Unlike the colorless atoms H, C, N, and O that constitute the bulk of biological molecules, transition metal ions are often brightly colored, which makes the proteins that contain them easy to study by spectroscopic methods using visible light. One family of such colored proteins, the cytochromes, contains a bound heme group, in which an iron atom is tightly held by four nitrogen atoms at the corners of a square in a porphyrin ring (Figure 14–15). Similar porphyrin rings are responsible both for the red color of blood and for the green color of leaves, binding an iron in hemoglobin or a magnesium in chlorophyll, respectively.
Iron–sulfur proteins contain a second major family of electron-transfer cofactors. In this case, either two or four iron atoms are bound to an equal number of sulfur atoms and to cysteine side chains, forming iron–sulfur clusters in the protein (Figure 14–16). Like the cytochrome hemes, these clusters carry one electron at a time.
The simplest of the electron-transfer cofactors in the respiratory chain—and the only one that is not always bound to a protein—is a quinone (called ubiquinone, or coenzyme Q). A quinone (Q) is a small hydrophobic molecule that is freely mobile in the lipid bilayer. This electron carrier can accept or donate either one or two electrons. Upon reduction (note that reduced quinones are called quinols), it picks up a proton from water along with each electron (Figure 14–17).
In the mitochondrial electron-transport chain, six different cytochrome hemes, eight iron–sulfur clusters, three copper atoms, a flavin mononucleotide (another electron-transfer cofactor), and ubiquinone work in a defined sequence to carry electrons from NADH to O2. In total, this pathway involves more than 60 different polypeptides arranged in three large membrane protein complexes, each of which binds several of the above electron-carrying cofactors.
As we would expect, the electron-transfer cofactors have increasing affinities for electrons (higher redox potentials) as the electrons move along the respiratory chain. The redox potentials have been fine-tuned during evolution by the protein environment of each cofactor, which alters the cofactor’s normal affinity for electrons. Because iron–sulfur clusters have a relatively low affinity for electrons, they predominate in the first half of the respiratory chain; in contrast, the heme cytochromes predominate further down the chain, where a higher electron affinity is required.
NaDh transfers Its Electrons to Oxygen through three Large Enzyme Complexes Embedded in the Inner Membrane
Membrane proteins are difficult to purify because they are insoluble in aqueous solutions, and they are easily disrupted by the detergents that are required to solubilize them. But by using mild nonionic detergents, such as octylglucoside or dodecyl maltoside (see Figure 10–28), they can be solubilized and purified in their native form, and even crystallized for structure determination. Each of the three different detergent-solubilized respiratory-chain complexes can be re-inserted
Figure 14–16 The structure of an iron–sulfur cluster. these dark brown clusters consist either of four iron and four sulfur atoms, as shown here, or of two irons and two sulfurs linked to cysteines in the polypeptide chain via covalent sulfur bridges, or to histidines. although they contain several iron atoms, each iron–sulfur cluster can carry only one electron at a time. Nine different iron– sulfur clusters participate in electron transport in the respiratory chain.
into artificial lipid bilayer vesicles and shown to pump protons across the membrane as electrons pass through them.
In the mitochondrion, the three complexes are linked in series, serving as electron-transport-driven H+ pumps that pump protons out of the matrix to acidify the crista space (Figure 14–18): 1.