Chemiosmosis

A couple of posts ago I showed how an unfavourable reaction, EH synthesis, could be driven by the hydrolysis of AD by coupling the two reactions because the reaction: AD + E + H ⇌ A + D + EH was favourable overall (with the given concentrations). Now we have defined transporters we can actually use the EH hydolysis reaction to drive AD synthesis, despite the overall reaction being unfavourable. We do this by making use of a chemical gradient to drive a reaction, which is effectively chemiosmosis. Note that this is an emergent property of the simulation and not something I specifically coded into it.

The cell diagrammed has an EH-pore which allows it to take up EH and hydrolyse it with an EHase. This results in a build up of E and H; the concentration gradient of H is then used to drive AD synthesis by an H-dependent AD synthase.

Diagram of cell illustrating chemiosmosis

If the cell is initialised with 1% AD and EH, and 0.5% A, D, E and H, and put in a solution of 1% EH, and 0.5% A and D, the AD hydrolysis reaction is more favourable than the EH hydrolysis reaction, but the concentration gradient of H can be used as an intermediate to drive the ADase reaction in reverse.

The reason this cell is able use the EHase reaction to drive the more favourable ADase reaction in reverse is that by going through the intermediate of the H concentration gradient, it can alter the stoichiometry of the reactions. The graph shows that over the course of 50 000 time units, the concentration of chemical E increase from 0.5% to over 2%. In this simulation, that corresponds to ~1000 units of E, meaning that the net EHase reaction, EH → E + H, has occured ~1000 times. The concentration of AD increases from 1% to ~1.4%, which corresponds to ~250 units. The overall reaction within the cell is therefore roughly:

A + D + 4 EH ⇌ AD + 4 E + 4 H

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