The final steps of cocaine biosynthesis in the Erythroxylaceae provide insight into the biochemistry, physiology and evolution of tropane alkaloid biosynthesis
Tropane alkaloids are medicinally-valued plant secondary metabolites found in ten angiosperm plant families. Despite their medicinal value and socioeconomic impact on mankind, their biosynthesis and ecological function remains to be understood. The most biochemically investigated angiosperm family producing tropane alkaloids is the Solanaceae, predominantly known for atropine and scopolamine from belladonna (Atropa belladonna), datura (Datura stramonium), henbane (Hyoscyamus niger) and mandrake (Mandragora officinalis). The Erythroxylaceae, another important tropane alkaloid producing angiosperm family, is predominantly known for cocaine from the coca plant (Erythroxylum coca). In this thesis, I review the current state of tropane alkaloid biosynthesis in plants, their occurrence in the angiosperms and their ecological functions. Besides the Solanaceae, biochemical investigations on tropane alkaloid production in other plant families have been neglected. Therefore the last two steps of tropane alkaloid biosynthesis in E. coca were investigated. Interestingly, the penultimate step in cocaine biosynthesis in Erythroxylaceae is performed by a different family of oxidoreductase enzymes, than reported from the Solanaceae. Short chain reductases / dehydrogenases (SDR) reduce tropinone in Solanaceae and an aldo-keto reductase (AKR) reduces 2-carbomethoxy-3-tropinone in the Erythroxylaceae. The utilization of both SDR and AKR enzymes in tropane alkaloid biosynthesis in angiosperms is an example of convergent evolution. In addition, the enzyme responsible for the last step of cocaine biosynthesis, the esterification of 2-carbomethoxy-3β-tropine and benzoyl-CoA, in E. coca was characterized and belongs to the BAHD acyltransferase enzyme superfamily. The importance of biochemical investigations of plant secondary metabolite pathways is further reviewed in the context of metabolic engineering.