Sulfate radicals enable a non-enzymatic Krebs cycle precursor

Abstract

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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Figure 1: TCA intermediates were stable in water but showed reactivity in the presence of transition metals frequently found in Archaean sediments.
Figure 2: Peroxydisulfate enables the non-enzymatic interconversion of TCA intermediates.
Figure 3: Non-enzymatic Krebs-cycle-like reactions in the presence of peroxydisulfate and peroxydisulfate/ferrous sulfide.
Figure 4: Peroxydisulfate enables TCA-like reactivity by providing sulfate radicals.

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Acknowledgements

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).

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M.A.K. and M.R. designed the research. M.A.K., D.K. and S.A.H. performed the research. M.A.K. and M.R. wrote the first draft of the paper, and all authors contributed to finalizing the manuscript.

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Correspondence to Markus Ralser.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Tables 6–13, Supplementary Figures 1–7. (PDF 1019 kb)

Supplementary Table 1

Reaction rate data for controls, Fe(II), peroxydisulfate and peroxydisulfate/ferrous sulfide. (XLS 45 kb)

Supplementary Table 2

Metal dependency rate data. (XLS 30 kb)

Supplementary Table 3

Z-score data. (XLS 505 kb)

Supplementary Table 4

Complete reaction list. (XLS 39 kb)

Supplementary Table 5

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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