However, many other organisms, all of them microbes (and include bacterial, archaeal and eykaryotic members) can use other compounds as terminal electron acceptors. These other compounds include common ions as nitrate (NO
3- ), reduction potential of +0.42, and nitrite (NO
2- ), reduction potential of +0.72, or tetrathionate (S
4 O
62- ) reduction potential of +0.024. When the terminal electron acceptor is
not molecular oxygen (O
2 ) then the process is considered
anaerobic and is referred to as
anaerobic respiration . The ability of an organism to vary its terminal electron acceptor provides metabolic flexibility and can ensure better survival if any given terminal acceptor is in limited supply. Think about this, in the absence of oxygen we die; but an organism that can use a different terminal electron acceptor can survive.
A generic example of a simple, 2 complex etc
Figure 1 shows a generic electron transport chain, composed of two integral membrane complexes; Complex I
ox and complex II
ox . A reduced high energy electron donor, designated HD (such as NADH or FADH
2 ) reduces complex 1
ox giving rise to the oxidized form D (such as NAD or FAD). Simultaneously, a prosthetic group within complex I is now reduced (accepts the electrons) the energy released is used to translocate a proton from one side of the membrane to the other. The net result is that one surface becomes more negatively charged, due to an excess of hydroxyl ions (OH
- ) and the other side becomes positively charged due to an increase in protons on the other side. Complex I
red can now reduce the prosthetic group in Complex II
red while simultaneously oxidizing Complex I
red . Electrons pass from Complex I to Complex II via red/ox reactions, regenerating Complex I
ox which can repeat the process. Complex II
red reduces A, the terminal electron acceptor to regenerate Complex II
ox and create the reduced form of the terminal electron acceptor. In this case, Complex II can also translocate a proton during the process. If A is molecular oxygen, water (AH) will be produced. This reaction would then be considered a model of an aerobic ETC. However, if A is nitrate, NO
3- then Nitrite, NO
2- is produced (AH) and this would be an example of an anaerobic ETC.
Generic 2 complex electron transport chain. In the figure, DH is the electron donor (donor reduced) and D is the donor oxidized. A is the oxidized terminal electron acceptor and AH is the final product, the reduced form of the acceptor. As DH is oxidized to D, protons are translocated across the membrane, leaving an excess of hydroxyl ions (negatively charged) on one side of the membrane and protons (positively charged) on the other side of the membrane. The same reaction occurs in Complex II as the terminal electron acceptor is reduced to AH.
Based on Figure 2 above and using the electron tower in Figure 1, what is the difference in the electrical potential if (A) DH is NADH and A is O
2 and (B) DH is NADH and A is NO
3- . Which pairs (A or B) provides the most amount of usable energy?
