New Clues on Origins of Life As Peptides Produced in Space-Like Conditions
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A recent study by scientists at the Friedrich Schiller University Jena and the Max Planck Institute for Astronomy found that peptides can form in conditions that mimic outer space.
The origins of life on Earth
How did life come to be on planet Earth? It’s arguably one of those “wormhole” questions, the type that you avoid pondering over too much (particularly before bedtime) as it can prompt a never-ending cycle of questioning.
Perhaps a key starting point is to address: What is life, from a molecular viewpoint? Living things comprise cells. Cells serve unique and specific functions across the different categories of biological life. This function is conducted by the “workhorses” of cells, the proteins that are expressed.
The process by which proteins are created is known as the “central dogma”. It describes the formation of proteins via the transcription of DNA to mRNA and the translation of mRNA to amino acids, peptides and proteins.
Every living biological entity is the sum of its biomolecular building blocks. But where did these biomolecular building blocks come from in the first place? Our planet, or beyond?
“There is no clear understanding on how peptides and other biopolymers were first formed,” says Dr. Serge Krasnokutski, physicist at the Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the University of Jena. “It is commonly expected that some catalytic activity of different minerals may play a role, but it is not known what the first molecules of life were.”
Previous analysis of meteoritic substances highlighted the presence of amino acids, sugars and nucleobases. Consequently, a potential extraterrestrial source of life was considered, whereby biomolecules were delivered to our planet when meteorites crashed into the Earth’s surface. However, a barrier to this hypothesis has been that the reactions underlying peptide formation – the polymerization of amino acids – require specific conditions, assumed to have been of Earthly origin. But what if there was another way to create peptides?
Krasnokutski is the lead author of a new study, published in Nature Astronomy, that demonstrates a novel pathway for peptide formation. It can occur on dust particles in a laboratory environment created to mimic outer space.
A new way for peptide formation
Expanding on the “central dogma” of biology, the formation of peptides involves a biochemical reaction, which joins the amino group of one amino acid with the carboxyl group of its neighbor amino acid. This reaction extracts a water molecule, has a high energy barrier and is therefore improbable to occur even at elevated temperatures, never mind the temperatures in space, Krasnokutski explains.
Quantum chemical calculations predict that the amino acid glycine – the simplest amino acid, from a structural perspective – could be formed by combining a chemical precursor (amino ketene) with a water molecule.
Thus, the formation of a peptide requires the addition of a water molecule to produce the amino acid from its precursor, but the subsequent removal of a water molecule to create the peptide. Krasnokutski wanted to explore whether there was a “detour” route that could be taken, whereby amino ketene molecules could be combined directly to form peptides. “We did this under the conditions that prevail in cosmic molecular clouds, that is to say on dust particles in a vacuum, where the corresponding chemicals are present in abundance: carbon, ammonia and carbon monoxide,” explains Krasnokutski.
The scientists brought together carbon, ammonia and carbon monoxide to model the surface of dust particles in an ultra-high vacuum chamber. The chamber was set at a pressure level approximately one quadrillionth of standard air pressure, and at a temperature of – 263 °C. Krasnokutski told Technology Networks that he previously studied low-temperature chemistry relevant to molecule clouds: “So, it was quite natural for me to plan these studies at the relevant conditions,” he said.
Under these conditions, the condensation of carbon atoms located on the surface of the “cosmic dust” led polyglycine monomers – aminoketene molecules – to form. Encounters between these aminoketene molecules subsequently led to their polymerization and the production of the peptide polyglycine.
“The key finding [of this research] is a new way of peptide formation,” says Krasnokutski. “Aminoketene, being smaller than the simplest amino acid - glycine, is easier to form, and easy to polymerize. Therefore, it is much more likely that the first peptides and proteins were formed in this chemical way. This chemistry can take place even at very low temperatures found in the interstellar medium and involves the [chemical] species that are the most abundant in molecular clouds and protoplanetary disks.”
This study has clear implications for studying the beginnings of life – whether it began on Earth or extraterritorially. However, Krasnokutski emphasizes that there is still so much that we do not know about this newly discovered chemical pathway.
“We cannot currently completely rule out that it played a key role in the origin of life,” he said. “We will explore whether larger peptides that can already be called proteins can be formed in this chemical way. In the current experiments, relatively short glycine peptides, comprising less than 11 amino acids in the chains, were detected. However, their formation was completely spontaneous at low temperatures. Adding energetic photons, as present in the interstellar medium, or adding other chemicals that can act as catalysts or participate in the reaction could lead to the formation of much larger and more complex molecules,” Krasnokutski concludes.
Reference: Krasnokutski SA, Chuang KJ, Jäger C, Ueberschaar N, Henning Th. A pathway to peptides in space through the condensation of atomic carbon. Nat Astron. 2022. doi: 10.1038/s41550-021-01577-9.