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.

Functional iridoid synthases from iridoid producing and non-producing Nepeta species (subfam. Nepetoidae, fam. Lamiaceae)

Iridoids, a class of atypical monoterpenes, exhibit exceptional diversity within the Nepeta genus (subfam. Nepetoidae, fam. Lamiaceae).The majority of these plants produce iridoids of the unique stereochemistry, with nepetalactones (NLs) predominating; however, a few Nepeta species lack these compounds. By comparatively analyzing metabolomics, transcriptomics, gene co-expression, and phylogenetic data of the iridoid-producing N. rtanjensis Diklić & Milojević and iridoid-lacking N. nervosa Royle & Bentham, we presumed that one of the factors responsible for the absence of these compounds in N. nervosa is iridoid synthase (ISY). Two orthologues of ISY were mined from leaves transcriptome of N. rtanjensis (NrPRISE1 and NrPRISE2), while in N. nervosa only one (NnPRISE) was identified, and it was phylogenetically closer to the representatives of the Family 1 isoforms, designated as P5βRs. Organ-specific and MeJA-elicited profiling of iridoid content and co-expression analysis of IBG candidates, highlighted NrPRISE2 and NnPRISE as promising candidates for ISY orthologues, and their function was confirmed using in vitro assays with recombinant proteins, after heterologous expression of recombinant proteins in E. coli and their His-tag affinity purification. NrPRISE2 demonstrated ISY activity both in vitro and likely in planta, which was supported by the 3D modeling and molecular docking analysis, thus reclassification of NrPRISE2 to NrISY is accordingly recommended. NnPRISE also displays in vitro ISY-like activity, while its role under in vivo conditions was not here unambiguously confirmed. Most probably under in vivo conditions the NnPRISE lacks substrates to act upon, as a result of the loss of function of some of the upstream enzymes of the iridoid pathway. Our ongoing work is conducted towards re-establishing the biosynthesis of iridoids in N. nervosa.

Identification and functional characterization of three iridoid synthases in Gardenia jasminoides

The discovery of three iridoid synthases (GjISY, GjISY2 and GjISY4) from Gardenia jasminoides and their functional characterization increase the understanding of iridoid scaffold/iridoid glycoside biosynthesis in iridoid-producing plants. Iridoids are a class of noncanonical monoterpenes that are found naturally in the plant kingdom mostly as glycosides. Over 40 iridoid glycosides (e.g., geniposide, gardenoside and shanzhiside) have been isolated from Gardenia jasminoides. They have multiple pharmacological properties and health-promoting effects. However, their biosynthetic pathway is poorly understood, and the iridoid synthase (ISY) responsible for the cyclization of the core scaffold remains unclear. In this study, three homologs of ISYs from G. jasminoides (GjISY, GjISY2 and GjISY4) were identified on the basis of transcriptomic data and functionally characterized. The genomic structure and intron-exon arrangement revealed that all three ISYs contained an intron. Biochemical assays indicated that all three recombinant enzymes reduced 8-oxogeranial to nepetalactol and its open forms (iridodials) as the products of the classical CrISY (Catharanthus roseus). In addition, all three enzymes reduced progesterone to 5-β-prognane-3,20-dione. However, only GjISY2 and GjISY4 reduced 2-cyclohexen-1-one to cyclohexanone. Overall, the GjISY2 expression levels in the flowers and fruits were similar to the GjISY and GjISY4 expression levels. By contrast, the GjISY2 expression levels in the upper and lower leaves were substantially higher than the GjISY and GjISY4 expression levels. Among the three, GjISY2 exhibited the highest catalytic efficiency for 8-oxogeranial. GjISY2 might be the major contributor to iridoid biosynthesis in G. jasminoides. Collectively, our results advance the understanding of iridoid scaffold/iridoid glycoside biosynthesis in G. jasminoides and provide a potential target for metabolic engineering and breeding.

Knockout of Arabidopsis thaliana VEP1, Encoding a PRISE (Progesterone 5β-Reductase/Iridoid Synthase-Like Enzyme), Leads to Metabolic Changes in Response to Exogenous Methyl Vinyl Ketone (MVK)

Small or specialized natural products (SNAPs) produced by plants vary greatly in structure and function, leading to selective advantages during evolution. With a limited number of genes available, a high promiscuity of the enzymes involved allows the generation of a broad range of SNAPs in complex metabolic networks. Comparative metabolic studies may help to understand why—or why not—certain SNAPs are produced in plants. Here, we used the wound-induced, vein patterning regulating VEP1 (AtStR1, At4g24220) and its paralogue gene on locus At5g58750 (AtStR2) from Arabidopsis to study this issue. The enzymes encoded by VEP1-like genes were clustered under the term PRISEs (progesterone 5β-reductase/iridoid synthase-like enzymes) as it was previously demonstrated that they are involved in cardenolide and/or iridoid biosynthesis in other plants. In order to further understand the general role of PRISEs and to detect additional more “accidental” roles we herein characterized A. thaliana steroid reductase 1 (AtStR1) and compared it to A. thaliana steroid reductase 2 (AtStR2). We used A. thaliana Col-0 wildtype plants as well as VEP1 knockout mutants and VEP1 knockout mutants overexpressing either AtStR1 or AtStR2 to investigate the effects on vein patterning and on the stress response after treatment with methyl vinyl ketone (MVK). Our results added evidence to the assumption that AtStR1 and AtStR2, as well as PRISEs in general, play specific roles in stress and defense situations and may be responsible for sudden metabolic shifts.

