Molecular cloning and expression of progesterone 5beta-reductase (5beta-POR) from Isoplexis canariensis

Transcriptome and Metabolite analysis reveal candidate genes of the cardiac glycoside biosynthetic pathway from Calotropis procera

Calotropis procera is a medicinal plant of immense importance due to its pharmaceutical active components, especially cardiac glycosides (CG). As genomic resources for this plant are limited, the genes involved in CG biosynthetic pathway remain largely unknown till date. Our study on stage and tissue specific metabolite accumulation showed that CG's were maximally accumulated in stems of 3 month old seedlings. De novo transcriptome sequencing of same was done using high throughput Illumina HiSeq platform generating 44074 unigenes with average mean length of 1785 base pair. Around 66.6% of unigenes were annotated by using various public databases and 5324 unigenes showed significant match in the KEGG database involved in 133 different pathways of plant metabolism. Further KEGG analysis resulted in identification of 336 unigenes involved in cardenolide biosynthesis. Tissue specific expression analysis of 30 putative transcripts involved in terpenoid, steroid and cardenolide pathways showed a positive correlation between metabolite and transcript accumulation. Wound stress elevated CG levels as well the levels of the putative transcripts involved in its biosynthetic pathways. This result further validated the involvement of identified transcripts in CGs biosynthesis. The identified transcripts will lay a substantial foundation for further research on metabolic engineering and regulation of cardiac glycosides biosynthesis pathway genes.

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.

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.

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.

Occurrence of progesterone and related animal steroids in two higher plants

Previously, the presence of a wide variety of chemically diverse steroids has been identified in both flora and fauna. Despite the relatively small differences in chemical structures and large differences in physiological function of steroids, new discoveries indicate that plants and animals are more closely related than previously thought. In this regard, the present study gathers supporting evidence for shared phylogenetic roots of structurally similar steroids produced by these two eukaryotic taxa. Definitive proof for the presence of progesterone (3) in a vascular plant, Juglans regia, is provided. Additional evidence is gleaned from the characterization of five new plant steroids from Adonis aleppica: three 3-O-sulfated pregnenolones (6a/ b, 7), a sulfated H-5beta cardenolide, strophanthidin-3-O-sulfate (8), and spirophanthigenin (10), a novel C-18 oxygenated spirocyclic derivative of strophanthidin. The ab initio isolation and structure elucidation (NMR, MS) of these genuine minor plant steroids offers information on preparative metabolomic profiling at the ppm level and provides striking evidence for the conserved structural space of pregnanes and its congeners across the phylogenetic tree.

Synthesis of a putative substrate for malonyl-coenzyme A: 21-hydroxypregnane 21-O-malonyltransferase and development of an HPLC method for the quantification of the enzyme reaction

The butenolide ring is the main common characteristic of all cardenolides. Its formation is supposed to be initiated by the transfer of a malonyl moiety from malonyl-coenzyme A to an appropriate 21-hydroxypregnane. A new, reliable, fast and sensitive method to determine malonyl-coenzyme A: 21-hydroxypregnane 21-O-malonyltransferase activity had to be developed since previous attempts employing HPLC, TLC or GC did not prove successful. A surrogate substrate was synthesized containing a side chain resembling the sugar side chain attached to C-3 of putative cardenolide precursors and containing a chromophor allowing UV detection. 3beta-benzoyloxy-5beta-pregnane-14beta,21-dihydroxy-20-one and its 21-O-malonylated derivative were synthesized, the latter being the expected product of the enzyme reaction. The new substrate was well accepted by the enzyme. An HPLC method has been established to detect and quantify 3beta-benzoyloxy-5beta-pregnane-14beta,21-dihydroxy-20-one and its 21-O-malonylated derivative, 3beta-benzoyloxy-5beta-pregnane-14beta-hydroxy-20-one 21-O-malonylhemiester. The method was validated.