Expression of 3beta-HSD and P5betaR, genes respectively coding for Delta5-3beta-hydroxysteroid dehydrogenase and progesterone 5beta-reductase, in leaves and cell cultures of Digitalis lanata EHRH.

Plants of the genus Digitalis produce 5 beta-cardenolides that are used in the therapy of cardiac insufficiency in humans. 3 beta-Hydroxysteroid dehydrogenase (3 beta-HSD) and progesterone 5 beta-reductase (P5 betaR) are both supposed to be important enzymes in the biosynthesis of these natural products. Activity and gene expression were demonstrated for both enzymes in cardenolide-accumulating leaves of Digitalis lanata but also in cardenolide-free permanent cell suspension cultures initiated from D. lanata leaf tissue. Enzyme activities were determined and quantified by HPLC and GC-MS methods. Expression of the respective genes, namely AY585867.1 (P5betaR gene) and DQ466890.1 (3beta-HSD gene), was made evident by real-time polymerase chain reaction (qPCR) analysis. We demonstrate for the first time that the P5betaR gene, encoding an enzyme described as a key enzyme in cardenolide biosynthesis, is also expressed in cardenolide-free tissues of cardenolide-containing plants.

Molecular cloning and heterologous expression of progesterone 5beta-reductase from Digitalis lanata Ehrh.

A full-length cDNA clone that encodes progesterone 5beta-reductase (5beta-POR) was isolated from Digitalis lanata leaves. The reading frame of the 5beta-POR gene is 1170 nucleotides corresponding to 389 amino acids. For expression, a Sph1/Sal1 5beta-POR fragment was cloned into the pQE vector and was transformed into Escherichia coli strain M15[pREP4]. The recombinant gene was functionally expressed and the recombinant enzyme was characterized. The K(m) and v(max) values for the putative natural substrate progesterone were calculated to be 0.120 mM and 45 nkat mg(-1) protein, respectively. Only 5beta-pregnane-3,20-dione but not its alpha-isomer was formed when progesterone was used as the substrate. Kinetic constants for cortisol, cortexone, 4-androstene-3,17-dione and NADPH were also determined. The molecular organization of the 5beta-POR gene in D. lanata was determined by Southern blot analysis. The 5beta-POR is highly conserved within the genus Digitalis and the respective genes and proteins share considerable homology to putative progesterone reductases from other plant species.

Crystallization and preliminary crystallographic analysis of selenomethionine-labelled progesterone 5beta-reductase from Digitalis lanata Ehrh.

Progesterone 5beta-reductase (5beta-POR) catalyzes the reduction of progesterone to 5beta-pregnane-3,20-dione and is the first stereospecific enzyme in the putative biosynthetic pathway of Digitalis cardenolides. Selenomethionine-derivatized 5beta-POR from D. lanata was successfully overproduced and crystallized. The crystals belong to space group P4(3)2(1)2, with unit-cell parameters a = 71.73, c = 186.64 A. A MAD data set collected at 2.7 A resolution allowed the identification of six out of eight possible Se-atom positions. A first inspection of the MAD-phased electron-density map shows that 5beta-POR is a Rossmann-type reductase and the quality of the map is such that it is anticipated that a complete atomic model of 5beta-POR will readily be built.

The crystal structure of progesterone 5beta-reductase from Digitalis lanata defines a novel class of short chain dehydrogenases/reductases.

Progesterone 5beta-reductase (5beta-POR) catalyzes the stereospecific reduction of progesterone to 5beta-pregnane-3,20-dione and is a key enzyme in the biosynthetic pathway of cardenolides in Digitalis (foxglove) plants. Sequence considerations suggested that 5beta-POR is a member of the short chain dehydrogenase/reductase (SDR) family of proteins but at the same time revealed that the sequence motifs that in standard SDRs contain the catalytically important residues are missing. Here we present crystal structures of 5beta-POR from Digitalis lanata in complex with NADP(+) at 2.3A and without cofactor bound at 2.4A resolution together with a model of a ternary complex consisting of 5beta-POR, NADP(+), and progesterone. Indeed, 5beta-POR displays the fold of an extended SDR. The architecture of the active site is, however, unprecedented because none of the standard catalytic residues are structurally conserved. A tyrosine (Tyr-179) and a lysine residue (Lys-147) are present in the active site, but they are displayed from novel positions and are part of novel sequence motifs. Mutating Tyr-179 to either alanine or phenylalanine completely abolishes the enzymatic activity. We propose that the distinct topology reflects the fact that 5beta-POR reduces a conjugated double bond in a steroid substrate via a 1-4 addition mechanism and that this requires a repositioning of the catalytically important residues. Our observation that the sequence motifs that line the active site are conserved in a number of bacterial and plant enzymes of yet unknown function leads us to the proposition that 5beta-POR defines a novel class of SDRs.

Delta 5-3beta-hydroxysteroid dehydrogenase (3 beta HSD) from Digitalis lanata. Heterologous expression and characterisation of the recombinant enzyme.

During the biosynthesis of cardiac glycosides, Delta (5)-3beta-hydroxysteroid dehydrogenase (3 beta HSD, EC 1.1.1.51) converts pregnenolone (5-pregnen-3beta-ol-20-one) to isoprogesterone (5-pregnene-3,20-dione). A 3 beta HSD gene was isolated from leaves of Digitalis lanata. It consisted of 870 nucleotides containing a 90 nucleotide long intron. A full-length cDNA clone that encodes 3 beta HSD was isolated by RT-PCR from the same source. A SPH I /KPN I 3 beta HSD cDNA was cloned into the pQE30 vector and then transferred into E. COLI strain M15[pREP4]. 3 beta HSD cDNA was functionally expressed as a His-tagged fusion protein (pQ3 beta HSD) composed of 273 amino acids (calculated molecular mass 28,561 Da). pQ3 beta HSD was purified by metal chelate affinity chromatography on Ni-NTA. Pregnenolone and other 3beta-hydroxypregnanes but not cholesterol were 3beta-oxidised by pQ3 beta HSD when NAD was used as the co-substrate. Testosterone (4-androsten-17beta-ol-3-one) was converted to 4-androstene-3,17-dione indicating that the pQ3 beta HSD has also 17beta-dehydrogenase activity. pQ3 beta HSD was able to reduce 3-keto steroids to their corresponding 3beta-hydroxy derivatives when NADH was used as the co-substrate. For comparison, 3 beta HSD genes were isolated and sequenced from another 6 species of the genus DIGITALIS. Gene structure and the deduced 3 beta HSD proteins share a high degree of similarity.

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

A full-length cDNA clone that encodes progesterone 5beta-reductase (5beta-POR, EC 1.3.1.3) was isolated from ISOPLEXIS CANARIENSIS leaves. The reading frame of the IC5beta-POR gene is 1170 nucleotides corresponding to 389 amino acids. The SPHI /SALI IC5beta-POR fragment was cloned into the pQE vector system and then transformed into ESCHERICHIA COLI strain M15[pREP4]. The gene was functionally expressed and the recombinant enzyme was characterised. K(m) and V(max) were calculated to be 0.215 mM and 46.4 nkat/mg protein, respectively, using progesterone as the substrate. Kinetic constants for cortisol, cortexone, 4-androstene-3,17-dione and NADPH were also determined. The 5beta-POR from I. CANARIENSIS shows a significant homology to the putative progesterone 5beta-reductases isolated from other plant species, such as DIGITALIS LANATA and ARABIDOPSIS THALIANA.