The biocatalytic capacity of tropinone reductase-like enzymes from plants

Tropinone reductases (TRs) are short-chain dehydrogenases/reductases (SDRs) involved in tropane alkaloid biosynthesis. TRs reduce tropinone stereospecifically either to tropine (3a-tropanol), precursor of classical tropane alkaloids like hyoscyamine and scopolamine, or to pseudotropine (3b-tropanol), precursor of nortropane alkaloids e.g. calystegines. TRs were cloned and expressed from several tropane and nortropane alkaloid containing Solanaceae. In addition, one tropane alkaloid forming species of the Brassicaceae, Cochlearia officinalis, contains a special tropinone reductase (CoTR), which reduces tropinone to pseudotropine and tropine in vitro.

Based on sequence similarity, a large number of SDR genes from diverse plants were annotated to code for putative TRs or TR-like enzymes (TRLs). Many of those plants do not contain tropane or nortropane alkaloids at all (e.g. Arabidopsis thaliana, Brassicaceae) or they accumulate calystegines only, e.g. Solanum species. The metabolic role is not known for any of those TRLs.

SDRs accept a large variety of substrates, and one individual SDR often accepts several substrates. Accordingly, many tropinone reductase-like enzymes reduce various carbonyl compounds in vitro, among them terpene ketones, but no tropinone. Reduction was stereospecifically and the corresponding alcohol was oxidised stereoselectively. Thus, application of TRLs for directed biocatalytic conversions appears attractive. Carbonyl structures that were identified in vitro to serve as substrates for TRLs were used for pharmacophore searches in silico. A data base screening with those pharmacophores led to the identification of additional potential substrates that will be confirmed by turnover experiments in vitro.

Fig. 1