Though they have been mentioned frequently in the earlier parts of this chapter, the structures of the electron transport chain participants, and particularly of the moieties that temporarily hold extra electrons, have not been addressed. So, now is the time to do so. The major players are the flavin mononucleotide (FMN) that plays a role in complex I, ubiquinone (Coenzyme Q), the lipid-soluble electron carrier, the heme groups of the cytochromes, and iron-sulfur clusters, found in complexes I, II, and III.
Figure 11. Flavin mononucleotide and Ubiquinone are electron carriers.
Flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD), are pictured in figure 11. Note the triple-ring structure and the three possible oxidation states. All three states are stable - the semiquinone state is not merely a transient form. This stability allows the conversion from carriers that can only handle one electron to carriers that can handle two electrons, and vice versa. The same holds true for ubiquinone - stable as ubiquinone (fully oxidized), semiubiquinone (radical state), and ubiquinol (fully reduced). Alternative nomenclature for these molecules is Coenzyme Q, CoQH+, and CoQH2. Note the aromaticity gained by ubiquinone when it is reduced. This enhances its stability and its suitability as a receiver of electrons from NADH.
Figure 12. Left: The heme group shows one of the possible combinations of the groups attached to the outside of the porphyrin ring. Right: The Fe-S centers also have variations, such as 4 Fe and 4 S, or even just 1 Fe surrounded by the S atoms of the four cysteines.
Heme groups (figure 12) are considerably larger, encompassing a porphyrin ring with an iron ion held in its center. This iron ion alternates between ferric (Fe3+) and ferrous (Fe2+) states as the heme group is oxidized and reduced, respectively. In the case of complex IV, the iron ion can form a complex with O2, which can then receive the electrons being held by the ring structure. This large structure is particularly important because it needs to be able to transfer a total of 4 electrons to reduce O2 to 2 H2O.
Finally, Fe-S clusters (figure 12) can also act as electron carrying moieties. Like in the heme group, the iron atom can readily switch between the ferric and ferrous states.