Identification of key sites determining the cofactor specificity and improvement of catalytic activity of a steroid 5β-reductase from Capsella rubella

Progesterone 5β-reductases (P5βRs) are involved in 5β-cardenolide formation by stereo-specific reduction of the △^4,5 double bond of steroid precursors. In this study a steroid 5β-reductase was identified in Capsella rubella (CrSt5βR1) and its function in steroid 5β-reduction was validated experimentally. CrSt5βR1 is capable of enantioselectively reducing the activated CC bond of broad substrates such as steroids and enones by using NADPH as a cofactor and therefore has the potential as a biocatalyst in organic synthesis. However, for industrial purposes the cheaper NADH is the preferred cofactor. By applying rational design based on literature and complementary mutagenesis strategies, we successfully identified two key amino acid residues determining the cofactor specificity of the enzyme. The R63 K mutation enables the enzyme to convert progesterone to 5β-pregnane-3,20-dione with NADH as cofactor, whereas the wild-type CrSt5βR1 is strictly NADPH-dependent. By further introducing the R64H mutation, the double mutant R63K_R64H of CrSt5βR1 was shown to increase enzymatic activity by13.8-fold with NADH as a cofactor and to increase the NADH/NADPH conversion ratio by 10.9-fold over the R63 K single mutant. This finding was successfully applied to change the cofactor specificity and to improve activity of other members of the same enzyme family, AtP5βR and DlP5βR. CrSt5βR1 mutants are expected to have the potential for biotechnological applications in combination with the well-established NADH regeneration systems.

A draft genome and transcriptome of common milkweed (Asclepias syriaca) as resources for evolutionary, ecological, and molecular studies in milkweeds and Apocynaceae

Milkweeds (Asclepias) are used in wide-ranging studies including floral development, pollination biology, plant-insect interactions and co-evolution, secondary metabolite chemistry, and rapid diversification. We present a transcriptome and draft nuclear genome assembly of the common milkweed, Asclepias syriaca. This reconstruction of the nuclear genome is augmented by linkage group information, adding to existing chloroplast and mitochondrial genomic resources for this member of the Apocynaceae subfamily Asclepiadoideae. The genome was sequenced to 80.4× depth and the draft assembly contains 54,266 scaffolds ≥1 kbp, with N50 = 3,415 bp, representing 37% (156.6 Mbp) of the estimated 420 Mbp genome. A total of 14,474 protein-coding genes were identified based on transcript evidence, closely related proteins, and ab initio models, and 95% of genes were annotated. A large proportion of gene space is represented in the assembly, with 96.7% of Asclepias transcripts, 88.4% of transcripts from the related genus Calotropis, and 90.6% of proteins from Coffea mapping to the assembly. Scaffolds covering 75 Mbp of the Asclepias assembly formed 11 linkage groups. Comparisons of these groups with pseudochromosomes in Coffea found that six chromosomes show consistent stability in gene content, while one may have a long history of fragmentation and rearrangement. The progesterone 5β-reductase gene family, a key component of cardenolide production, is likely reduced in Asclepias relative to other Apocynaceae. The genome and transcriptome of common milkweed provide a rich resource for future studies of the ecology and evolution of a charismatic plant family.

Biosynthetic approach to combine the first steps of cardenolide formation in Saccharomyces cerevisiae

A yeast expression plasmid was constructed containing a cardenolide biosynthetic module, referred to as CARD II, using the AssemblX toolkit, which enables the assembly of large DNA constructs. The genes cloned into the vector were (a) a Δ⁵‐3β‐hydroxysteroid dehydrogenase gene from Digitalis lanata, (b) a steroid Δ⁵‐isomerase gene from Comamonas testosteronii, (c) a mutated steroid‐5β‐reductase gene from Arabidopsis thaliana, and (d) a steroid 21‐hydroxylase gene from Mus musculus. A second plasmid bearing an ADR/ADX fusion gene from Bos taurus was also constructed. A Saccharomyces cerevisiae strain bearing these two plasmids was generated. This strain, termed “CARD II yeast”, was capable of producing 5β‐pregnane‐3β,21‐diol‐20‐one, a central intermediate in 5β‐cardenolide biosynthesis, starting from pregnenolone which was added to the culture medium. Using this approach, five consecutive steps in cardenolide biosynthesis were realized in baker's yeast.

