Researchers discover new 'origin of life' chemical reaction

Four billion years ago, Earth looked very different from today, lifeless and covered by a vast ocean. Over millions of years, in that primordial soup, life arose. Researchers have long theorized how molecules come together to trigger this transition. Now, scientists at Scripps Research have discovered a new set of chemical reactions that use cyanide, ammonia, and carbon dioxide — all thought to be common on the early Earth — to produce amino acids and nucleic acids, the building blocks of protein and DNA.

“We have come up with a new paradigm to explain the shift from prebiotics to biotic chemistry,” says Ramanarayanan Krishnamurthy, PhD, a professor of chemistry at Scripps Research, and lead author of the new paper, published July 28, 2022 in the journal Nature Chemistry. “We think the type of reaction we’ve described is probably what could have happened on the early Earth.”

In addition to providing researchers with insight into early Earth chemistry, the newly discovered chemical reactions are also useful in certain manufacturing processes, such as the manufacture of specially labeled biomolecules from inexpensive starting materials.

Earlier this year, Krishnamurthy’s group demonstrated how cyanide can activate chemical reactions that convert prebiotic molecules and water into basic organic compounds necessary for life. Unlike the previously proposed reactions, this reaction works at room temperature and over a wide pH range. The researchers wondered if, under the same conditions, there was a way to produce amino acids, the more complex molecules that make up the proteins in all known living cells.

In today’s cells, amino acids are produced from precursors called -keto acids using nitrogen and special proteins called enzymes. Researchers have found evidence that -keto acids may have existed early in Earth’s history. However, many hypothesize that before the advent of cellular life, amino acids must have been produced from a completely different precursor, the aldehyde, than the -keto acid, because the enzymes to carry out the conversion did not yet exist. But the idea has sparked debate about how and when the switch from aldehydes to -keto acids is the main ingredient for making amino acids.

After their successful use of cyanide to drive other chemical reactions, Krishnamurthy and his colleagues suspected that cyanide, even without the enzyme, might also help convert -keto acids into amino acids. Because they knew nitrogen would be needed in some form, they added ammonia — a form of nitrogen that would have existed in the early earth. Then, through trial and error, they discovered a third main ingredient: carbon dioxide. With this mixture, they quickly began to see the form of amino acids.

“We thought it would be quite difficult to figure this out, and it turned out to be simpler than we thought,” Krishnamurthy said. “If you just mix keto acids, cyanide and ammonia, it’s just there. As soon as you add carbon dioxide, even a small amount, the reaction speeds up.”

Because the new reaction is relatively similar to what’s happening today inside cells — except driven by cyanide instead of protein — it seems more likely to be the source of early life, rather than a drastically different reaction, the researchers said. The research also helped unify the two sides of the longstanding debate about the importance of carbon dioxide for early life, concluding that carbon dioxide was the key, but only in combination with other molecules.

In the process of studying their chemical soup, Krishnamurthy’s group discovered that a by-product of the same reaction was orotate, a precursor to the nucleotides that make up DNA and RNA. This suggests that the same primordial soup, under the right conditions, can give rise to a large number of molecules necessary for life’s key elements.

“What we want to do next is continue to investigate what kind of chemistry can emerge from this mixture,” Krishnamurthy said. “Can amino acids start to form small proteins? Could one of those proteins come back and start acting as an enzyme to make more of these amino acids?”

This work was supported by funding from the NSF Center for Chemical Evolution (CHE-1504217), the NASA Exobiology grant (80NSSC18K1300) and a grant from the Simons Foundation (327124FY19).

Reference:

1. Pulletikurti, S., Yadav, M., Springsteen, G. et al. Prebiotic synthesis of -amino and orotic acids from -keto acids potentiates the transition to extant metabolic pathways. Nat. Chemistry, 2022 DOI: 10.1038/s41557-022-00999-w

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