Coacervates: Early Earth's Proto-Enzymes for Life's Chemistry

Researchers at UC Santa Barbara have discovered that naturally forming liquid droplets, known as coacervates, create an environment that promotes reduction and oxidation (redox) reactions, which are fundamental to the development of life. These findings suggest that coacervates could have functioned as 'proto-enzymes' on early Earth, enabling the formation of complex organic molecules. The study was published in the Proceedings of the National Academy of Sciences.

The Research Approach

Co-lead author Nick Watkins elaborated on the team's methodology:

"The team developed a method using electrochemistry to analyze the internal conditions of these biologically significant liquid droplets, revealing their role in facilitating chemical reactions."

This research builds upon previous work by UCSB professors Herbert Waite, Daniel Morse, and Mike Gordon.

The study investigated the hypothesis that the chemical processes leading to the first lifeforms on pre-biotic Earth occurred within small droplets. Coacervates are formed from macromolecules such as proteins, RNA, or other polymers that coalesce within a solution, distinct from oil droplets.

To determine if coacervates foster biologically relevant reactions, the team focused on redox reactions. These reactions involve the transfer of electrons and are integral to approximately one-third of all biochemical processes. For their experimental setup, researchers utilized an RNA molecule (polyuridylic acid) and a peptide (poly-L-lysine) to form the coacervate. They then studied the reduction of ferricyanide to ferrocyanide, a well-understood redox pair, to measure the coacervates' Gibbs energy, which indicates the spontaneity of a reaction. Measurements were taken at varying temperatures to differentiate between entropic and enthalpic contributions.

Key Insights

The research revealed that the environment within coacervates significantly increased the likelihood of redox reactions occurring spontaneously. While previous studies have shown that these droplets facilitate redox reactions, this research is the first to explain the underlying mechanism.

The team found that the internal environment of the coacervate alters the Gibbs energy of the reaction. This means the chemistry inside these droplets differs fundamentally from that in ordinary water, making electron donation easier within the droplet. Raman spectroscopy provided molecular-level evidence, linking changes in voltage to shifts in the internal environment of the droplet by tracking vibrational modes of iron-containing molecules.

Professor Lior Sepunaru emphasized the distinct nature of this internal environment:

"The chemistry inside coacervates is distinct from normal water, allowing for chemical and biochemical reactions that would otherwise be improbable in aqueous solutions, a factor deemed crucial for the origin of life."

Coacervates as 'Proto-Enzymes'

The authors propose that coacervates acted as 'proto-enzymes,' actively catalyzing certain reactions. Like enzymes, coacervates are composed of polymers and modify their microenvironment to promote specific reactions. However, coacervates are simpler, naturally occurring droplets rather than complex, evolved proteins. Their function as proto-enzymes could have facilitated the emergence of more complex biomolecules.

Looking Ahead

Future work will investigate how coacervates influence reaction speed, particularly for electron-transfer reactions where Gibbs energy and rate are correlated. The team also plans to examine more complex redox reactions to further understand their potential role in the origin of life.