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.

De novo transcriptome analysis deciphered polyoxypregnane glycoside biosynthesis pathway in Gymnema sylvestre

Gymnema sylvestre is an important medicinal plant containing antidiabetic activity. Through de novo transcriptomic study, the pathways of polyoxypregnane glycosides were explored and candidate genes of these pathways were identified in G. sylvestre. High-quality raw reads were assembled into transcripts which resulted in 193,615 unigenes. These unigenes further decoded 58,274 coding DNA sequences (CDSs). Functional annotation of predicted CDSs was carried out using the protein databases, i.e., NCBI's non-redundant, Uniprot and Pfam. Eukaryotic orthologous group (KOG) classification and transcription factor analysis has revealed most CDS-enriched categories as "Signal transduction mechanism" and "Basic Helix loop helix" (bHLH) transcription factor family, respectively. A total of 16,569 CDSs were assigned minimum one Gene Ontology (GO) term. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis disclosed 235 CDSs which represented total 27 genes of pregnane glycoside pathways and 19 CDSs represented 10 important enzymes of polyoxypregnane glycoside biosynthesis, i.e., sterol 24-C-methyltransferase, cycloeucalenol cycloisomerase, Δ¹⁴-sterol reductase, C-8,7 sterol isomerase, sterol methyltransferase 2, C-5 sterol desaturase, sterol Δ⁷ reductase, Δ²⁴ sterol reductase, 3β-hydroxysteroid dehydrogenase and progesterone 5β reductase (5βPOR). This transcriptome analysis provided an important resource for future functional genomic studies in G. sylvestre.

Identification of iridoid synthases from Nepeta species: Iridoid cyclization does not determine nepetalactone stereochemistry

Nepetalactones are iridoid monoterpenes with a broad range of biological activities produced by plants in the Nepeta genus. However, none of the genes for nepetalactone biosynthesis have been discovered. Here we report the transcriptomes of two Nepeta species, each with distinctive profiles of nepetalactone stereoisomers. As a starting point for investigation of nepetalactone biosynthesis in Nepeta, these transcriptomes were used to identify candidate genes for iridoid synthase homologs, an enzyme that has been shown to form the core iridoid skeleton in several iridoid producing plant species. Iridoid synthase homologs identified from the transcriptomes were cloned, heterologously expressed, and then assayed with the 8-oxogeranial substrate. These experiments revealed that catalytically active iridoid synthase enzymes are present in Nepeta, though there are unusual mutations in key active site residues. Nevertheless, these enzymes exhibit similar catalytic activity and product profile compared to previously reported iridoid synthases from other plants. Notably, four nepetalactone stereoisomers with differing stereochemistry at the 4α and 7α positions - which are generated during the iridoid synthase reaction - are observed at different ratios in various Nepeta species. This work strongly suggests that the variable stereochemistry at these 4α and 7α positions of nepetalactone diastereomers is established further downstream in the iridoid pathway in Nepeta. Overall, this work provides a gateway into the biosynthesis of nepetalactones in Nepeta.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Cloning and Characterization of Two Iridoid Synthase Homologs from Swertia Mussotii

Swertia mussotii is an important medicinal plant found on the Qinghai Tibetan Plateau that has great economic and medicinal value. This plant has enjoyed a long history of use as a curative for hepatitis. The biological activity of secoiridoids, including gentiopicroside and swertiamarin, has been mainly tested for its anti-hepatitis effects. Here, we identify two candidate genes (SmIS1 and SmIS2) that are homologues of iridoid synthase and that are components of the secoiridoid pathway in S. mussotii. Using sequencing and phylogenetic analyses, we confirm that SmIS1 and SmIS2 contain six conserved short-chain dehydrogenases/reductase (SDR) motifs and thus belong to the P5βRs group. The two purified Escherichia coli-expressed proteins reduced 8-oxogeranial to both nepetalactol and iridodials. A comparison of the kinetic parameters of SmIS1 and SmIS2 recombinant proteins revealed that SmIS2 has a lower affinity than SmIS1 for 8-oxogeranial. Transcript levels of the two genes were analysed in three different tissues of S. mussotii using semi-quantitative RT-PCR and RT-qPCR. SmIS1 and SmIS2 expression levels were more abundant in leaves and stems. This investigation adds to our knowledge of P5βRs genes in the secoiridoid synthesis pathway and provides candidate genes for genetically improving S. mussotii by enhancing secondary metabolite production.

