Progesterone 5␤-reductase (5␤-POR) catalyzes the stereospecific reduction of progesterone to 5␤-pregnane-3,20-dione and is a key enzyme in the biosynthetic pathway of cardenolides in Digitalis (foxglove) plants. Sequence considerations suggested that 5␤-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 5␤-POR from Digitalis lanata in complex with NADP ؉ at 2.3 Å and without cofactor bound at 2.4 Å resolution together with a model of a ternary complex consisting of 5␤-POR, NADP ؉ , and progesterone. Indeed, 5␤-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 5␤-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 5␤-POR defines a novel class of SDRs.The beneficial effects of cardenolides, also known as cardiac glycosides or cardiotonic steroids, are well documented, and they have been applied for the treatment of cardiac insufficiencies for centuries (1-3). On a molecular level, these steroids are potent inhibitors of the sodium/potassium pump (Na ϩ /K ϩ -ATPase) that is present in almost all cells in higher organisms (4). Digitalis plants are still the major source for cardenolides, and as a step in the biosynthetic pathway, the Digitalis enzyme progesterone 5␤-reductase (5␤-POR) 2 catalyzes the stereospecific NADPH-dependent reduction of the ⌬ 4 -double bond in progesterone to 5␤-pregnane-3,20-dione. Because all Digitalis cardenolides share the characteristic 5␤-configuration, the enzyme 5␤-POR catalyzes a central step during their biosynthesis (5-7).NADH/NADPH-dependent reductases as well as the related dehydrogenases, dehydratases, and epimerases can be classified into two major protein families: the (␣/␤) 8 -barrel containing aldo-keto-reductases (AKRs) (8) and the Rossman fold containing short chain dehydrogenases/reductases (SDRs) (9 -11). Additional families such as the long and medium chain dehydrogenases/reductases are related to SDRs because they share with the latter the dinucleotide-binding double Rossman fold (12)(13)(14). SDRs are about 250 residues long and form a large family with over 2000 members (15). Because their central feature consists of an all-p...

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.

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Cardiac glycosides, known ligands of the sodium pump, are widely used in the treatment of heart failure, such as digoxin and digitoxin. Besides this important activity, other biological activities, such as the antiviral activity, have been described for this group. HSV are responsible for many infections of oral, ocular and genital regions. Treatment with nucleoside analogs such as acyclovir is effective in most cases; however drug-resistance may arise due to prolonged treatment mainly in immunocompromised individuals. In this study, an antiherpes screening was performed with 65 cardenolide derivatives obtained from different sources, and one natural cardenolide, glucoevatromonoside, inhibited HSV-1 and HSV-2 replication at very low concentrations. This cardenolide showed viral inhibitory effects if added up to 12h p.i. and these effects appear to take place by the inhibition of viral proteins synthesis (ICP27, U(L)42, gB, gD), the blockage of virus release and the reduction of viral cell-to-cell spread. This compound also showed synergistic antiviral effects with acyclovir and anti-Na(+)K(+)ATPase activity, suggesting that cellular electrochemical gradient alterations might be involved in the mechanism of viral inhibition. These results suggest that cardenolides might be promising for future antiviral drug design.