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Metabolism-like chemistry may be possible without enzymes: in a 2018 laboratory study, researchers demonstrated two linked, enzyme-free reaction cycles that convert glyoxylate through pathways resembling selected parts of the modern citric acid cycle. The result makes a possible pre-life chemical route more plausible; it does not show that these cycles operated on early Earth or that they produced life.
What did the researchers discover?
Springsteen and colleagues reported two four-step reaction cycles, called the malonate cycle and the 4-hydroxy-2-ketoglutarate (HKG) cycle. Both use glyoxylate as a carbon source and hydrogen peroxide as an oxidant. Their reactions generate compounds also found in the modern citric acid cycle, including oxaloacetate and malate, but the pathways are simpler analogs—not reconstructions of the full modern cycle.
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The experiments were described as uncatalyzed: they did not rely on enzymes. The work demonstrated turnover under controlled conditions through sequential additions of glyoxylate and hydrogen peroxide. Depending on the pathway, reactions could begin from malonate, oxaloacetate, or pyruvate in the presence of glyoxylate. The original 2018 study in Nature Communications describes the chemistry as protometabolic analogs of the citric acid cycle.
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| Feature | Malonate cycle | HKG cycle |
|---|---|---|
| Starting route | Malonate reacts with glyoxylate to form 3-carboxymalonate. | Oxaloacetate and glyoxylate form oxalomalate, which leads to HKG and then malate. |
| Cycle logic | Oxidative decarboxylation steps regenerate malonate. | The sequence proceeds through oxaloacetate, oxalomalate, HKG and malate, with malonate production in the reported sequence. |
| Notable reported step yield | At least 98% 3-carboxymalonate formation after 24 hours at 50°C; 51% malonate regeneration after peroxide treatment and 48 hours at 50°C. | At least 98% HKG formation from oxaloacetate and glyoxylate via oxalomalate; 55% malonate production from malate after 24 hours at 50°C. |
| Reported bottleneck | Oxidation of 3-carboxymalonate limits the cycle. | Oxidation of malate’s secondary hydroxyl limits rate and yield. |
These percentages describe particular steps in laboratory experiments, not overall cycle yields or the efficiency of a natural early-Earth process. In the HKG sequence, the researchers reported that ferrous sulfate accelerated the malate oxidation step from 24 hours to three hours with a similar result. That condition-specific acceleration does not mean every step was equally fast or efficient without assistance.
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What conditions did the reactions require?
The reactions were carried out in aqueous buffers at pH 7.0–8.5 and temperatures up to 50°C. Although the cycles were enzyme-free, the experiments used controlled laboratory conditions and carefully sequenced reagent additions. When glyoxylate and hydrogen peroxide were added together, peroxide reacted with glyoxylate to form formate, showing that the order of addition mattered.
Does this mean metabolism began before enzymes?
It shows that selected metabolism-like reactions can be linked into cycles without enzymes under the tested conditions. That is relevant to origin-of-life research because it offers an example of chemical pathways that could, in principle, resemble aspects of metabolism before enzyme-based systems existed.
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It does not establish that the cycles occurred on the early Earth. Laboratory buffers, selected starting compounds and controlled feeding are not evidence that the same chemistry arose or persisted in a natural setting. Nor does the study explain how life, enzymes or informational systems first emerged. Its contribution is narrower: it demonstrates a chemical possibility worth distinguishing from a historical claim.
The paper’s framing echoes Leslie Orgel’s earlier observation that complex cycles analogous to metabolic ones, if they operated before enzymes or other informational polymers, could ease some obstacles in origin-of-life scenarios. That is a broader motivation for investigating protometabolism, not a conclusion proven by this experiment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the finding matters—and what remains open
The study makes a useful case for studying metabolism-like chemistry from the bottom up. Rather than requiring a complete modern biochemical network, the researchers linked smaller reactions that share intermediates and reaction logic with the citric acid cycle. The observed turnover suggests such linked chemistry is not inherently dependent on enzymes in every step.
The unresolved question is whether comparable chemistry could arise and sustain itself in plausible natural environments, with available reactants and without laboratory control. The reported limiting oxidation steps and sensitivity to reagent order show why demonstrating a cycle in a flask is an important but incomplete step toward explaining prebiotic chemistry.
PubMed’s record for the paper identifies the article as published in 2018. A contemporaneous Chemistry World report includes interpretations from researchers, but those comments should be distinguished from what the experiments themselves established.
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