Identification of a Noroxomaritidine Reductase with Amaryllidaceae Alkaloid Biosynthesis Related Activities

Amaryllidaceae alkaloids are a large group of plant natural products with over 300 documented structures and diverse biological activities. Several groups of Amaryllidaceae alkaloids including the hemanthamine- and crinine-type alkaloids show promise as anticancer agents. Two reduction reactions are required for the production of these compounds: the reduction of norcraugsodine to norbelladine and the reduction of noroxomaritidine to normaritidine, with the enantiomer of noroxomaritidine dictating whether the derivatives will be the crinine-type or hemanthamine-type. It is also possible for the carbon-carbon double bond of noroxomaritidine to be reduced, forming the precursor for maritinamine or elwesine depending on the enantiomer reduced to an oxomaritinamine product. In this study, a short chain alcohol dehydrogenase/reductase that co-expresses with the previously discovered norbelladine 4'-O-methyltransferase from Narcissus sp. and Galanthus spp. was cloned and expressed in Escherichia coli Biochemical analyses and x-ray crystallography indicates that this protein functions as a noroxomaritidine reductase that forms oxomaritinamine from noroxomaritidine through a carbon-carbon double bond reduction. The enzyme also reduces norcraugsodine to norbelladine with a 400-fold lower specific activity. These studies identify a missing step in the biosynthesis of this pharmacologically important class of plant natural products.

Identification and Characterization of the Iridoid Synthase Involved in Oleuropein Biosynthesis in Olive (Olea europaea) Fruits

The secoiridoids are the main class of specialized metabolites present in olive (Olea europaea L.) fruit. In particular, the secoiridoid oleuropein strongly influences olive oil quality because of its bitterness, which is a desirable trait. In addition, oleuropein possesses a wide range of pharmacological properties, including antioxidant, anti-inflammatory, and anti-cancer activities. In accordance, obtaining high oleuropein varieties is a main goal of molecular breeding programs. Here we use a transcriptomic approach to identify candidate genes belonging to the secoiridoid pathway in olive. From these candidates, we have functionally characterized the olive homologue of iridoid synthase (OeISY), an unusual terpene cyclase that couples an NAD (P)H-dependent 1,4-reduction step with a subsequent cyclization, and we provide evidence that OeISY likely generates the monoterpene scaffold of oleuropein in olive fruits. OeISY, the first pathway gene characterized for this type of secoiridoid, is a potential target for breeding programs in a high value secoiridoid-accumulating species.

Structure of iridoid synthase in complex with NADP(+)/8-oxogeranial reveals the structural basis of its substrate specificity

Iridoid synthase (IS), as a vegetal enzyme belonging to the short-chain dehydrogenase/reductase (SDR) superfamily, produces the ring skeletons for downstream alkaloids with various pharmaceutical activities, including the commercially available antineoplastic agents, vinblastine and vincristine. Here, we present the crystal structures of IS in apo state and in complex with NADP(+)/8-oxogeranial, exhibiting an active center that lacks the classical Tyr/Lys/Ser triad spatially conserved in SDRs, with only the catalytically critical function of triad tyrosine remained in Tyr178. In consistent, mutation of Tyr178 to a phenylalanine residue significantly abolished the catalytic activity of IS. Within the substrate binding pocket, the linear-shaped 8-oxogeranial adopts an entirely extended conformation with its two aldehyde ends hydrogen-bonded to Tyr178-OH and Ser349-OH, respectively. In addition, the intermediate carbon chain of bound substrate is harbored by a well-ordered hydrophobic scaffold, involving residues Ile145, Phe149, Leu203, Met213, Phe342, Ile345 and Leu352. Mutagenesis studies showed that both Ser349 and the hydrophobic residues around are determinant to the substrate specificity and, consequently, the catalytic activity of IS. In contrast, the Gly150-Pro160 loop previously proposed as a factor involved in substrate binding might have very limited contribution, because the deletion of residues Ile151-His161 has only slight influence on the catalytic activity. We believe that the present work will help to elucidate the substrate specificity of IS and to integrate its detailed catalytic mechanism.

Structural determinants of reductive terpene cyclization in iridoid biosynthesis

The carbon skeleton of ecologically and pharmacologically important iridoid monoterpenes is formed in a reductive cyclization reaction unrelated to canonical terpene cyclization. Here we report the crystal structure of the recently discovered iridoid cyclase (from Catharanthus roseus) bound to a mechanism-inspired inhibitor that illuminates substrate binding and catalytic function of the enzyme. Key features that distinguish iridoid synthase from its close homolog progesterone 5β-reductase are highlighted.

Structures of Iridoid Synthase from Cantharanthus roseus with Bound NAD(+) , NADPH, or NAD(+) /10-Oxogeranial: Reaction Mechanisms

Structures of the iridoid synthase nepetalactol synthase in the presence of NAD(+) , NADPH or NAD(+) /10-oxogeranial were solved. The 10-oxogeranial substrate binds in a transoid-O1-C3 conformation and can be reduced by hydride addition to form the byproduct S-10-oxo-citronellal. Tyr178 Oζ is positioned 2.5 Å from the substrate O1 and provides the second proton required for reaction. Nepetalactol product formation requires rotation about C1-C2 to form the cisoid isomer, leading to formation of the cis-enolate, together with rotation about C4-C5, which enables cyclization and lactol production. The structure is similar to that of progesterone-5β-reductase, with almost identical positioning of NADP, Lys146(147), Tyr178(179), and F342(343), but only Tyr178 and Phe342 appear to be essential for activity. The transoid 10-oxogeranial structure also serves as a model for β-face hydride attack in progesterone 5β-reductases and is of general interest in the context of asymmetric synthesis.

