Small enzyme-mimicking polymers may have helped start life

Most difficulty in origin of life research is focused on understanding the prebiotic formation of biological building blocks. However, it is possible that early biological evolution was affected by various chemical structures and processes, and these were gradually replaced over time by aeons of evolution. Recently, chemists Irena Mamajanov, Melina Caudan, and Tony Jia at the Earth-Life Science Institute (ELSI) in Japan borrowed ideas from polymer science, drug delivery, and biomimetics to investigate this possibility. Surprisingly, they found that even small, high branch polymers could serve as effective catalysts, and these may have helped life in the beginning.

In modern biology, encoded protein enzymes do most of the catalytic work in cells. These enzymes consist of linear polymers of amino acids, which fold up and double on their own to form solid three-dimensional shapes. With these preformed forms, they can interact very specifically with the chemicals whose reactions they catalyze. Catalysts help prevent reactions much faster than they would otherwise, but are not consumed in the reaction itself, so a single catalyst molecule can cause the same reaction many times. In these three-dimensional filling states, most of the structure of the catalyst does not directly interact with the chemicals it does, and it simply helps the enzyme structure retain its shape.

In the present work, ELSI researchers studied hyperbranch polymers – empowered structures with a high degree and density of branches that are intrinsically globular without the need for informed folding – required for modern enzymes. Hyperbranch polymers, such as enzymes, are able to position catalysts and reagents, and modulate local chemistry in precise ways.

Most effort in origins of life research is focused on understanding the prebiotic formation of modern biological structures and building blocks. The logic is that these connections now exist, and so understanding how they could have been made in the environment can help explain how they came to be. However, we know of only one example of life, and we know that life is constantly evolving, which means that only the most successful variants of organisms survive. So it may be reasonable to assume modern organisms may not be very similar to the first organisms, and it is possible prebiotic chemistry and early biological evolution rely on different chemical structures and processes than modern biology to reproduce themselves. As an analogy with technological evolution, early cathode ray TV sets have more or less the same function as modern high definition screens, but they are basically different technologies. One technology led in one way or another to the creation of the other, but it was not necessarily the logical and direct forerunner of the other.

If this kind of ‘scaffolding’ model for biochemical evolution is true, then the question becomes what kind of simpler structures, besides those used in modern biological systems, might have helped to perform the same kinds of catalytic functions that modern life requires? Mamajanov and her team argued that hyper-industry polymers could be good candidates.

The team synthesized some of the hyperbranch polymers they studied from chemicals that could reasonably be expected to have been present on early Earth before life began. The team then showed that these polymers could form small naturally occurring inorganic clusters of atoms, known as zinc sulfide nanoparticles. Such nanoparticles are known to be unusually catalytic on their own.

As Chief Scientist Mamajanov remarks, ‘We examined two different types of hyperbranched polymer stems in this study. To make them work, all we needed was to mix a zinc chloride solution and a solution of polymer, then add sodium sulfide, and ‘voila,’ we got a stable and effective catalyst on nanoparticle. ‘

The next challenge of the team was to prove that these hyperbranched hybrids of polymer nanoparticle 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 they catalyzed the reaction by as much as a factor of 20. As Mamajanov says, ‘So far we have only two possible investigated scaffolding and only one dopant. Without a doubt, there are many, many more examples of this remaining to be discovered. ‘

The researchers further noted this chemistry may be relevant to an origin of the life model known as the ‘Zinc World’. According to this model, the first metabolism was driven by photochemical reactions catalyzed by zinc sulfide minerals. They think that with some modifications such hyperbranched scaffolds could be adapted to study analogs of iron as molybdenum-containing protein enzymes, including important ones involved in modern biological nitrogen fixation. Mamajanov says: ‘The other question that this raises is, if we assume that life as predestination uses this kind of stilling process, why does life ultimately depend on enzymes? Is there an advantage to using linear polymers over branches? How, when and why did this transition occur? ‘