The Electron-Transport Chain CHAPTER 20 The Electron-Transport Chain The cheetah, whose capacity for aerobic metabolism makes it one of the fastest animals

The role of Oxidative Phosphorylation and Electron-transport in Mitochondria Oxidative phosphorylation is the process by which NADH and FADH2 (QH2) are oxidized and ATP is formed Glycolysis and citric acid cycle are carried out to produce the reduced forms of NAD+ and FAD (Q) from oxidation of glucose. The membrane-associated electron transport system is a series of enzyme complexes embedded in the inner mitochondrial membrane, which oxidize NADH and QH2. Oxidation energy is used to transport protons across the inner mitochondrial membrane, creating a proton gradient The electrons from the oxidation NADH and QH2 are passed to a terminal electron acceptor usually oxygen (O2) to produce water ATP synthase (ATPase) is a key enzyme that used the proton gradient energy to produce ATP

Figure 20.2 Structure of the mitochondrion Outer membrane has few proteins. Channels are present that allow free diffusion of ions and water soluble metabolites. Inner membrane is very rich in protein (protein : lipid ratio of 4:1) Permeable to neutral molecules (O2 and CO2) Is a barrier to protons and large polar and ionic substances Polar substances must be actively transported (pyruvate transferase) Intermembrane space is where the protons are transported during the membrane-associated electron transport process Matrix contents include the enzymes associated with production of acetyl-CoA and the citric acid cycle (except succinate dehydrogenase complex). Protons are removed from the matrix during electron transport Figure 14.2 Structure of the mitochondrion. The outer mitochondrial membrane is freely permeable to small molecules but the inner membrane is impermeable to polar and ionic substances. The inner membrane is highly folded and convoluted forming structures called cristae. The protein complexes that catalyze the reactions of membrane-associated electron transport and ATP synthesis are located in the inner membrane. (a) Illustration. (b) Electron micrograph: longitudinal section from bat pancreas cell.

Figure 20.15: The electron transport chain V 3 H+ / ATP Complexes I, III, and IV pump protons across the inner membrane as electrons are transferred Mobile coenzymes: ubiquinone (Q) and cytochrome c serve as links between complexes Complex IV reduces O2 to water Complex V is ATP synthase, which uses the proton gradient across the membrane to make ATP

Figure 20.15: The electron transport chain V NADH donates electrons two at a time to complex I (NADH-Q reductase complex) of the electron transport chain 4 H+ are pumped across the inner mitochondrial membrane

X Figure 20.15: The electron transport chain 4 H+ / 2e- 4 H+ / 2e- V Complex I donates 2 e- to Ubiquinone (Q), forming QH2 QH2 donates 2 e- to Complex III (cytochrome c reductase complex)

Mobile electron carrier 1 Ubiquinone (Q) Q is a lipid soluble molecule that diffuses within the lipid bilayer of the inner mitochondrial membrane, accepting electrons from Complex I and Complex II and passing them to Complex III

Figure 20.15: The electron transport chain V Complex III donates one e- to cytochrome c cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex) This one e- transfer is repeated to transfer both electrons

Mobile electron carrier 2 Cytochrome c A protein associated with the outer face of the inner mitochondrial membrane. Transports electrons from complex III to complex IV.

Figure 20.15: The electron transport chain V A total of 10 H+ are pumped across the inner mitochondrial membrane for every two electrons donated to Complex I and the electrons transferred to oxygen to make H2O.

X Figure 20.15: The electron transport chain V FADH2 is bound to Complex II (succinate dehydrogenase)

Complex II. Succinate dehydrogenase Transfers electrons from succinate to flavin adenine dinucleotide (FAD) as a hydride ion (H:-), to an Fe-S complex (one electron at a time), to ubiquinone (Q), making QH2 - Complex II does not pump protons Figure 14.9 Electron transfer in Complex II. A pair of electrons is passed from succinate to FAD as part of citric acid cycle. Electrons are transferred one at a time from FADH2 to three Fe-S clusters and then to Q. (Only one Fe-S cluster is shown in the figure.) Two protons are taken up from the interior to form QH2. Complex II does not directly contribute to the proton concentration gradient but serves as a tributary that supplies electrons (as QH2) to the rest of the electron transport chain.

Structure of E. Coli succinate dehydrogenase complex Figure 14.8 Structure of the E. coli succinate dehydrogenase complex. A single copy of the enzyme showing the positions of FAD, the three Fe–S clusters, QH2, and heme b. Complex II contains three copies of this multisubunit enzyme. [PDB 1NEK]

X Figure 20.15: The electron transport chain V Complex II donates 2 e- to Ubiquinone (Q), forming QH2 QH2 donates 2 e- to Complex III (cytochrome c reductase complex)

X Figure 20.15: The electron transport chain 4 H+ / 2e- V Complex III donates one e- to cytochrome c cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex) This one e- transfer is repeated to transfer both electrons

X Figure 20.15: The electron transport chain 4 H+ / 2e- 2 H+ / 2e- V A total of 6 H+ are pumped across the inner mitochondrial membrane for every two electrons of FADH2 and the two electrons transferred to oxygen to make H2O.

involving Complexes I-IV Electron Transport involving Complexes I-IV Figure 20.6 Figure 14.6 Electron transport.Each of the four complexes of the electron-transport chain, composed of several protein subunits and cofactors, undergoes cyclic reduction and oxidation. The complexes are linked by the mobile carriers ubiquinone (Q) and cytochrome c. The height of each complex indicates the E°' between its reducing agent (substrate) and its oxidizing agent (which becomes the reduced product).

Iron and Copper in metalloenzymes are important in electron transport Iron can undergo reversible oxidation and reduction: Enzyme heme groups and cytochromes contain iron and are important in the electron transport process Nonheme iron exists in iron-sulfur clusters. iron is bound by sulfide ions and S- groups from cysteine (iron-sulfur clusters can accept only one e- in a reaction) Copper (Cu) assists in the electron transport in Complex IV Cu2+ Cu+

Figure 20.7 Iron – sulfur proteins Iron atoms are complexed with an equal number of sulfide ions (S2-) and with thiolate groups of Cys side chains Each can undergo reduction-oxidation reactions

Figure 20.8 Heme Fe(II)-protoporphyrin component of cytochrome c oxidase Heme consists of a tetrapyrrole porphyrin ring system complexed with iron

Figure 20.15: The electron transport chain V 3 H+ / ATP Next: Use of proton gradient for synthesis of ATP by ATP synthase (Complex V).

Assignment Read Chapter 20 Read Chapter 21 Topics not covered: Standard Reduction Potentials (Fig 20.1) Details of the inner workings of the Complexes