Living organisms arose from lifeless chemistry. But how? Perhaps the chemicals that support life as we know it also supported primordial life. If so, primordial life relied on RNA and DNA, as the RNA World and DNA World hypotheses would have it. But primordial life might have relied on something quite different—a chemical world that no longer exists.

This possibility looks a little more plausible in light of new research published by scientists from the Tokyo Institute of Technology’s Earth-Life Science Institute (ELSI). According to these scientists, primordial life may have relied on protoenzymes, specifically, hyperbranched polymer-scaffolded metal-sulfide nanocrystals. These scientists also suggest that protoenzymes were slowly replaced by more sophisticated enzymes, through chemical evolution, after life got started.

Details of the scientists’ work appeared August 13 in the journal Life, in an article titled, “Protoenzymes: The Case of Hyperbranched Polymer-Scaffolded ZnS Nanocrystals.” The article describes the formation of protoenzyme candidates by a “straightforward process.” These candidates include hyperbranched polyethyleneimine (HyPEI) and glycerol citrate polymer-supported ZnS nanocrystals (NCs).

The study’s authors—the ELSI’s Irena Mamajanov, Melina Caudan, and Tony Jia—asserted that their protoenzyme candidates demonstrate properties that may have facilitated the emergence of life. For example, the protoenzyme candidates possess high photocatalytic activity and resist excessive aggregation.

“The structure of these materials, a catalytic agent scaffolded by globular hyperbranched polymers, is superficially reminiscent of the enzymatic structure and therefore is a compelling model for the study of the chemical evolution of enzymes,” the article’s authors wrote. “The hyperbranched polymer scaffold could have been the early primitive augmenting scaffold to be replaced with more sophisticated ones throughout chemical evolution.”

In modern biology, coded protein enzymes do most of the catalytic work in cells. These enzymes, which consist of linear polymers of amino acids, fold up and double back on themselves to form three-dimensional shapes. In a typical enzyme, just a small part of the three-dimensional structure interacts with substrates. Most of the structure just plays a supporting role, preserving the enzyme’s overall shape.

In contrast, the ELSI researchers’ protoenzymes are tree-like structures with a high degree and density of branching. These hyperbranched polymer scaffolds are intrinsically globular without the need for the sort of informed folding required for modern enzymes. Hyperbranched polymer scaffolds, like enzymes, are capable of positioning catalysts and reagents, and modulating local chemistry in precise ways.

Functionally analogous and, in a gross sense, structurally analogous to modern enzymes, the ELSI’s hyperbranched polymer scaffolds may have fulfilled a role that modern enzymes ultimately took over.

ZnS hyperbranched polymers may have been superseded by modern enzymes. For example, as shown in this diagram, globular metal-sulfide/hyperbranched polymer particles may have been part of an evolutionary process that led to metal sulfide enzymes. [Irena Mamajanov, ELSI]
The ELSI team synthesized some of the hyperbranched 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 bind small naturally occurring inorganic clusters of atoms known as zinc sulfide (ZnS) nanoparticles. Such nanoparticles are known to be unusually catalytic on their own.

“We tried two different types of hyperbranched polymer scaffolds in this study,” lead scientist Mamajanov commented. “To make them work, all we needed to do was to mix a zinc chloride solution and a solution of polymer, then add sodium sulfide, and ‘voila,’ we obtained a stable and effective nanoparticle-based catalyst.”

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 especially active in the presence of light, in some cases, they catalyzed the reaction by as much as a factor of 20.

“So far, we have only explored two possible scaffolds and only one dopant,” Mamajanov pointed out. “Undoubtedly there are many, many more examples of this remaining to be discovered.”

The researchers further noted this chemistry may be relevant to an origins-of-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 adjusted to study analogues of iron- or molybdenum-containing protein enzymes, including important ones involved in modern biological nitrogen fixation.

The ELSI team, Mamajanov declared, is now entertaining deeper questions: “Assuming life or pre-life used this kind of scaffolding process, why did life ultimately settle upon enzymes? Is there an advantage to using linear polymers over branched ones? How, when, and why did this transition occur?”

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