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. 2023 Jan 19;19(6):695–702. doi: 10.1038/s41589-022-01232-y

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© The Author(s) 2023

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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Extended Data Fig. 8 — a, Electrostatic charge profile of MtFsr shown in surface is coloured in red and blue to represent acidic and basic patches, respectively. The siroheme is accessible via a positively charged solvent channel. Carbon, oxygen, nitrogen, sulfur and iron are coloured in green, red, blue, yellow and orange, respectively. b and c, Close up of the axial ligands bound on the siroheme of MjFsr (b) and MtFsr (c). The 2F_o-F_c map of the siroheme and SO₃²⁻ are contoured to 1.5-σ in MjFsr, while the siroheme and HS⁻ is contoured to 3-σ in MtFsr. In MjFsr the Fe-siroheme is equidistant (2.3 Å) to the sulfur from the modelled SO₃²⁻ and the bridging-sulfur of the cysteine 472, suggesting a tight covalent binding. In MtFsr, the bridging-sulfur of the cysteine 472 is at a distance of 2.6 Å to the Fe-siroheme and the sulfur from the modelled HS⁻ is 2.9 Å distant to the Fe-siroheme, indicating a loose binding of the HS⁻, which might result from a reduction event by X-ray radiation⁶⁰. d, Siroheme superposition between aSirs (1AOP, 5H92), dSirs (3MM5, 2V4J) and Fsrs. Siroheme from aSirs and Fsr are coloured in green, structural siroheme/sirohydrochlorin from dSirs in black and dSirs functional sirohemes in blue. Superposition analysis shows that the functional sirohemes are arranged in a highly similar manner, whereas the conformation of the structural siroheme or sirohydrochlorin differ, which highlights the strong influence of the protein environment on the siroheme geometry.