CHAPTER 20The 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 NADHand 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 mitochondrialmembrane, which oxidize NADH and QH2. Oxidation energyis used to transport protons across the inner mitochondrialmembrane, 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 20.2Structure of themitochondrion

- 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 ofacetyl-CoA and the citric acid cycle (except succinate dehydrogenase complex). Protons are removed from the matrix during electron transport

Figure 20.15: The electron transport chain

4 H+ / 2e- 4 H+ / 2e-2 H+ / 2e-

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

V

Figure 20.15: The electron transport chain

4 H+ / 2e-

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

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)

X

Mobile electron carrier 1

Ubiquinone (Q)Q is a lipid soluble molecule that diffuses withinthe lipid bilayer of the inner mitochondrial membrane,accepting electrons from Complex I and Complex IIand passing them to Complex III

Figure 20.15: The electron transport chain

4 H+ / 2e- 4 H+ / 2e-2 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

Mobile electron carrier 2

Cytochrome cA 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

4 H+ / 2e- 4 H+ / 2e-2 H+ / 2e-

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.

V

Figure 20.15: The electron transport chain

V

- FADH2 is bound to Complex II (succinate dehydrogenase)

X

Complex II. Succinate dehydrogenase

- Transfers electrons from succinate to flavin adenine dinucleotide

(FAD) as a hydride ion (H:-), to an Fe-S complex (one electronat a time), to ubiquinone (Q), making QH2

- Complex II does not pump protons

Structure of E. Coli succinate dehydrogenase complex

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-

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.

VX

Electron Transport involving Complexes I-IV

Figure 20.6

Iron and Copper in metalloenzymes are important in electron transport

Iron can undergo reversible oxidation and reduction:

Fe3+ + e- (reduced substrate) Fe2+ + (oxidized substrate)

- 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 withan equal number of sulfide ions(S2-) and with thiolate groupsof Cys side chains

Each can undergo reduction-oxidationreactions

Figure 20.8 Heme Fe(II)-protoporphyrin component ofcytochrome c oxidase

Heme consists of a tetrapyrroleporphyrin ring system complexedwith iron

Figure 20.15: The electron transport chain

4 H+ / 2e- 4 H+ / 2e-2 H+ / 2e-

3 H+ / ATP

Next: Use of proton gradient for synthesis of ATP by ATP synthase (Complex V).

V

Assignment

Read Chapter 20Read Chapter 21

Topics not covered:Standard Reduction Potentials (Fig 20.1)Details of the inner workings of the Complexes