Small polymers that mimic enzymes may have helped start life

Most of the origins of life research effort has focused on understanding the prebiotic formation of basic biological elements. However, it is possible that early biological evolution relied on different chemical structures and processes, and these were gradually replaced over time by eons of evolution. Recently, chemists Irena Mamajanov, Melina Caudan and Tony Jia of the Earth-Life Science Institute (ELSI) in Japan borrowed ideas from polymer science, drug delivery, and biomimicry to explore this possibility. Surprisingly, they found that even small, highly branched polymers could serve as effective catalysts, which could have helped start life.

In modern biology, encoded protein enzymes do most of the catalytic work in cells. These enzymes are made up of linear polymers of amino acids, which fold and fold on themselves to form fixed three-dimensional shapes. These preformed shapes allow them to interact very specifically with the chemicals whose reactions they catalyze. Catalysts help reactions to happen much faster than they otherwise would, but are not consumed in the reaction themselves, so a single catalyst molecule can help the same reaction happen multiple times. . In these three-dimensional folded states, most of the catalyst structure does not directly interact with the chemicals it acts on, and simply helps the enzyme structure maintain its shape.

In the present work, ELSI researchers investigated hyperbranched polymers – tree-like structures with a high degree and density of branching that are inherently globular without the need for informed folding – which is necessary for modern enzymes. Hyperbranched polymers, like enzymes, are able to position catalysts and reagents, and modulate local chemistry precisely.

Most research efforts on the origins of life have focused on understanding the prebiotic formation of modern biological structures and building blocks. The logic is that these compounds exist now, and so understanding how they might be made in the environment could help explain how they were born. However, we only know of one example of life and we know that life is constantly changing, which means that only the most successful variants of organisms survive. Thus, it may be reasonable to assume modern organisms may not be very similar to first organisms, and it is possible that prebiotic chemistry and early biological evolution relied on different chemical structures and processes than modern biology to reproduce. By analogy with technological development, the first cathode ray televisions performed more or less the same function as modern high definition displays, but they are fundamentally different technologies. One technology led to the creation of the other in a way, but it was not necessarily the logical and direct precursor of the other.

If this type of “ scaffolding ” model for biochemical evolution is true, the question becomes what type of simpler structures, in addition to those used in contemporary biological systems, could have helped achieve the same? types of catalytic functions that modern life demands? Mamajanov and his team felt that hyperbranched polymers could be good candidates.

The team synthesized some of the hyperbranched polymers they studied from chemicals that could reasonably be expected to be present on early Earth before life began. The team then showed that these polymers can bind to small natural inorganic clusters of atoms known as zinc sulfide nanoparticles. These nanoparticles are known to be exceptionally catalytic in themselves.

As lead scientist Mamajanov comments, “We tried two different types of hyperbranched polymer scaffolds in this study. To get them to work, all we had to do was mix a zinc chloride solution and a polymer solution, then add sodium sulfide, and “voila” we got a nanoparticle catalyst. stable and efficient.

The team’s next challenge was to demonstrate that these hyperbranched polymer-nanoparticle hybrids could actually do something interesting and catalytic. They found that these metal sulfide doped polymers that degrade small molecules were particularly active in the presence of light, in some cases catalyzing the reaction up to a factor of 20. As Mamajanov says, “Until now, we only explored two possibilities. scaffolding and a single dope. There are undoubtedly many more examples to be discovered.

The researchers further noted that this chemistry may be relevant to a model of the origin of living things known as the “zinc world”. According to this model, the first metabolism was driven by photochemical reactions catalyzed by zinc sulfide minerals. They believe that with some modifications, these hyperbranched scaffolds could be adjusted to study analogs of protein enzymes containing iron or molybdenum, including important enzymes involved in modern biological nitrogen fixation. Mamajanov said, “The other question this raises is, assuming life or pre-life used this type of scaffolding process, why did life ultimately settle on enzymes? Is there an advantage to using linear polymers over branched polymers? How, when and why did this transition occur? ”