Electron Transport

The electron transport system is the place in the cell where electrons generated by oxidation are transferred. Passage of the electrons through the system generates potential energy that is used to make ATP in oxidative phosphorylation.

The E 0' values for the electron carriers in the mitochondrial membrane increase in the same order as the sequence in which they are used in electron transport. The order is consistent with being exergonic for the redox reactions. Figure 15.10 lists the contents of the various multiprotein complexes described below:

> NADH and NADH Dehydrogenase (Complex I) - NADH is generated by numerous dehydrogenases in the cell. NADH is reoxidized to NAD+ by complex I of the mitochondria (also called NADH dehydrogenase). NADH dehydrogenase contains flavin mononucleotide (FMN) as a tightly bound prosthetic group and catalyzes the following reaction

> > NADH + H+ + FMN <=> NAD+ + FMNH 2

> Complex I contains about 25 separate polypeptide chains. It also contains iron-sulfur centers, which transfer electrons from FMNH 2 to the next carrier, coenzyme Q. Figure 15.4 shows four known structures of the non-heme iron sulfur complexes in Complex I. Iron centers undergo cyclic oxidoreduction between ferrous and ferric states, as shown here. Complex I is also called NADH-coenzyme Q reductase because the electrons are used to reduce coenzyme Q. The passage through Complex I can be blocked by the compounds rotenone and amytal and the artificial electron acceptor methylene blue can accept electrons from FMNH 2 Figure 15.9.

> Complex II (succinate dehydrogenase) - Complex II is not in the path traveled by electrons from Complex I (Figure 15.3). Instead, it is a point of entry of electrons from FADH 2 produced by the enzyme succinate dehydrogenase in the citric acid cycle. Both complexes I and II donate their electrons to the same acceptor, coenzyme Q. Complex II, like complex I, contains iron-sulfur proteins, which participate in electron transfer. It is also called succinate-coenzyme Q reductase because its electrons reduce coenzyme Q.

> Coenzyme Q (CoQ) - CoQ is a benzoquinone linked to a number of isoprene units (usually 10 in mammalian cells and 6 in bacteria). The isoprenoid tail gives the molecule its apolar character, which allows CoQ to diffuse rapidly through the inner mitochondrial membrane. CoQ has the ability to accept electrons in pairs and pass them one at a time through a semiquinone intermediate to Complex III (see here). This cycle is referred to as the Q cycle.

> Complex III - Complex III contains a diversity of electron carrying proteins. They include cytochrome b, iron sulfur centers, and cytochrome c 1. Cytochrome b is the first of the heme-carrying proteins (Figure 15.6) involved in electron transport. Passage of electrons from cytochrome b to the iron sulfur centers can be blocked by antimycin A. Also, the artificial electron acceptor phenazine methosulfate can accept electrons from cytochrome b and 2,6-dichlorophenol-indophenol can accept electrons from the iron sulfur proteins (Figure 15.9). The crystal structure of the redox components of complex III from bovine heart mitochondria is shown in Figure 15.16

> Cytochrome c - This small protein is the only one from the electron transport system not in a complex. It accepts electrons from complex III and shuttles them to complex IV. The artificial electron carrier, tetramethyl-p-phenylene diamine can accept electrons from cytochrome c (Figure 15.9).

> Complex IV - Complex IV is also known as cytochrome oxidase, because it takes electrons from cytochrome c. Complex IV contains cytochromes a and a 3. Cytochromes a and a 3 evidently represent two identical heme A moieties, attached to the same polypeptide chain. They are within different environments in the inner membrane, however, so they have different reduction potentials. Each of the hemes is associated with a copper ion, located close to the heme iron. Electrons that pass through complex IV can be blocked by cyanide, azide, and carbon monoxide and the artificial electron carrier, ferricyanide, can accept electrons from cytochrome a in the complex (Figure 15.9). A model for the final stages in proton pumping by cytochrome oxidase is shown in Figure 15.18.

See also: Difference Spectra, Inhibitors and Artificial Electron Acceptors, FMN, FMNH 2, NAD+, NADH, Rotenone, Amytal, Coenzyme Q, Cytochromes, Antimycin A, Cyanide, Azide, Carbon Monoxide.