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.

Steroidogenesis in plants--Biosynthesis and conversions of progesterone and other pregnane derivatives

In plants androstanes, estranes, pregnanes and corticoids have been described. Sometimes 17β-estradiol, androsterone, testosterone or progesterone were summarized as sex hormones. These steroids influence plant development: cell divisions, root and shoot growth, embryo growth, flowering, pollen tube growth and callus proliferation. First reports on the effect of applicated substances and of their endogenous occurrence date from the early twenties of the last century. This caused later on doubts on the identity of the compounds. Best investigated is the effect of progesterone. Main steps of the progesterone biosynthetic pathway have been analyzed in Digitalis. Cholesterol-side-chain-cleavage, pregnenolone and progesterone formation as well as the stereospecific reduction of progesterone are described and the corresponding enzymes are presented. Biosynthesis of androstanes, estranes and corticoids is discussed. Possible progesterone receptors and physiological reactions on progesterone application are reviewed.

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.

Terpenoid biosynthesis off the beaten track: unconventional cyclases and their impact on biomimetic synthesis

Terpene and terpenoid cyclizations are counted among the most complex chemical reactions occurring in nature and contribute crucially to the tremendous structural diversity of this largest family of natural products. Many studies were conducted at the chemical, genetic, and biochemical levels to gain mechanistic insights into these intriguing reactions that are catalyzed by terpene and terpenoid cyclases. A myriad of these enzymes have been characterized. Classical textbook knowledge divides terpene/terpenoid cyclases into two major classes according to their structure and reaction mechanism. However, recent discoveries of novel types of terpenoid cyclases illustrate that nature's enzymatic repertoire is far more diverse than initially thought. This Review outlines novel terpenoid cyclases that are out of the ordinary.

Conversion of substrate analogs suggests a Michael cyclization in iridoid biosynthesis

The core structure of the iridoid monoterpenes is formed by a unique cyclization reaction. The enzyme that catalyzes this reaction, iridoid synthase, is mechanistically distinct from other terpene cyclases. Here we describe the synthesis of two substrate analogs to probe the mechanism of iridoid synthase. Enzymatic assay of these substrate analogs along with clues from the product profile of the native substrate strongly suggest that iridoid synthase utilizes a Michael reaction to achieve cyclization. This improved mechanistic understanding will facilitate the exploitation of the potential of iridoid synthase to synthesize new cyclic compounds from nonnatural substrates.

5β-Reduced steroids and human Δ(4)-3-ketosteroid 5β-reductase (AKR1D1)

5β-Reduced steroids are non-planar steroids that have a 90° bend in their structure to create an A/B cis-ring junction. This novel property is required for bile-acids to act as emulsifiers, but in addition 5β-reduced steroids have remarkable physiology and may act as potent tocolytic agents, endogenous cardiac glycosides, neurosteroids, and can act as ligands for orphan and membrane bound receptors. In humans there is only a single 5β-reductase gene AKR1D1, which encodes Δ(4)-3-ketosteroid-5β-reductase (AKR1D1). This enzyme is a member of the aldo-keto reductase superfamily, but possesses an altered catalytic tetrad, in which Glu120 replaces the conserved His residue. This predominant liver enzyme generates all 5β-dihydrosteroids in the C19-C27 steroid series. Mutations exist in the AKR1D1 gene, which result in loss of protein stability and are causative in bile-acid deficiency.

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.

Complexity generation in fungal polyketide biosynthesis: a spirocycle-forming P450 in the concise pathway to the antifungal drug griseofulvin

Griseofulvin (1) is a spirocyclic fungal natural product used in treatment of fungal dermatophytes. Formation of the spirocycle, or the grisan scaffold, from a benzophenone precursor is critical for the activity of 1. In this study, we have systematically characterized each of the biosynthetic enzymes related to the biogenesis of 1, including the characterization of a new polyketide synthase GsfA that synthesizes the benzophenone precursor and a cytochrome P450 GsfF that performs oxidative coupling between the orcinol and the phloroglucinol rings to yield the grisan structure. Notably, the finding of GsfF is in sharp contrast to the copper-dependent dihydrogeodin oxidase that performs a similar reaction in the geodin biosynthetic pathway. The biosynthetic knowledge enabled the in vitro total biosynthesis of 1 from malonyl-CoA using all purified enzyme components. This work therefore completely maps out the previously unresolved enzymology of the biosynthesis of a therapeutically relevant natural product.

