The
electron transport chain , or
ETC , is made up of a group of protein complexes that undergo a series of red/ox reactions to translocate protons across the membrane to generate a PMF. Electrons enter the ETS from a high energy electron donor, often times this is in the form of NADH or FADH
2 , which are generated during catabolism (oxidation of carbon compounds, such as sugars or proteins or fats). Use the electron tower below (figure 1) as a reference guide to orient you as to where each component sits. Depending on the complexity of the ETC being used, electrons can enter at a variety of places depending upon the energy level of those entering electrons. To enter the ETC (electrons being donated to a red/ox complex within the chain), the electron donor must have a lower electronegativity than the electron acceptor (the complex that is taking the electrons). The donor will become oxidized and the acceptor will become reduced. The difference in the reduction potential between the donor and acceptor is the measure of energy released. If sufficient energy is released the cell can use it to do work, and in the case of an ETC that work would include translocating a proton from one side of the membrane to the other, setting up a PMF.
Electron Tower
Note : electrons entering the ETC do not have to come from NADH or FADH
2 . Many other compounds can serve as electron donors, the only requirement is that there exists an enzyme that can oxidize the electron donor and then reduce another compound. Even a small amount of energy can add up. For example there are bacteria that use H
2 as an electron donor. This is not too difficult to believe because the half reaction 2H
+ + 2 e
- /H
2 has a reduction potential (E
0' ) of -0.42 eV. If these electrons are eventually donated to oxygen then the ΔE
0' of the reaction is 1.24 eV and that is equivalent to a lot of energy, a large negative ΔG (-ΔG). Alternatively, there are some bacteria that can oxidize iron, Fe
2+ at pH 7 to Fe
3+ with a reduction potential (E
0' ) of +0.2 eV. These bacteria use oxygen as their terminal electron acceptor and in this case, the ΔE
0' of the reaction is approximately 0.62. Not so great, but still produces a -ΔG. The bottom line is that depending on the electron donor and acceptor that the organism uses, a little or a lot of energy can be harvested and used by the cell per electrons donated to the electron transport chain.
What are the complexes of the etc?
ETCs are made up of a series (at least one) of membrane associated (some are integral) red/ox complexes that move electrons from a donor source, such as NADH, to a final acceptor, such as oxygen (that's what we use). Each requires a reduced substrate as an electron donor and an oxidized substrate as the electron acceptor. In most cases the electron acceptor is a member of the enzyme complex. Once the complex is reduced, the complex can serve as the substrate (source of electrons) for the next reaction. In other words, think of the ETC as a series of complexes that passes electrons to the next complex, which eventually uses some oxidized compound as the final substrate (referred to as the
terminal electron acceptor ).
