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. 2019 Aug 8;10:3566. doi: 10.1038/s41467-019-11470-9

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© The Author(s) 2019

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. 6 — Proposed model of Fe-S cluster biosynthesis. The upper part describes physiologically relevant Fe-S cluster assembly by Fe-ISCU. Upon reaction with L-cysteine, a persulfide is generated on the catalytic cysteine of NFS1 that is transferred to the cysteine C104 of Fe-ISCU and FXN accelerates this reaction. The persulfide of Fe-ISCU is then reduced into sulfide by FDX2, which leads to formation of a [2Fe2S] cluster. The [2Fe2S] cluster is putatively formed by dimerization of ISCU that is assisted by the dimeric structure of the NIA complex. The bridged [2Fe2S] cluster subsequently segregates on one subunit and is transferred to recipient apo-proteins by the HSPA9/HSC20 chaperone system. Reloading of ISCU with iron ions, by a still ill-defined chaperone, allows subsequent turnovers. The lower part describes the reaction with Zn-ISCU that is not physiologically relevant. Upon reaction with L-cysteine, a persulfide is generated on NFS1 and is transferred to the cysteine C104 of Zn-ISCU. FXN enhances the rate of this reaction, but the persulfide of Zn-ISCU is not reduced by FDX2. Upon reaction with a second molecule of L-cysteine, the persulfide of NFS1 is regenerated. This persulfide is reduced by thiols such as L-cysteine which leads to formation of free sulfide that combines with free iron to form a Fe-S cluster in Zn-ISCU in a slow and poorly efficient process akin to chemical reconstitution (orange background). FXN accelerates the reduction of the persulfide of NFS1 by thiols