The evolutionary origins of the Krebs cycle (tricarboxylic acid cycle) are not currently clear. Despite the existence of a simple non-enzymatic Krebs cycle catalyst being dismissed only a few years ago as ‘an appeal to magic’, citrate and other intermediates have since been discovered on a carbonaceous meteorite and do interconvert non-enzymatically. To identify a metabolism-like non-enzymatic Krebs cycle catalyst, we used combinatorial, quantitative high-throughput metabolomics to systematically screen iron and sulfate compounds in a reaction mixture that orients on the typical components of Archaean sediment. Krebs cycle intermediates were found to be stable in water and in the presence of most molecule species, including simple iron sulfate minerals. However, in the presence of sulfate radicals generated from peroxydisulfate, the intermediates underwent 24 interconversion reactions. These non-enzymatic reactions covered the critical topology of the oxidative Krebs cycle, the glyoxylate shunt and the succinic-semialdehyde pathway. Assembled in a chemical network, the reactions achieved over 90% carbon recovery. Our results show that a non-enzymatic precursor of the Krebs cycle is biologically sensible, efficient, and forms spontaneously in the presence of sulfate radicals.
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We thank G. Averill and T. Littmann for helping with experiments. This work was supported by the Francis Crick Institute, which receives its core funding from Cancer Research UK (FC001134), the UK Medical Research Council (FC001134) and the Wellcome Trust (FC001134). M.R. is supported by a Wellcome Trust grant, RG 093735/Z/10/Z, and a European Research Council Starting Grant, 260809. M.A.K. is supported by an Erwin Schrödinger postdoctoral fellowship (FWF, Austria, J3341). D.K. is supported by an Ad Futura studentship (Slovene Scholarship Fund).
The authors declare no competing financial interests.
Supplementary Tables 6–13, Supplementary Figures 1–7. (PDF 1019 kb)
Reaction rate data for controls, Fe(II), peroxydisulfate and peroxydisulfate/ferrous sulfide. (XLS 45 kb)
Metal dependency rate data. (XLS 30 kb)
Z-score data. (XLS 505 kb)
Complete reaction list. (XLS 39 kb)
Scavenger experiment reaction rate data. (XLS 91 kb)
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Keller, M., Kampjut, D., Harrison, S. et al. Sulfate radicals enable a non-enzymatic Krebs cycle precursor. Nat Ecol Evol 1, 0083 (2017). https://doi.org/10.1038/s41559-017-0083
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