An alternative route to cyclic terpenes by reductive cyclization in iridoid biosynthesis

The iridoids comprise a large family of distinctive bicyclic monoterpenes that possess a wide range of pharmacological activities, including anticancer, anti-inflammatory, antifungal and antibacterial activities. Additionally, certain iridoids are used as sex pheromones in agriculturally important species of aphids, a fact that has underpinned innovative and integrated pest management strategies. To harness the biotechnological potential of this natural product class, the enzymes involved in the biosynthetic pathway must be elucidated. Here we report the discovery of iridoid synthase, a plant-derived enzyme that generates the iridoid ring scaffold, as evidenced by biochemical assays, gene silencing, co-expression analysis and localization studies. In contrast to all known monoterpene cyclases, which use geranyl diphosphate as substrate and invoke a cationic intermediate, iridoid synthase uses the linear monoterpene 10-oxogeranial as substrate and probably couples an initial NAD(P)H-dependent reduction step with a subsequent cyclization step via a Diels-Alder cycloaddition or a Michael addition. Our results illustrate how a short-chain reductase was recruited as cyclase for the production of iridoids in medicinal plants. Furthermore, we highlight the prospects of using unrelated reductases to generate artificial cyclic scaffolds. Beyond the recognition of an alternative biochemical mechanism for the biosynthesis of cyclic terpenes, we anticipate that our work will enable the large-scale heterologous production of iridoids in plants and microorganisms for agricultural and pharmaceutical applications.

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.

Conversion of human steroid 5β-reductase (AKR1D1) into 3β-hydroxysteroid dehydrogenase by single point mutation E120H: example of perfect enzyme engineering

Human aldo-keto reductase 1D1 (AKR1D1) and AKR1C enzymes are essential for bile acid biosynthesis and steroid hormone metabolism. AKR1D1 catalyzes the 5β-reduction of Δ(4)-3-ketosteroids, whereas AKR1C enzymes are hydroxysteroid dehydrogenases (HSDs). These enzymes share high sequence identity and catalyze 4-pro-(R)-hydride transfer from NADPH to an electrophilic carbon but differ in that one residue in the conserved AKR catalytic tetrad, His(120) (AKR1D1 numbering), is substituted by a glutamate in AKR1D1. We find that the AKR1D1 E120H mutant abolishes 5β-reductase activity and introduces HSD activity. However, the E120H mutant unexpectedly favors dihydrosteroids with the 5α-configuration and, unlike most of the AKR1C enzymes, shows a dominant stereochemical preference to act as a 3β-HSD as opposed to a 3α-HSD. The catalytic efficiency achieved for 3β-HSD activity is higher than that observed for any AKR to date. High resolution crystal structures of the E120H mutant in complex with epiandrosterone, 5β-dihydrotestosterone, and Δ(4)-androstene-3,17-dione elucidated the structural basis for this functional change. The glutamate-histidine substitution prevents a 3-ketosteroid from penetrating the active site so that hydride transfer is directed toward the C3 carbonyl group rather than the Δ(4)-double bond and confers 3β-HSD activity on the 5β-reductase. Structures indicate that stereospecificity of HSD activity is achieved because the steroid flips over to present its α-face to the A-face of NADPH. This is in contrast to the AKR1C enzymes, which can invert stereochemistry when the steroid swings across the binding pocket. These studies show how a single point mutation in AKR1D1 can introduce HSD activity with unexpected configurational and stereochemical preference.

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

Erysimum is a genus of the Brassicaceae family closely related to the genus Arabidopsis. Several Erysimum species accumulate 5β-cardenolides. Progesterone 5β-reductases (P5βRs) first described in Digitalis species are thought to be involved in 5β-cardenolide biosynthesis. P5βRs belong to the dehydrogenase/reductase super-family of proteins. A full length cDNA clone encoding a P5βR was isolated from Erysimum crepidifolium leaves by 5'/3' RACE-PCR (termed EcP5βR). Subsequently, the P5βR cDNAs of another nine Erysimum species were amplified by RT-PCR using 5' and 3' end primers deduced from the EcP5βR cDNA. The EcP5βR cDNA is 1170bp long and encodes for 389 amino acids. The EcP5βR cDNA was ligated into the vector pQE 30 UA and the recombinant His-tagged protein (termed rEcP5βR) was over-expressed in Escherichia coli and purified by Ni-chelate affinity chromatography. Kinetic constants were determined for progesterone, 2-cyclohexen-1-one, isophorone, and NADPH. The by far highest specificity constant (k(cat)K(M)⁻¹) was estimated for 2-cyclohexen-1-one indicating that this monocyclic enone may be more related to the natural substrate of the enzyme than progesterone. The atomic structure of rEcP5βR was modelled using the crystal structure of P5βR from Digitalis lanata 2V6G as the template. All sequence motifs specific for SDRs as well as the NFYYxxED motif typical for P5βR-like enzymes were present and the protein sequence fitted into the template smoothly.

