Four billion years ago, the Earth looked very different than it does today, devoid of life and covered in a vast ocean. Over millions of years, in that primordial soup, life appeared. Researchers have long theorized about how molecules combine to cause this transformation. 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 in the early Earth — to generate amino acids and nucleic acids, the building blocks of proteins and DNA.
“We have come up with a new paradigm to explain this shift from biochemistry to biochemistry,” says Ramanarayanan Krishnamurthy, assistant professor of chemistry at Scripps Research, and lead author of the new paper, published July 28. , 2022 in the magazine nature chemistry. “We think the type of reactions we’ve described are most likely what would have happened on early Earth.”
In addition to giving researchers insight into the chemistry of the early Earth, the newly discovered chemical reactions are also useful in some manufacturing processes, such as generating custom-labeled biomolecules from inexpensive starting materials.
Earlier this year, Krishnamurthy’s team demonstrated how cyanide can enable chemical reactions that convert prebiotic molecules and water into essential organic compounds required for life. In contrast to the previously proposed reactions, these reactions worked at room temperature and in a wide range of pH. The researchers wondered whether, under the same conditions, there was a way to generate amino acids, the more complex molecules that make up the proteins in all known living cells.
In cells today, amino acids are created from precursors called alpha-keto acids using both nitrogen and specialized proteins called enzymes. Researchers have found evidence that alpha-keto acids may have existed early in Earth’s history. However, many assumed that before the advent of cellular life, the amino acids must have been created from an entirely different precursor, aldehydes, rather than alpha-keto acids, because the enzymes needed to make the conversion did not yet exist. But this idea has led to controversy over how and when the switch from aldehydes to alpha-keto acids occurred as a major component of making amino acids.
After successfully using cyanide to trigger other chemical reactions, Krishnamurthy and his colleagues suspected that cyanide, even without enzymes, might also help convert alpha-keto acids into amino acids. Knowing that nitrogen would be required in some form, they added ammonia – a form of nitrogen that would have been present on the early Earth. Then, through trial and error, they discovered a third key ingredient: carbon dioxide. With this mixture, they soon began to see the formation of amino acids.
“We were expecting it to be very difficult to figure out, and it turned out to be simpler than we had imagined,” says Krishnamurthy. “If you just mix keto acid, cyanide, and ammonia, it’s still there. Once you add carbon dioxide, even trace amounts, the reaction speeds up.”
Because the new reaction is relatively similar to what happens today inside cells — except that it is driven by cyanide rather than protein — it appears likely to be the source of early life, rather than radically different reactions, the researchers say. The research also helps bring together two aspects of a long-running debate about the importance of carbon dioxide in early life, concluding that carbon dioxide was key, but only in combination with other molecules.
In the process of studying their chemical soup, Krishnamurthy’s team discovered that a byproduct of the reaction itself is a gradient, a precursor to the nucleotides that make up DNA and RNA. This suggests that the same primordial soup, under the right conditions, could have given rise to a large number of molecules necessary for the basic elements of life.
“What we want to do next is keep looking for the kind of chemistry that can emerge from this mixture,” Krishnamurthy says. “Can amino acids start forming small proteins? Can one of these proteins go back and start acting as an enzyme to produce more of these amino acids?”
In addition to Krishnamurthy, the authors of the study, “Prebiotic Synthesis of α-Amino Acids and Orotate from α-Ketoacids Potentiates Transition to Extant Metabolic Pathways” are Sunil Politikurti, Mahipal Yadav and Greg Springsteen.
A new role for cyanide in the early Earth and the search for extraterrestrial life
Ramanarayanan Krishnamurthy, Prebiotic synthesis of α-amino acids and orotate from α-ketoacids stimulates transition to existing metabolic pathways, nature chemistry (2022). DOI: 10.1038/s41557-022-00999-w. www.nature.com/articles/s41557-022-00999-w
Submitted by The Scripps Research Institute
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