Let There be Morphine: Structural Insights into Functional and Evolutionary Relationships of Morphine Biosynthesis in Opium Poppy
Creators: name: Carr, Samuel Clyde
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Opium Poppy
subject: Opiates
subject: Morphine
subject: Plant Specialized Metabolism
subject: Biology
subject: Biochemistry
subject: Structural Biology
subject: Evolution
subject: Benzylisoquinoline Alkaloids
subject: Biochemistry
subject: Botany

description: Benzylisoquinoline alkaloids are a large and diverse class of plant specialized metabolites known for their pharmaceutical properties, including the tumor suppressant noscapine, vasodilator papaverine, antimicrobial sanguinarine, and the important analgesics codeine, morphine, and their semisynthetic derivatives. The latter group of analgesics are known as morphinan alkaloids or opiates and are produced solely in select members of the genus Papaver, most importantly Papaver somniferum commonly known as opium poppy. This thesis highlights the use of structural biology in multidisciplinary approaches to understand the biosynthesis of morphinan alkaloids in opium poppy and related species. Dehydroreticuline reductase and codeinone reductase are closely related enzymes from the aldo-keto reductase superfamily catalyzing the second and second-last steps, respectively, in morphine biosynthesis in opium poppy. The elucidation of the crystal structure of codeinone reductase reveals novel structural features allowing for the in-depth analysis of substrate binding and catalysis leading to the engineering of substrate specificity. This structure also provides the means for the homology modeling of dehydroreticuline reductase giving insight into its novel catalytic mechanism. Transcriptomic analysis of representative Papaver species reveals putative morphinan biosynthetic enzyme orthologues and the ubiquitous distribution of dehydroreticuline reductase and codeinone reductase, among other morphinan biosynthetic enzymes. Structural analysis and functional/kinetic characterization of aldo-keto reductases demonstrates the ubiquitous conservation of dehydroreticuline reductase and codeinone reductase activity in the genus Papaver. These results challenge the current evolutionary narrative of morphinan biosynthesis suggesting: [1] a more ancient neofunctionalization of early enzymatic steps of morphinan biosynthesis than previously believed and [2] that morphinan biosynthesis evolved via the patchwork evolutionary model. Enzymatic latency and ligand binding affinity both provide a platform for the neofunctionalization of novel enzymatic activities. The alkaloid binding capabilities of pathogenesis related 10 proteins provides a means to understand the neofunctionalization of the morphine biosynthetic enzymes thebaine synthase and neopinone isomerase, alongside a biological role in alkaloid storage for abundant non-catalytic pathogenesis related 10 proteins. Their binding to alkaloids is predicted to promote the formation of protein-alkaloid aggregates based on binding-induced conformational changes observed through X-ray crystallography and dramatic changes in sucrose gradient fractionation.