A Hypothesis as to how ETC may have evolved
A proposed link between SLP/Fermentation and the evolution of ETCs
When we last discussed energy metabolism, it was in context of substrate level phosphorylation (SLP) and fermentation reactions. One of the questions in the Discussion points was what would be the consequences of SLP, both short-term and long-term to the environment? We discussed how cells would need to co-evolve mechanisms to remove protons from the cytosol (interior of the cell), which lead to the evolution of the F0F1ATPase, a multi-subunit enzyme that translocates protons from the inside of the cell to the outside of the cell by hydrolyzing ATP as shown in figure 6 below. This arrangement works as long as small reduced organic molecules are freely available, making SLP and fermentation advantageous. As these biological process continue, the small reduced organic molecules begin to be used up and their concentration decreases, putting a demand on cells to be more efficient. One source of potential "ATP waste" is in the removal of protons from the cell's cytosol, organisms that could find other mechanisms could have a selective advantage. Such selective pressure could have led to the first membrane-bound proteins that could use Red/Ox reactions as their energy source, as depicted in figuire 7. In other words use the energy from a Red/Ox reaction to move protons. Such enzymes and enzyme complexes exist today in the form of the electron transport complexes, like Complex I, the NADH dehydrogenase.
Figure 1: Proposed evolution of an ATP dependent proton translocator
Figure 2: As small reduced organic molecules become limiting organisms that can find alternative mechanisms to remove protons from the cytosol may have had and advantage. The evolution of a proton translocator that uses the energy in a Red/Ox reaction could substitute for the ATAase.
Continuing with this line of logic, there are organisms that can now use Red/Ox reactions to translocate protons across the membrane, instead of an ATP driven proton pump. With protons being being translocated by Red/Ox reactions, this would now cause a build up of protons on the outside of the membrane, separating both charge (positive on the outside and negative on the inside; an electrical potential) and pH (low pH outside, higher pH inside). With excess protons on the outside of the cell membrane, and the F0F1ATPase no longer consuming ATP to translocate protons, the pH and charge gradients can be used to drive the F0F1ATPase "backwards"; that is to form or produce ATP by using the energy in the charge and pH gradients set up by the Red/Ox pumps as depicted in figure 8. This arrangement is called an electron transport chain (ETC).
Figure 3: The evolution of the ETC; the combination of the Red/Ox driven proton translocators coupled to the production of ATP by the F0F1ATPase.