Figure 14–20 The structure of NADH dehydrogenase. (a) the model of the mitochondrial complex shown here is based on the x-ray structure of the smaller bacterial complex, which works in the same way. the matrix arm of NaDh dehydrogenase (also known as Complex I) contains eight iron–sulfur (FeS) clusters that appear to participate in electron transport. the membrane contains more than 70 transmembrane helices, forming three distinct proton-pumping modules, while the matrix arm contains the electron-transport cofactors. (B) NaDh donates two electrons, via a bound flavin mononucleotide (FMN; yellow), to a chain of seven iron–sulfur clusters (red and yellow spheres). From the terminal iron–sulfur cluster, the electrons pass to ubiquinone (orange). Electron transfer results in conformational changes (black arrows) that are thought to be transmitted to a long amphipathic α helix (purple) on the matrix side of the membrane arm, which pulls on discontinuous transmembrane helices (red) in three membrane subunits, each of which resembles an antiporter (see Chapter 11). this movement is thought to change the conformation of charged residues in the three proton channels, resulting in the translocation of three protons out of the matrix. a fourth proton may be translocated at the interface of the two arms (dotted line). (C) this shows the symbol for NaDh dehydrogenase used throughout this chapter. (adapted from r.G. Efremov, r. Baradaran and L.a. Sazanov, Nature 465:441–445, 2010. pDB code: 3M9S.) the respiratory chain, ubiquinol tranfers electrons from NADH dehydrogenase to cytochrome c reductase. Because the protons in this QH2 molecule are taken up from the matrix and released on the opposite side of the crista membrane, two protons are transferred from the matrix into the crista space per pair of electrons transferred (Figure 14–21). This vectorial transfer of protons supplements the electrochemical proton gradient that is created by the NADH dehydrogenase proton pumping just discussed.
Cytochrome c reductase is a large assembly of membrane protein subunits. Three subunits form a catalytic core that passes electrons from ubiquinol to cytochrome c, with a structure that has been highly conserved from bacterial ancestors (Figure 14–22). It pumps protons by a vectorial transfer of protons that involves a binding site for a second molecule of ubiquinone; the elaborate
Figure 14–21 How a directional release and uptake of protons by a quinone pumps protons across a membrane.
two protons are picked up on the matrix side of the inner mitochondrial membrane when the reaction Q + 2e – + 2h+ → Qh2 is catalyzed by the NaDh dehydrogenase complex. this molecule of ubiquinol (Qh2) diffuses rapidly in the plane of the membrane, becoming bound to the crista side of cytochrome c reductase. When its oxidation by cytochrome c reductase generates two protons and two electrons (see Figure 14–17), the two protons are released into the crista space. the flow of electrons is not shown in this diagram.
770 Chapter 14:  Energy Conversion: Mitochondria and Chloroplasts electrons out to
Figure 14–22 The structure of cytochrome c reductase. Cytochrome c reductase (also known as the cytochrome b-c1 complex) is a dimer of two identical 240,000-dalton halves, each composed of 11 different protein molecules in mammals.