Biosynthesis of the central tricyclic skeleton of trichothecene mycotoxins

Authors: GAO, JINMIN - University Of California; LIU, DONG - University Of California; NGUYEN, CAROLYN - University Of California; McCormick, Susan; Proctor, Robert; LUO, SHENGGAN - Shanghai Jiaotong University; ZOU, YIKE - Shanghai Jiaotong University; HAI, YANG - University Of California

Submitted to: Journal of the American Chemical Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2025
Publication Date: 3/12/2025
Citation: Gao, J., Liu, D., Nguyen, C., Mccormick, S.P., Proctor, R.H., Luo, S., Zou, Y., Hai, Y. 2025. Biosynthesis of the central tricyclic skeleton of trichothecene mycotoxins. Journal of the American Chemical Society. https://doi.org/10.1021/jacs.4c16973.
DOI: https://doi.org/10.1021/jacs.4c16973

Interpretive Summary: Trichothecenes are among the fungal toxins of most concern to food and feed safety because of their potent toxicity and their frequent occurrence in crops used to produce human food and animal feed. All trichothecenes share the same basic chemical structure, which is critical for toxicity of trichothecenes and, therefore, their concern to food and feed safety. For decades, the formation of this basic structure was thought to include a spontaneous chemical reaction. But this reaction was too slow to account for rapid production of trichothecenes by fungi. Therefore, scientists at ARS in Peoria, Illinois, and at the University of California at Santa Barbara used a combination of genetic and chemical analyses to identify enzymes that would increase the speed of formation of the basic structure. The scientists identified a novel two-enzyme mechanism that rapidly forms the basic chemical structure and in turn explains why fungi can produce trichothecene toxins as rapidly as they do. These findings provide information on a critical step in trichothecene production and identify targets to control of trichothecene contamination in food and feed crops.

Technical Abstract: Trichothecenes are a widespread family of sesquiterpenoid toxins that can pose significant risks to food and feed safety as well as environmental health. A defining feature of all trichothecenes is their central tricyclic 12,13-epoxytrichothec-9-ene (EPT) motif. Although the formation of the EPT central skeleton has long been presumed to be a spontaneous process, the nonenzymatic cyclization reaction forming the tetrahydropyran ring in EPT requires acid-catalysis; otherwise, it occurs too slowly to sustain efficient trichothecene biosynthesis under physiological conditions. Here, we resolved this decades-old problem by identifying the missing enzymes for EPT biosynthesis. We demonstrate that the C11 hydroxyl group of universal trichothecene precursors, isotrichodiol and isotrichotriol, must be acetylated by a strictly conserved O-acetyltransferase Tri3 to furnish a better leaving group. These acetylated intermediates preferentially undergo spontaneous allylic rearrangement with water to give shunt products, trichodiol and trichotriol. Therefore, a novel cyclase, Tri14, which was previously annotated as a hypothetical protein, is required to overcome the kinetically unfavored oxide bridge closure and meanwhile suppress spontaneous formation of any shunt products.