The Vein Patterning 1 (VEP1) gene family laterally spread through an ecological network

Lateral gene transfer (LGT) is a major evolutionary mechanism in prokaryotes. Knowledge about LGT— particularly, multicellular— eukaryotes has only recently started to accumulate. A widespread assumption sees the gene as the unit of LGT, largely because little is yet known about how LGT chances are affected by structural/functional features at the subgenic level. Here we trace the evolutionary trajectory of VEin Patterning 1, a novel gene family known to be essential for plant development and defense. At the subgenic level VEP1 encodes a dinucleotide-binding Rossmann-fold domain, in common with members of the short-chain dehydrogenase/reductase (SDR) protein family. We found: i) VEP1 likely originated in an aerobic, mesophilic and chemoorganotrophic α-proteobacterium, and was laterally propagated through nets of ecological interactions, including multiple LGTs between phylogenetically distant green plant/fungi-associated bacteria, and five independent LGTs to eukaryotes. Of these latest five transfers, three are ancient LGTs, implicating an ancestral fungus, the last common ancestor of land plants and an ancestral trebouxiophyte green alga, and two are recent LGTs to modern embryophytes. ii) VEP1's rampant LGT behavior was enabled by the robustness and broad utility of the dinucleotide-binding Rossmann-fold, which provided a platform for the evolution of two unprecedented departures from the canonical SDR catalytic triad. iii) The fate of VEP1 in eukaryotes has been different in different lineages, being ubiquitous and highly conserved in land plants, whereas fungi underwent multiple losses. And iv) VEP1-harboring bacteria include non-phytopathogenic and phytopathogenic symbionts which are non-randomly distributed with respect to the type of harbored VEP1 gene. Our findings suggest that VEP1 may have been instrumental for the evolutionary transition of green plants to land, and point to a LGT-mediated ‘Trojan Horse’ mechanism for the evolution of bacterial pathogenesis against plants. VEP1 may serve as tool for revealing microbial interactions in plant/fungi-associated environments.

Digitalis purpurea P5 beta R2, encoding steroid 5 beta-reductase, is a novel defense-related gene involved in cardenolide biosynthesis

The stereospecific 5 beta-reduction of progesterone is a required step for cardiac glycoside biosynthesis in foxglove plants. Recently, we have isolated the gene P5 beta R, and here we investigate the function and regulation of P5 beta R2, a new progesterone 5 beta-reductase gene from Digitalis purpurea. P5 beta R2 cDNA was isolated from a D. purpurea cDNA library and further characterized at the biochemical, structural and physiological levels. Like P5 beta R, P5 beta R2 catalyzes the 5 beta-reduction of the Delta(4) double bond of several steroids and is present in all plant organs. Under stress conditions or on treatment with chemical elicitors, P5 beta R expression does not vary, whereas P5 beta R2 is highly responsive. P5 beta R2 expression is regulated by ethylene and hydrogen peroxide. The correlation between P5 beta R2 expression and cardenolide formation demonstrates the key role of this gene in cardenolide biosynthesis, and therefore in the chemical defense of foxglove plants.

Cyclopiazonic acid biosynthesis in Aspergillus sp.: characterization of a reductase-like R* domain in cyclopiazonate synthetase that forms and releases cyclo-acetoacetyl-L-tryptophan

The fungal neurotoxin alpha-cyclopiazonic acid (CPA), a nanomolar inhibitor of Ca2+-ATPase, has a pentacyclic indole tetramic acid scaffold that arises from one molecule of tryptophan, acetyl-CoA, malonyl-CoA, and dimethylallyl pyrophosphate by consecutive action of three enzymes, CpaS, CpaD, and CpaO. CpaS is a hybrid, two module polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) that makes and releases cyclo-acetoacetyl-L-tryptophan (cAATrp), the tetramic acid that serves as substrate for subsequent prenylation and oxidative cyclization to the five ring CPA scaffold. The NRPS module in CpaS has a predicted four-domain organization of condensation, adenylation, thiolation, and reductase* (C-A-T-R*), where R* lacks the critical Ser-Tyr-Lys catalytic triad of the short chain dehydrogenase/reductase (SDR) superfamily. By heterologous overproduction in Escherichia coli of the 56 kDa Aspergillus flavus CpaS TR* didomain and the single T and R* domains, we demonstrate that CpaS catalyzes a Dieckmann-type cyclization on the N-acetoacetyl-Trp intermediate bound in thioester linkage to the phosphopantetheinyl arm of the T domain to form and release cAATrp. This occurs without any participation of NAD(P)H, so R* does not function as a canonical SDR family member. Use of the T and R* domains in in trans assays enabled multiple turnovers and evaluation of specific mutants. Mutation of the D3803 residue in the R* domain, conserved in other fungal tetramate synthetases, abolished activity both in in trans and in cis (TR*) activity assays. It is likely that cyclization of beta-ketoacylaminoacyl-S-pantetheinyl intermediates to released tetramates represents a default cyclization/release route for redox-incompetent R* domains embedded in NRPS assembly lines.

