Exobiology and Evolutionary Biology

Prebiotic formation of membranes and the role of membranes in energy uptake: An essential step in the origin of protocellular life (2)

PI: Pierre-Alain Monnard

It is likely that early forms of cellular life, so called protocells, possessed membranous boundaries composed of self-assembled amphiphile bilayers just as contemporary cell do. Early membranes must have played an essential role in the functioning of protocells, by preserving their internal content and directing the exchanges between them and their environment. In particular, the uptake of both energy (such as solar irradiation) and nutrients to yield useful products must have been mediated by early membranes. Energy conversion that yielded new chemical bonds would have represented the first crucial step in the evolution toward cells because it would have allowed for a semi-autonomous, controlled metabolism to emerge.

This project proposes a coordinated three-step effort to achieve the prebiotic formation of membranes capable of energy uptake, in particular light energy.
i) We will investigate the formation of plausible protocell membranes composed of mixtures of short-chain fatty acid and their derivatives. Studies of the organics available on the early Earth have established that these compounds were present as complex mixtures. The “mixed” membrane character was presumably essential to the functioning of a protocell lacking an evolved protein machinery because it would have allowed enhanced diffusion of nutrients. Furthermore, it might have played a triggering role in protocellular division since mixed membranes have an increased probability of forming domains along which membrane budding can take place.
ii) The role of primitive membranes in the metabolism and energy conversion will then be emphasized. We will encapsulate an experimental photochemical system within membraneous compartments. We have recently demonstrated that this photochemical system, which includes a ruthenium metal complex with a single nucleobase attached to a bipyridine ligand can catalyze the formation of fatty acid molecules from picolyl ester precursors. Unlike the fatty acid products, these precursors do not form any stable structure in water, but can absorb into preformed membranes. The integral production of amphiphiles and its impact on the growth and replication of the protocell by division will be established both at the level of bulk suspensions and single protocell compartments.
iii) Mixtures of fatty acids, polycyclic aromatic hydrocarbons (PAH) and alkanes as precursors will be investigated for their ability to yield membraneous protocell compartments that could be used to study PAH energy conversion aimed at synthesizing amphiphiles from alkanes. The role of PAHs as primitive energy conversion systems in protocells has been postulated in the literature, however the effect of PAHs and amphiphile precursors on the protocell integrity is still lacking.
These studies will represent a significant advance in the study of the emergence of autonomous protocells capable of processing nutrients into their own building blocks using an external energy source (e.g., light).