The mitochondrial electron transport chain is composed of three main membrane-associated electron carriers flavoproteins (FMN, FAD), cytochromes, and quinones (coenzyme Q, also known as ubiquinone because it is a ubiquitous quinone in biological systems).
All these electron carriers reside within the inner membrane of the mitochondria and operate together to transfer electrons from donors, like NADH and FADH2, to acceptors, such as O2. The, electrons flow from carriers with more negative reduction potentials to those with more positive reduction potentials and eventually combine with O2 and H to form water.
However, the mitochondrial electron transport system is arranged into four enzyme complexes of carriers, each capable of transporting electrons part of the way to O2 (Fig. 24.5). Coenzyme Q and cytochrome c connect the complexes with each other.
The four enzyme complexes of carriers are: NADH-Q oxidoreductase, succinate-Q-reductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase. These complexes are the enzyme complex and each of them consists of different prosthetic groups (Table 24.2).
The process of mitochondrial electron transport chain is summarized in Figure 24.6, which shows the flow of electrons and protons through the four enzyme complexes of the transport chain.
The whole process of mitochondrial electron transport can be represented in brief in the following manner:
1. Electrons donated by NADH enter the chain at complex I (NADH-Q-oxidoreductase) and pass through a flavoprotein (FMN) to a series of iron-sulphur-proteins (FeS) and then to ubiquinone (Q).
2. Electrons donated by succinate enter the chain at Complex II (succinate-Q-reductase) and pass through a flavoprotein (FAD) and FeS centres and then to ubiquinone (Q).
3. Ubiquinone (Q) serves as a mobile carrier of electrons received from complexes I and II and passes them to complex III (Q-cytochrome c oxidoreductase).
4. Complex III called Q-cytochrome c oxidoreductase or cytochrome bc1 complex passes the electrons through its prosthetic groups Cyt bL (Heme bL), Cyt bH (heme bH), FeS, and Cyt cL (Heme cL) to cytochrome c.
5. Cytochrome c (Cyt c), a mobile connecting link between complex III and IV, passes electrons to complex IV (cytochrome c oxidase). The latter carries electrons through its prosthetic groups Cyt a (Heme a), Cyt a3 (Heme a3) CuA and CuB and transfers them to molecular oxygen, reducing it to H2O.
6. Electron flow through complexes I, III and IV is accompanied by proton flow from the mitochondrial matrix (which becomes negatively charged) to inter membrane space or cytosolic side (which becomes positively charged). The number of protons (H+) moved across the membrane at each site per pair of electrons transported is still somewhat uncertain; the current consensus is that at least 10 protons move outward during NADH oxidation.