Plant progesterone 5β-reductase is not homologous to the animal enzyme. Molecular evolutionary characterization of P5βR from Digitalis purpurea

Abstract

Plants of the genus Digitalis produce cardiac glycosides, i.e. digoxin, which are widely used for congestive heart failure. Progesterone 5β-reductase (P5βR) is a key enzyme in the biosynthesis of these natural products. Here, we have carried out the purification and partial amino acid sequencing of the native P5βR from foxglove ( Digitalis purpurea), and isolated a cDNA encoding this enzyme. Similarly to other steroid 5β-reductases, the recombinant P5βR catalyzes the stereospecific reduction of the Δ 4-double bond of several steroids with a 3-oxo,Δ 4,5 structure. The gene encoding P5βR is expressed in all plant organs, and maximally transcribed in leaves and mature flowers. P5βR belongs to the short-chain dehydrogenase/reductase (SDR) superfamily, bearing no structural homology to its mammalian counterpart, which is a member of the aldo-keto reductase (AKR) superfamily. A similar situation occurs with 3β-hydroxy-Δ 5-steroid dehydrogenase (3βHSD), the gene immediately preceding P5βR in the cardenolide pathway, which suggests that the entire route has evolved independently in animals and plants. P5βR is retained only in plants, where it is ubiquitous, and a few distantly related bacterial lineages after its diversification from the last universal common ancestor. Evolutionary conserved changes in its putative active site suggest that plant P5βR is a member of a novel subfamily of extended SDRs, or a new SDR family.