoid indole alkaloids, including the anti-cancer drugs vinblastine and vincristine. The monoterpenoid branch of the alkaloid pathway leads to the secoiridoid secologanin and involves the enzyme iridoid synthase (IS), a member of the progesterone 5β-reductase (P5βR) family. IS reduces 8-oxogeranial to iridodial. Through transcriptome mining, we show that IS belongs to a family of six C. roseus P5βR genes. Characterization of recombinant CrP5βR proteins demonstrates that all but CrP5βR3 can reduce progesterone and thus can be classified as P5βRs. Three of them, namely CrP5βR1, CrP5βR2, and CrP5βR4, can also reduce 8-oxogeranial, pointing to a possible redundancy with IS (corresponding to CrP5βR5) in secoiridoid synthesis. In-depth functional analysis by subcellular protein localization, gene expression analysis, in situ hybridization, and virus-induced gene silencing indicate that besides IS, CrP5βR4 may also participate in secoiridoid biosynthesis. We cloned a set of P5βR genes from angiosperm plant species not known to produce iridoids and demonstrate that the corresponding recombinant proteins are also capable of using 8-oxogeranial as a substrate. This suggests that IS activity is intrinsic to angiosperm P5βR proteins and has evolved early during evolution.

Carbon-carbon double-bond reductases in nature

Reduction of C = C bonds by reductases, found in a variety of microorganisms (e.g. yeasts, bacteria, and lower fungi), animals, and plants has applications in the production of metabolites that include pharmacologically active drugs and other chemicals. Therefore, the reductase enzymes that mediate this transformation have become important therapeutic targets and biotechnological tools. These reductases are broad-spectrum, in that, they can act on isolation/conjugation C = C-bond compounds, α,β-unsaturated carbonyl compounds, carboxylic acids, acid derivatives, and nitro compounds. In addition, several mutations in the reductase gene have been identified, some associated with diseases. Several of these reductases have been cloned and/or purified, and studies to further characterize them and determine their structure in order to identify potential industrial biocatalysts are still in progress. In this study, crucial reductases for bioreduction of C = C bonds have been reviewed with emphasis on their principal substrates and effective inhibitors, their distribution, genetic polymorphisms, and implications in human disease and treatment.

Identification and stress-induced expression of three 3β-hydroxysteroid dehydrogenases from Erysimum crepidifolium Rchb. and their putative role in cardenolide biosynthesis

3β-Hydroxysteroid dehydrogenases (3βHSD) are supposed to be involved in cardenolide biosynthesis in plants. Erysimum crepidifolium Rchb., a member of the Brassicaceae accumulating cardenolides, is a close relative to Arabidopsis thaliana. Full length cDNAs encoding for three individual 3βHSDs (EcHSD1, EcHSD2, EcHSD3) were isolated from E. crepidifolium leaves. EcHSD1 and EcHSD2 encode proteins assembled from 257 amino acids whereas EcHSD3 encodes a protein assembled from 260 amino acids. All three proteins qualify as members of the short-chain dehydrogenases/reductases family of proteins (SDRs). EcHSD1 and EcHSD2 shared a high amino acid sequence identity of about 86% and 91% with putative 3βHSDs of A. thaliana (AT2G47140 and AT2G47130). EcHSD3 showed high homology to the A. thaliana SDRs AT2G47150 (74%) and AT2G47120 (81%). All three EcHSD genes were expressed in Escherichia coli and the recombinant enzymes were characterized biochemically. All three recombinant EcHSDs catalyzed the dehydrogenation of pregnenolone and the 3-reduction of 5α/β-pregnane-3,20-dione when NAD and NADH were used as cosubstrates, respectively. After exposure to different stress conditions, no increased transcription was seen for EcHSD1 whereas EcHSD2 was expressed four times higher under osmotic stress than under control conditions. EcHSD3 expression was 10 times and 6 times higher after osmotic stress and MeJA treatment, respectively, than in controls.

Truncation of N-terminal regions of Digitalis lanata progesterone 5β-reductase alters catalytic efficiency and substrate preference

N-Terminal truncated forms of progesterone 5β-reductase (P5βR) were synthesized taking a full-length cDNA encoding for Digitalis lanata P5βR with a hexa-histidine tag attached at the C-terminus (rDlP5βRc) as the starting point. Four pETite-c-His/DlP5βR constructs coding for P5βR derivatives truncated in the N-terminal region, termed rDlP5βRcn-10, rDlP5βRcn-20, rDlP5βRcn-30, and rDlP5βRcn-40 were obtained by site-directed mutagenesis. The cDNAs coding for full-length rDlP5βRc, rDlP5βRcn-10 and rDlP5βRcn-20 were over-expressed in Escherichia coli and the respective enzymes were soluble and catalytically active (progesterone and 2-cyclohexen-1-one as substrates). GST-tagged recombinant DlP5βR (rDlP5βR-GST) and rDlP5βR-GSTr, with the GST-tag removed by protease treatment were produced as well and served as controls. The Km values and substrate preferences considerably differed between the various DlP5βR derivatives. As for the C-terminal His-tagged rDlP5βR the catalytic efficiency for progesterone was highest for the full-length rDlP5βRc whereas the N-terminal truncated forms preferred 2-cyclohexen-1-one as the substrate. Affinity tags and artifacts resulting from the cloning strategy used may alter substrate specificity. Therefore enzyme properties determined with recombinant proteins should not be used to infer in vivo scenarios and should be considered for each particular case.

Vein Patterning 1-encoded progesterone 5β-reductase: activity-guided improvement of catalytic efficiency

Progesterone 5β-reductases (P5βR; EC 1.3.99.6) encoded by Vein Patterning 1 (VEP1) genes are capable of reducing the CC double-bond of a variety of enones enantioselectively. Sequence and activity data of orthologous P5βRs were used to define a set of residues possibly responsible for the large differences in enzyme activity seen between rAtSt5βR and rDlP5βR, recombinant forms of P5βRs from Arabidopsis thaliana and Digitalis lanata, respectively. Tyrosine-156, asparagine-205 and serine-248 were identified as hot spots in the rDlP5βR responsible for its low catalytic efficiency. These positions were individually substituted for amino acids found in the strong rAtSt5βR in the corresponding sites. Kinetic constants were determined for rDlP5βR and its mutants as well as for rAtSt5βR using progesterone and 2-cyclohexen-1-one as substrates. Enzyme mutants in which asparagine-205 was substituted for methionine or alanine showed considerably lower km and higher K(cat)/k(m) values than the wild-type DlP5βR, approaching the catalytic efficiency of strong P5βRs. The introduced mutations not only lead to an improved capability to reduce progesterone but also to altered substrate preference. Our findings provided structural insights into the differences seen among the natural P5βRs with regard to their substrate preferences and catalytic efficiencies.