11 julio, 2024

What are coacervates? Theory, characteristics and formation

We explain what coacervates are, their characteristics and formation

What are coacervates?

Coacervates are defined as droplets or colloidal groups formed by macromolecules such as proteins, nucleic acids, synthetic polymers, etc., which form spontaneously and are held together by different types of chemical interactions.

The word colloidal refers to one of the most striking characteristics of these structures, since they are liquid drops that contain two immiscible phases, that is, they do not mix with each other, either due to charge repulsion or hydrophobic effects related to the macromolecules that make them up.

In these drops a liquid-liquid phase separation occurs, where a more compact region -rich in macromolecules- is in a thermodynamic equilibrium with a dilute or liquid phase, with which it does not mix.

Coacervates often form spontaneously in aqueous solutions or mixtures, allowing a kind of stable compartmentalization without the presence of a membrane, as occurs with living cells.

Consequently, these structures occur commonly in nature and are very important for different biological processes in a large number of organisms.

However, the study of coacervates is not limited solely to our modern times. In the early 1920s, the Russian biochemist Alexander Oparin proposed that these groupings of molecules may have given rise to the first cells on the early earth.

coacervate theory

Different theories have been proposed throughout the history of humanity to explain the origin of living beings as we know them today.

After the dismissal of the theories about the origin of life by spontaneous generation, many leaned towards the idea of ​​the physicochemical origin of living beings.

This theory was postulated at the beginning of the 20th century by the Russian biochemist Alexander Oparin (in his book entitled The Origin of Life) and by the British geneticist John Burdon Haldane, whose works, although independent, shared very similar ideas.

The main argument of the theory was based on the fact that life could have arisen for the first time on Earth inside the structures that we now call coacervates, made up of an immense variety of organic molecules.

Based on their observations that coacervates could form even in very dilute solutions of different molecules, Oparin and Haldane proposed that coacervation (formation of coacervates) might have been the way in which the separation of the fluid phase occurred in the «broth». ” primal of abiotic Earth.

In other words, these scientists thought that the first cells could have been formed from the spontaneous grouping of organic molecules contained in the waters of the seas, including proteins with enzymatic activity, which could become more complex and establish more ordered and autonomous structures.

Primal broth?: Abiotic synthesis

Although coacervates played a central role in Oparin’s and Haldane’s origin-of-life theory, it only made sense in light of the view that a long period of abiotic synthesis would have previously had to occur.

This synthesis referred to the production and accumulation of organic compounds precursors of proteins and nucleic acids, for example, thanks to:

The action of energy from ultraviolet radiation and electrical storms.
To the contribution of material from volcanoes and space bodies.
To the aqueous environment of the primitive seas.
Initial Earth atmospheric conditions.

Miller and Urey experiments

A few years after Oparin and Haldane presented their theories to the scientific community, the American biochemists Stanley Miller and Harold Urey, in the 1950s, designed a series of experiments to try to recreate primitive conditions on Earth and support or disprove the coacervate theory.

In their experiments, Stanley and Urey were able to obtain simple organic molecules, such as amino acids, supporting the idea that life could have arisen from relatively simple organic precursors and with the conditions hypothesized for the abiotic Earth.

Characteristics of coacervates

They are groups of organic macromolecules, usually proteins or nucleic acids.
They are colloids, since they have two immiscible phases, which do not mix with each other, and which are in suspension, isolated from the rest of the liquid that surrounds them.
They form spontaneously from associating molecules in suspension, and are very common in different cellular and environmental contexts.
They are organized structures, in which there is a balance between the dense cluster of molecules that form them and their aqueous portion.
They can be simple, made up of a single type of molecule, or complex, made up of more than one kind of molecule.
They are able to maintain their structure by themselves and even increase their complexity by introducing new molecules from the environment.
Different chemical reactions can occur inside it, varying its composition, weight and volume in a very short time.
They share certain properties of living cells, but they are not considered living entities, since they are not capable of reproducing or feeding themselves.

Formation of coacervates (coacervation)

The formation of coacervates is also known as coacervation and is the process that generally originates from a liquid-liquid phase separation, which consists of the reversible separation of a homogeneous liquid into two phases, one more concentrated than the other. other.

Coacervation depends on different physicochemical conditions such as temperature, pH, salt concentration, macromolecule concentration, etc. Furthermore, this process depends on the type of coacervate that is formed, whether it is simple or complex.

As we mentioned earlier, simple coacervates are made up of a single type of organic molecule, and complexes are made up of two or more different molecules.

During the formation of simple coacervates, a series of chemical interactions occur that depend on the characteristics of the macromolecule in question. For example, in the case of molecules of the same protein, the formation of coacervates has to do with the self-assembly of these molecules through electrostatic, van der Waals, or hydrophobic interactions.

Instead, the formation of complex coacervates depends mainly on electrostatic interactions, since electrostatic neutralization has been shown to favor the coalescence of macromolecules in an equilibrium solution.

Functions and applications of coacervates

For example, in some marine molluscs and polychaetes the formation of extracellular coacervates is essential for many of their primary functions.

In addition, these have been associated with intracellular protein aggregation, trafficking through the nuclear pore complex, and some neurodegenerative diseases in humans.

Coacervates are also exploited in the food industry, in the research fields of biophysics, biomaterials and cell biology, mainly due to their impressive diversity, composition and ordering or topology.

References

Astoricchio, E., Alfano, C., Rajendran, L., Temussi, PA, & Pastore, A. (2020). The Wide World of Coacervates: From the Sea to Neurodegeneration. Trends in biochemical sciences.
Clark, BC, & Kolb, VM (2020). Macrobiont: Cradle for the Origin of Life and Creation of a Biosphere. Life, 10(11), 278.
Lazcano A. (2010). Historical development of origins research. Cold Spring Harbor perspectives in biology, 2(11), a002089. https://doi.org/10.1101/cshperspect.a002089
Novak, VJ (1974). The coacervate-in-coacervate theory of the origin of life. In The Origin of Life and Evolutionary Biochemistry (pp. 355-368). Springer, Boston, MA.
Solomon, E.P., Berg, L.R., & Martin, D.W. (2011). Biology (9th edn). Brooks/Cole, Cengage Learning: USA.

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