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Possibly—but a 2018 laboratory study showed only that two simple, enzyme-free chemical cycles can run under controlled conditions, not that either one operated on early Earth. The cycles use glyoxylate and hydrogen peroxide, share some intermediates with the modern citric acid cycle, and offer a possible example of how metabolism-like chemistry could precede enzymes.
What did the researchers demonstrate?
Greg Springsteen and colleagues reported two linked cycles of oxidative decarboxylation: the malonate cycle and the 4-hydroxy-2-ketoglutarate (HKG) cycle. Both use glyoxylate as a carbon source and hydrogen peroxide as an oxidant. They produce intermediates also found in the modern citric acid cycle, including oxaloacetate and malate, but they are much simpler pathways—not reconstructions of the full modern cycle.
The researchers described the cycles as protometabolic analogs: laboratory chemistry that resembles selected features of metabolism without relying on enzymes. The reactions took place in aqueous buffers at pH 7.0–8.5 and temperatures up to 50°C. These are the conditions tested in the lab, not evidence that the same chemistry occurred in a natural early-Earth setting. The primary study, published in Nature Communications on January 8, 2018, reports turnover under controlled conditions.
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How do the two cycles compare?
| Feature | Malonate cycle | HKG cycle |
|---|---|---|
| Starting materials or intermediates | Malonate and glyoxylate | Oxaloacetate and glyoxylate; the sequence proceeds through oxalomalate, HKG, and malate |
| Cycle outcome | Malonate is regenerated | The sequence produces malonate from malate, linking back into the pathway |
| Reported high-yield step | At least 98% 3-carboxymalonate formation after 24 hours at 50°C | At least 98% HKG formation from oxaloacetate and glyoxylate through oxalomalate |
| Reported limiting step | Oxidation of 3-carboxymalonate | Oxidation of malate’s secondary hydroxyl |
| Important condition or result | In the reported cycle experiment, malonate regeneration was 51% after hydrogen peroxide treatment and 48 hours at 50°C | Malate yielded 55% malonate after 24 hours at 50°C; ferrous sulfate accelerated this step to three hours with a similar result |
These figures describe specific steps and experimental conditions, not overall cycle yields. The reported chemistry also began from malonate, oxaloacetate, or pyruvate in the presence of glyoxylate. The paper characterizes the reactions as uncatalyzed, while noting that ferrous sulfate accelerated one rate- and yield-limiting oxidation; it would be misleading to treat every step as equally fast or efficient.
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Why do the cycles matter to origin-of-life research?
Modern metabolism depends on enzymes, which are highly specialized catalysts. A central question in origin-of-life research is whether useful, organized chemical activity could have existed before enzyme-based systems. This study offers one limited example: a small set of reactions can form linked cycles and regenerate intermediates without enzymes in the tested setup.
The cycles borrow some reaction logic from metabolism, including aldol addition and oxidative decarboxylation. Their shared intermediates with the citric acid cycle make the resemblance chemically meaningful, but resemblance does not establish that one pathway evolved into the other. The result supports the plausibility of simpler metabolism-like chemistry; it does not identify the first metabolism or explain how life began.
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What limits the result?
- Specific laboratory conditions: The reactions were performed in buffered water with controlled reagent additions. The study does not show that the ingredients, concentrations, or conditions were available together in a natural early-Earth environment.
- Uneven performance: Some steps produced high yields, while the regeneration and oxidation steps were less productive or slower. A high yield for one reaction cannot be read as a high yield for the complete cycle.
- Reagent timing matters: Adding glyoxylate and hydrogen peroxide all at once led to hydrogen peroxide reacting with glyoxylate to form formate. The authors therefore used sequential feeding to observe turnover; uncontrolled mixing did not simply reproduce the cycle.
- No direct historical evidence: Demonstrating that a reaction network can operate in a laboratory does not show that it operated on early Earth.
Does this prove metabolism began before enzymes?
No. It demonstrates enzyme-free, metabolism-like cycles under selected laboratory conditions. Whether comparable chemistry occurred before life, whether it could persist in a natural setting, and how it might connect to biological metabolism remain open questions. The appropriate conclusion is that these experiments make one kind of prebiotic chemical cycle more plausible—not that they settle the origin of metabolism.
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Sources
- Springsteen et al., “Linked cycles of oxidative decarboxylation of glyoxylate as protometabolic analogs of the citric acid cycle,” Nature Communications, January 8, 2018.
- Melissae Fellet, Chemistry World, “Enzyme-free reaction cycles hint at primitive precursor to metabolism,” January 10, 2018.
- PubMed record for the Nature Communications paper.
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