The Earth was lifeless and completely covered in water four billion years ago, when compared to how it is today. Life emerged from that primordial mix over millions of years.
Scientists have long speculated on how molecules come together to cause this transformation.
Now, scientists at Scripps Research have identified a new set of chemical reactions that use cyanide, ammonia, and carbon dioxide, all of which are believed to have been common on the early earth, to produce amino acids and nucleic acids, the building blocks of proteins and DNA.
“We’ve come up with a new paradigm,” says lead author Ramanarayanan Krishnamurthy, “to explain this shift from prebiotic to biotic chemistry.”
“We think the kind of reactions we’ve described are probably what could have happened on early earth,” add the author of study published today in the journal Nature Chemistry.
The newly found chemical reactions are helpful in specific manufacturing processes, such as the production of bespoke labeled biomolecules from affordable starting materials, in addition to providing researchers with information on the chemistry of the early earth.
Krishnamurthy’s group demonstrated earlier this year how cyanide can facilitate the chemical reactions that convert primordial materials and water into the basic organic components essential for life. In contrast to previously proposed reactions, this one was effective at ambient temperature and across a broad pH range. Under the same conditions, the researchers wondered whether it was possible to produce amino acids, the complex chemicals that make up proteins in all known living cells.
Today, nitrogen and special proteins called enzymes work together to turn α-keto acids, which are building blocks, into amino acids. Researchers have discovered evidence that α-keto acids existed early in the history of Earth. However, many argue that prior to the emergence of cellular life, amino acids must have been synthesized from completely other precursors, aldehydes, rather than α-keto acids, because no enzymes existed to carry out the conversion. But this idea has led to arguments about how and when amino acids switched from being made from aldehydes to being made from α-keto acids.
In light of their success in using cyanide to drive other chemical reactions, Krishnamurthy and his colleagues hypothesized that cyanide, even in the absence of enzymes, could also be used to turn α-keto acids to amino acids. They added ammonia, a type of nitrogen that would have been available on the early earth, because they knew nitrogen would be needed in some form. The third essential component was later identified through trial and error as carbon dioxide. With this mixture, they rapidly saw the formation of amino acids.
“We were expecting it to be quite difficult to figure this out, and it turned out to be even simpler than we had imagined,” adds the lead author.
“If you mix only the keto acid, cyanide and ammonia, it just sits there. As soon as you add carbon dioxide, even trace amounts, the reaction picks up speed.”
The novel reaction is more likely to be the origin of early life than vastly different reactions, the researchers argue, because it is relatively similar to what happens inside cells today—with the exception that it is driven by cyanide instead of a protein. The study also reconciles opposing views on the significance of carbon dioxide for the development of early life, coming to the conclusion that carbon dioxide was essential, but only in conjunction with other chemicals.
Krishnamurthy’s group identified orotate, a precursor to nucleotides that make up DNA and RNA, as a result of the same event while examining their chemical soup. This shows that the same primordial soup may have produced a high number of the molecules necessary for the essential components of life under the correct circumstances.
“What we want to do next is continue probing what kind of chemistry can emerge from this mixture,” adds Krishnamurthy. “Can amino acids start forming small proteins? Could one of those proteins come back and begin to act as an enzyme to make more of these amino acids?”
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