Characterization of norbelladine synthase and noroxomaritidine/norcraugsodine reductase reveals a novel catalytic route for the biosynthesis of Amaryllidaceae alkaloids including the Alzheimer's drug galanthamine

Amaryllidaceae alkaloids (AAs) are a large group of pharmacological plant-specialized metabolites. Norbelladine is the entry compound in AAs biosynthesis. Two enzymes are capable of catalyzing this reaction in-vitro, both with low-yield; 1) norbelladine synthase (NBS) condenses tyramine and 3,4-dihydroxybenzaldehyde, while 2) noroxomaritidine/norcraugsodine reductase (NR), catalyzes a reduction reaction. To clarify the mechanisms involved in this controversial step, both NBS and NR were characterized from Narcissus papyraceus and Leucojum aestivum. Assays with each enzyme suggested that NBS and NR function together for norbelladine formation. Using molecular-homology modeling and docking studies, we predicted models for the binding of substrates to NBS and NR. Moreover, NBS and NR physically interact, localize to the cell cytoplasm and nucleus and are expressed at high levels in bulbs. Our study establishes that both NBS and NR participate in the biosynthesis of norbelladine, catalyzing the first key steps involved in the biosynthesis of the Alzheimers drug galanthamine.