Chemists at the Scripps Research Institute (TSRI) have developed a fascinating new theory about how life on Earth may have begun.
Their experiments, described in the journal Nature Communications, show that the key chemical reactions that support life today could have been carried out with ingredients probably present on the planet four billion years ago.
"This was a black box for us," said Ramanarayanan Krishnamurthy, PhD, associate professor of chemistry at TSRI and lead author of the new study. 19659002] "But if you focus on chemistry, questions of the origins of life become less intimidating."
For the new study, Krishnamurthy and his coauthors, all members of the National Science Foundation / National Aeronautics and Space Administration Center for Chemical Evolution, focused on a series of chemical reactions that make up what researchers call the citric acid cycle.
All aerobic organisms, from flamingos to fungi, depend on the citric acid cycle to release energy stored in the cells. In previous studies, researchers imagined early life using the same molecules for the citric acid cycle as current life.
The problem with that approach, Krishnamurthy explains, is that these biological molecules are fragile and the chemical reactions used in the cycle have not existed in the first billion years of Earth; the ingredients simply did not exist yet.
The leaders of the new study began with chemical reactions first. They wrote the recipe and then they determined which molecules present on the primitive Earth could have functioned as ingredients.
The new study describes how two non-biological cycles – called the HKG cycle and the malonate cycle – could have come together to kick-start. a crude version of the citric acid cycle. The two cycles use reactions that perform the same fundamental chemistry of a-keto acids and b-keto acids as in the citric acid cycle.
These shared reactions include aldol additions, which bring new source molecules to the cycles, as well as beta and oxidative decarboxylations, which release molecules such as carbon dioxide (CO2).
As these reactions followed one another, the researchers discovered that they could produce amino acids in addition to CO2, which are also the final products of the citric acid cycle. Researchers think that as biological molecules, such as enzymes, become available, they could have led to the replacement of non-biological molecules in these fundamental reactions to make them more elaborate and efficient.
"Chemistry may have stayed the same over time, it was simply the nature of the molecules that changed," says Krishnamurthy.
"Molecules evolved to be more complicated over time based on what biology needed."
"Modern metabolism has a precursor, a template, that is not biological," adds Greg Springsteen, PhD, first author of the new study and associate professor of chemistry at Furman University.
Making these reactions even more plausible is the fact that at the center of these reactions is a molecule called glyoxylate, which studies show could have been available on the early Earth and is part of the citric acid cycle nowadays (called "glyoxylate cycle or derivation").
Krishnamurthy says more research needs to be done on how these chemical reactions might have become as sustainable as the citric acid cycle is today.
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