Skip to main content
Biology LibreTexts

A Hypothesis for How ETC May Have Evolved*#

  • Page ID
    21254
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    A Hypothesis for How ETCs May Have Evolved

    A proposed link between SLP/fermentation and the evolution of ETCs:

    In a previous discussion of energy metabolism, we explored substrate level phosphorylation (SLP) and fermentation reactions.  While SLP and fermentation together are perfectly good ways to harvest energy, one byproduct of these reactions is the acidification of the cell.  Early cells that used these modes of energy harvested therefore needed to co-evolve mechanisms that helped remove protons accumulated from SLP and fermentation from the cytosol (interior of the cell). One solution to the "proton problem" may have been the evolution of the F0F1-ATPase, a multi-subunit enzyme that translocated protons from the inside of the cell to the outside of the cell by hydrolyzing ATP (see the figure below). This arrangement works as long as small reduced organic molecules are abundant and freely available to generate ATP can through SLP that can "fuel" the business of the cell. However, as these biological processes continue, the small reduced organic molecules will become depleted. The resulting scarcity of fuel, therefore, puts a demand on cells to find alternative mechanisms to harness energy and/or to become more efficient.

    In the scheme proposed above, one potential source of "wasted ATP" is its use in the removal of protons from the cell's cytosol; organisms that could find other mechanisms to expel accumulating protons while still preserving ATP could have a selective advantage. We hypothesize that this selective evolutionary pressure potentially led to the evolution of the first membrane-bound proteins that used red/ox reactions as their energy source (depicted in a second picture) to pump out the accumulating protons. Enzymes and enzyme complexes with these properties exist today in the as electron transport complexes like Complex I, the NADH dehydrogenase.

    1a.jpg

    Figure 1. Proposed evolution of an ATP dependent proton translocator

    1b.jpg

    Figure 2. As small reduced organic molecules become limited, organisms that can find alternative mechanisms to remove protons from the cytosol may have an a selective advantage. The evolution of a proton translocator that uses red/ox reactions rather than ATP hydrolysis could substitute for the ATPase.

    Continuing with this line of logic, if organisms evolved that could now use red/ox reactions to translocate protons across the membrane they would create an electrochemical gradient, separating both charge (positive on the outside and negative on the inside, creating an electrical potential) and pH (low pH outside, higher pH inside). With excess protons on the outside of the cell membrane, and the F0F1-ATPase no longer consuming ATP to translocate protons, we hypothesize that the electrochemical gradient could then power the F0F1-ATPase "backwards" — that is, to form or produce ATP by using the energy in the charge/pH gradients set up by the red/ox pumps (as depicted below). We call this arrangement an electron transport chain (ETC).

    1d.jpg

    Figure 3. The evolution of the ETC; the combination of the red/ox driven proton translocators coupled to the production of ATP by the F0F1-ATPase.

    Note: Extended reading on the evolution of electron transport chains

    If you're interested in the the evolution of electron transport chains, check out this more in-depth discussion of the topic at NCBI.

     


    Possible NB Discussion nb-sticker.pngPoint

    Dinitrophenol (DNP) is a small chemical that serves to uncouple the flow of protons across the inner mitochondrial membrane to the ATP synthase, making the membrane leaky to protons. People used it until 1938 as a weight-loss drug. What effect would you expect DNP to have on the difference in pH across both sides of the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug? Why might it be dangerous? Can you think of any scenarios where it is non-harmful, or even beneficial, to uncouple proton flow with ATP synthase?


     

     


    This page titled A Hypothesis for How ETC May Have Evolved*# is shared under a not declared license and was authored, remixed, and/or curated by Marc Facciotti.

    • Was this article helpful?