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. 2017 Nov 21;8:1644. doi: 10.1038/s41467-017-01813-9

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 4 — Complete biosynthetic pathway of helvolic acid. Biosynthesis of helvolic acid includes nine steps from the universal triterpenoid progenitor (3 S)-2,3-oxidosqualene. Following the cyclization to the tetracyclic protosta-17(20)Z,24-dien-3β-ol (2) by HelA, HelB1-mediated and HelB2-mediated oxidation at C-4 and C-16, HelD2-dependent acetylation of 16-OH, oxidation of C-21 by HelB4, and HelC-dependent oxidative decarboxylation yield the fusidane skeleton 14, which is further modified in three additional steps mediated by HelB3, HelD1, and HelE to give helvolic acid. The premature decarboxylation (or dehydrogenation) by HelC prevents HelB1-mediated, HelB2-mediated, HelD2-mediated, and HelB4-mediated tailoring