Allenes and alkynes are versatile functional groups in total synthesis, medicinal chemistry, and bioorthogonal conjugation. The biosynthetic logic of how Nature installs allene or alkyne in natural products, especially that of allenes, is not well understood. Here we uncovered allenes and alkynes can be formed enzymatically through oxidative C(sp2)-demethylation of the common five-carbon prenyl group. Two fungal cytochrome P450 monooxygenases, PpnB and NseB, from the penipratynolene and sinuxylamide biosynthetic pathways, respectively, were shown to catalyze oxidative removal of a C(sp2)-methyl group in O-prenyl-L-tyrosine to afford O-homoallenyl-L-tyrosine and O-but-2-ynyl-L-tyrosine, respectively. Combining density functional theory calculations, heterologous expression, biotransformation and enzymatic assays with isotopically labeled substrates, a mechanism involving selective C-C bond cleavage followed by product-determining hydrogen atom abstraction is presented. An additional P450 enzyme from the penipratynolene pathway, PpnD, acts as an oxidative isomerase that converts the four-carbon terminal allene into a terminal alkyne. This unprecedented enzymatic editing strategy to install allene and alkyne expands the catalytic repertoire of P450 enzymes.
Liu, M., Ohashi, M., Han, W., Zhou, Q., Houk, K. N., Tang, Y.
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