Structure of Papaver somniferum O-Methyltransferase 1 Reveals Initiation of Noscapine Biosynthesis with Implications for Plant Natural Product Methylation

The opium poppy, Papaver somniferum, has been a source of medicinal alkaloids since the earliest civilizations, ca. 3400 B.C. The benzylisoquinoline alkaloid noscapine is produced commercially in P. somniferum for use as a cough suppressant, and it also has potential as an anticancer compound. The first committed step in the recently elucidated noscapine biosynthetic pathway involves the conversion of scoulerine to tetrahydrocolumbamine by 9-O-methylation, catalyzed by O-methyltransferase 1 (PSMT1). We demonstrate, through protein structures (obtained through rational crystal engineering at resolutions from 1.5 to 1.2 Å for the engineered variants) across the reaction coordinate, how domain closure allows specific methyl transfer to generate the product. SAM-dependent methyl transfer is central to myriad natural products in plants; analysis of amino acid sequence, now taking the three-dimensional structure of PSMT1 and low identity homologues into account, begins to shed light on the structural features that govern substrate specificity in these key, ubiquitous, plant enzymes. We propose how "gatekeeper" residues can determine acceptor regiochemistry, thus allowing prediction across the wide genomic resource.