Homology modeling and site-directed mutagenesis reveal catalytic key amino acids of 3beta-hydroxysteroid-dehydrogenase/C4-decarboxylase from Arabidopsis

Sterols become functional only after removal of the two methyl groups at C4 by a membrane-bound multienzyme complex including a 3beta-hydroxysteroid-dehydrogenase/C4-decarboxylase (3betaHSD/D). We recently identified Arabidopsis (Arabidopsis thaliana) 3betaHSD/D as a bifunctional short-chain dehydrogenase/reductase protein. We made use of three-dimensional homology modeling to identify key amino acids involved in 4alpha-carboxy-sterol and NAD binding and catalysis. Key amino acids were subjected to site-directed mutagenesis, and the mutated enzymes were expressed and assayed both in vivo and in vitro in an erg26 yeast strain defective in 3betaHSD/D. We show that tyrosine-159 and lysine-163, which are oriented near the 3beta-hydroxyl group of the substrate in the model, are essential for the 3betaHSD/D activity, consistent with their involvement in the initial dehydrogenation step of the reaction. The essential arginine-326 residue is predicted to form a salt bridge with the 4alpha-carboxyl group of the substrate, suggesting its involvement both in substrate binding and in the decarboxylation step. The essential aspartic acid-39 residue is in close contact with the hydroxyl groups of the adenosine-ribose ring of NAD+, in good agreement with the strong preference of 3betaHSD/D for NAD+. Data obtained with serine-133 mutants suggest close proximity between the serine-133 residue and the C4beta domain of the bound sterol. Based on these data, we propose a tentative mechanism for 3betaHSD/D activity. This study provides, to our knowledge, the first data on the three-dimensional molecular interactions of an enzyme of the postoxidosqualene cyclase sterol biosynthesis pathway with its substrate. The implications of our findings for studying the roles of C4-alkylated sterol precursors in plant development are discussed.

Aldo-keto reductases in which the conserved catalytic histidine is substituted

Aldo-keto reductases (AKRs) are a major superfamily of monomeric NADPH-dependent carbonyl oxidoreductases. They are characterized by an (alpha/beta)(8)-barrel structure, which at its base contains a conserved catalytic tetrad of Tyr, Lys, His and Asp. Two AKR subfamilies contain other residues substituted for the catalytic His and perform different functions. First, the steroid 5beta-reductase (AKR1D1), which reduces CC double bonds instead of carbonyl groups, has a Glu substituted for His. Second, the Kvbeta subunits (AKR6A3, AKR6A5 and AKR6A9) which modulate opening of the voltage-gated potassium channel (Kv1) by oxidizing NADPH, have an Asn substituted for the His. Previously, we noted that conserved catalytic residues in AKRs perform similar functions in the short-chain dehydrogenases (SDRs). With the availability of crystal structures of AKR1D1 and two SDRs that catalyze double-bond reduction reactions, Digitalis steroid 5beta-reductase and 2,4-dienoyl-CoA reductase, we have compared their active sites to outline the features that govern whether 1,2-, 1,4- or 1,6-hydride transfer occurs.

Crystal structure of human liver Delta4-3-ketosteroid 5beta-reductase (AKR1D1) and implications for substrate binding and catalysis

AKR1D1 (steroid 5beta-reductase) reduces all Delta(4)-3-ketosteroids to form 5beta-dihydrosteroids, a first step in the clearance of steroid hormones and an essential step in the synthesis of all bile acids. The reduction of the carbon-carbon double bond in an alpha,beta-unsaturated ketone by 5beta-reductase is a unique reaction in steroid enzymology because hydride transfer from NADPH to the beta-face of a Delta(4)-3-ketosteroid yields a cis-A/B-ring configuration with an approximately 90 degrees bend in steroid structure. Here, we report the first x-ray crystal structure of a mammalian steroid hormone carbon-carbon double bond reductase, human Delta(4)-3-ketosteroid 5beta-reductase (AKR1D1), and its complexes with intact substrates. We have determined the structures of AKR1D1 complexes with NADP(+) at 1.79- and 1.35-A resolution (HEPES bound in the active site), NADP(+) and cortisone at 1.90-A resolution, NADP(+) and progesterone at 2.03-A resolution, and NADP(+) and testosterone at 1.62-A resolution. Complexes with cortisone and progesterone reveal productive substrate binding orientations based on the proximity of each steroid carbon-carbon double bond to the re-face of the nicotinamide ring of NADP(+). This orientation would permit 4-pro-(R)-hydride transfer from NADPH. Each steroid carbonyl accepts hydrogen bonds from catalytic residues Tyr(58) and Glu(120). The Y58F and E120A mutants are devoid of activity, supporting a role for this dyad in the catalytic mechanism. Intriguingly, testosterone binds nonproductively, thereby rationalizing the substrate inhibition observed with this particular steroid. The locations of disease-linked mutations thought to be responsible for bile acid deficiency are also revealed.