The most fundamental requirement for the emergence of life is the existence of a closed compartment, and this manifests in modern life as the cell membrane. The cell is an extremely complex and highly evolved machine, and the oldest fossil records dating back several billion years show organisms with structures that are not so different to the modern versions. It is logical that these sophisticated cellular structures must have evolved from a more simple, less complex progenitor, but the lack of fossil evidence for these ‘protocells’ represents the ultimate missing link in the evolution of life.
Professor Stephen Mann and colleagues in the School of Chemistry have now attacked this intriguing problem by designing a chemical system which spontaneously assembles into water-containing micro-droplets, which contain the cellular energy molecule, adenosine triphosphate (ATP), and a short polymer of the natural amino acid lysine.
The droplets form via a process known as coacervation, and unlike modern cells they are membrane-free. Intriguingly, the droplets select and sequester molecules such as iron porphyrins, which are ancient molecules essential for oxygen binding, and the environment in the interior of the droplets is amenable to other chemical processes including nanoparticle and enzymatic catalysis powered by the ATP contained within the droplets.
In essence, Professor Mann’s system represents perhaps the simplest protocell model of cell formation on the early Earth, where the increasing complexity required for life is driven by chemical evolution via selective partitioning between the interior and exterior of the droplets without the need for a membrane.
“Taken together, our results suggest that peptide–nucleotide microdroplets can be considered as a new type of protocell model that could be used to develop novel bioreactors, primitive artificial cells and plausible pathways to prebiotic organization before the emergence of lipid-based compartmentalization on the early Earth,” the authors say.
‘Peptide–nucleotide microdroplets as a step towards a membrane-free protocell model’ by Shogo Koga, David S.Williams, AdamW. Perriman and Stephen Mann in Nature Chemistry