Modelling the chemiosmotic mode of ATP synthesis in protocells

All modern organisms feature chemiosmosis (ion-motive-force driven processes, including membrane transport and ATP production), and therefore the Last Common Universal Ancestor (LUCA) is believed to be chemiosmotic; although contradictorily. As for chemiosmosis to happen, cells utilise membranes relatively impermeable to the coupling ion (H+ or Na+).

It has previously been shown that, theoretically, LUCA could drive chemiosmosis using a geo-chemical H+-gradient but only if it had a leaky membrane. However, a leaky membrane rapidly achieves electrochemical equilibrium offsetting theH+ influx.

For the H+ influx, and therefore chemiosmosis, to continue, the membrane needs a system to pump H+.Paradoxically, pumping H+ across a leaky membrane is futile.

A potential solution has previously been hypothesised in the form of membranes leaky to H+ that harboured: an ATP-synthase coupled to both H+ and Na+, and an antiporter that pumps out Na+ for incoming H+. Such a Na+/H+ antiporter could convert the geochemical H+ gradient into a Na+-gradient.

It would be advantageous to do so as a Na+-gradient could be better sustained by the LUCA membrane and could drive ATP production via the promiscuous ATP-synthase. My project aims at obtaining empiricalmvalidation for this hypothesis.

To do so, a promiscuous ATP-synthase will be co-reconstituted, alongside a Na+/H+ antiporter, into fatty-acid-containing hybrid liposomes (made from fatty acids and other amphiphilic (phospho)lipids).

Ion-gradients will be imposed on the liposomes by pH-jump ordecarboxylase-catalysed alkalisation methods to test whether the system could: a) convert the H+-gradient into a Na+-gradient via the Na+/H+ antiporter, and b) use the Na+- gradient to drive ATP production via the promiscuous ATP-synthase.

This study is critical to understand the evolution of chemiosmotic systems and will have implications in evolutionary and synthetic biology as well as astrobiology.