Carnegie Mellon Chemists Help Discover Mechanism Behind Important Biosynthetic Reaction
By Ben Panko
Carnegie Mellon University chemists have helped discover the reaction mechanism behind the biosynthesis of kainic acid, an important natural product used to study neurodegenerative disorders. The research was featured as a supplementary cover article in the latest issue of the journal ACS Catalysis, designed by Rachel Keeney, Mellon College of Science publications manager and graphic designer.
First harvested from the red seaweed Digenea simplex nearly 70 years ago, kainic acid was originally used to treat intestinal worm infections in Japan, said Yisong (Alex) Guo, associate professor of chemistry and a co-author of this study. However, the compound was later discovered to be a potent neuroexcitant that activates glutamate, the primary neurotransmitter in the human nervous system. Nowadays, kainic acid is frequently used by neuroscientists to excite certain regions of model organisms’ brains in research that aims to better understand neurological conditions.
In recent years, however, the difficulty of harvesting natural kainic acid has made the product prohibitively expensive for many researchers to use, and scientists have not had much luck producing synthetic versions of it either. "Although there are more than 70 synthetic routes to make kainic acid, these methods are generally lengthy and complex, which make a large-scale synthesis difficult," Guo said. In close collaboration with researchers from North Carolina State University led by Wei-chen Chang, Guo has been working to shed light on the mechanism of kainic acid biosynthesis catalyzed by the iron-containing enzyme KabC in hopes of making the production of the substance easier, and to better understand how metalloenzymes catalyze oxidative cyclization reactions and carbon–carbon bond formation reactions.
The researchers deployed an array of experimental techniques, including molecular probe synthesis and various forms of spectroscopy, targeted at the final step of the biosynthesis, which consists of an oxidative cyclization reaction via intramolecular carbon-carbon bond formation. "We elucidated for the first time that the intramolecular carbon-carbon bond formation is initiated by a carbon–hydrogen bond activation on one side of the KabC substrate," Guo said. Then, a subsequently formed carbon radical attacks a carbon–carbon double bond on the other side of the substrate to form a new carbon–carbon bond, completing the cyclization step. The overall reaction finally finishes through another carbocation-triggered desaturation, and kainic acid is created.
"We are working on other carbon–carbon bond formation enzymes to understand whether the catalytic strategy utilized in KabC is one of the universal ways to install carbon–carbon bonds in enzymatic reactions," Guo said of the next steps for this research. In particular, he and his collaborators are working to understand what controls the options of enzymatic reactions, since many other enzymes like KabC lead to different outcomes than what was seen in this biosynthesis.
"We will try to unravel the molecular governing factors that determine the selectivity in KabC," Guo said. "This could help harness the power of enzymatic reactions to create new biocatalysts in order to access synthetically challenging reactions."
Other authors on this study include Shan Xue from Carnegie Mellon; Tzu-Yu Chen and Wei-chen Chang from North Carolina State University; and Wei-Chih Tsai and Tun-Cheng Chien from National Taiwan Normal University.
Funding for this research was provided by grants from North Carolina State University, Carnegie Mellon University, and Taiwan's Ministry of Science and Technology (MOST 109-2113-M-003-005) and its Ministry of Education's Higher Education Sprout Project.