Overview of the mitochondrial electron transport chain intermembrane space
or two electrons. NADH and a cofactor called FADH2 each carry two electrons. Reduced cytochromes carry single electrons as do the iron-sulfur clusters. Flavins and coenzyme Q can carry either one or two electrons.
Figure 12.7 shows an overview of the electron transport chain. Complex I (sometimes called NADH dehydrogenase or NADH-Q oxidoreductase) accepts electrons from NADH, thus oxidizing it, and uses these to reduce coenzyme Q (Fig. 12.8): In doing so it moves four hydrogen ions outward from the matrix to the mitochondrial intermembrane space. The reaction can be summarized as
The transport of the hydrogen ions is accomplished using coenzyme Q as it picks up a hydogen ion when it picks up an electron. Complex I has a bound coenzyme Q that collects two hydrogen ions from the matrix when it accepts two electrons from iron-sulfur clusters (which received them from a bound flavin which accepted them from NADH). The bound coenzyme QH2 formed then passes its electrons to iron-sulfur clusters, which finally reduce a mobile coenzyme Q, which again collects two hydrogen ions from the matrix. These are released to the intermembrane space when this mobile carrier is reoxidized. The machinery that produces this directionality is not yet clear.
Complex II (also known as succinate-Q reductase complex or succinate dehydroge-nase) is the only one of the four complexes that is not a hydrogen ion pump. As we will see in Chapter 13 (page 284) complex II is part of the Krebs cycle—it is the enzyme that oxidizes succinate to fumarate using FAD, which becomes FADH2. It passes its electrons to mobile coenzyme Q. Other mitochondrial enzymes that generate FADH2 share this property of being able to reduce coenzyme Q. Because hydrogen ions are not moved across the membrane in this process one gets less ATP formed from the electrons on FADH2 than from those on NADH.
Complex III (Q-cytochrome c oxidoreductase or cytochrome reductase) accepts electrons from reduced coenzyme Q (QH2) and uses them to reduce the other mobile electron matrix
Figure 12.8. Overview of the operation of the mitochondrial electron transport chain.
carrier cytochrome c. This process results in the movement of four hydrogen ions from the matrix to the intermembrane space. Complex II marks a change from two electrons being carried to one. It contains hemes, bound coenzyme Q, and iron-sulfur clusters.
Cytochrome c moves the electrons to complex IV (cytochrome c oxidase). This protein complex reduces oxygen to water and moves four hydrogen ions from the matrix to the intermembrane space. It contains two types of heme and copper ions. Oxygen binds to Fe2+ in a heme just as it binds to the heme in myoglobin or hemoglobin. The overall reaction is
4Cyto Creduced + 8H+atnx + O2 ^ 4Cyto Coxidized + 2H2O +
The electron transport chain is very similar in all organisms. In eukaryotes the complexes are found in the inner mitochondrial membrane, while in prokaryotes the complexes are found in the plasma membrane.
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