Chapter 20Electron Transport and

Oxidative Phosphorylation

Mary K. CampbellShawn O. Farrellhttp://academic.cengage.com/chemistry/campbell

Paul D. Adams • University of Arkansas

The Role of Electron Transport in Metabolism

• Electron transport is carried out by four closely related multisubunit membrane-bound complexes and two electron carriers, coenzyme ___ and cytochrome ___• In a series of oxidation-reduction reactions, electrons

from FADH2 and NADH are transferred from one complex to the next until they reach _____

• O2 is reduced to H2O

• As a result of electron transport, ____________ are pumped across the inner membrane to the intermembrane space, creating a pH gradient

ATP Production in the Mitochondrion

• The production of ATP in the mitochondria is the result of oxidative phosphorylation

• The proton gradient establishes a voltage gradient

• The proton and voltage gradients together provide the mechanism to couple electron transport with _______________________________________

Establishment of the Proton Gradient

Summary

• Electron transport from one carrier to another creates a proton gradient across the inner mitochondrial membrane

• The proton gradient is coupled to the production of ATP in aerobic metabolism

Reduction Potentials

• A useful way to look at electron transport is to consider the change in free energy associated with the movement of electrons from one carrier to another• If we have two electron carriers, for example NADH

and coenzyme Q, are electrons more likely to be transferred from NADH to coenzyme Q, or vice versa?

• What we need to know is the _____________ _____________ _____________ _____________ for each carrier

• A carrier of high reduction potential will tend to be _____________ _____________ if it is paired with a carrier of _____________ _____________ reduction potential

Reduction Potentials

Reduction Potentials

Summary

• Standard reduction potentials provide a basis for comparison among redox reactions

• The sequence of reactions in the electron transport chain can be predicted by using reduction potentials.

Electron Transport Complexes

• Complex I: NADH-CoQ oxidoreductase

• Electrons are passed from NADH to FMN

NADH + H+ E-FMN NAD+ + E-FMNH2

Electron Transport Complexes

• Electrons are then passed to the iron-sulfur clusters• The last step of Complex I involves electrons being

passed to coenzyme Q (also called ubiquinone)

Energetics of Electron Transport

• The transfer of electrons is strongly ____________ ____________ and sufficient to drive the phosphorylation of ADP

Electron Transport Complexes

Complex II: Succinate-coenzyme Q oxidoreductase

Succinate + E-FAD Fumarate + E-FADH2

• The overall reaction is exergonic (-13.5 kJ/mol), but not enough to drive ATP production

• No H+ is pumped out of the matrix during this step

Electron Transport Complexes

Complex III: CoQH2-cytochrome c oxidoreductase

CoQH2 + 2Cyt c[Fe(III)] CoQ + 2Cyt c[Fe(II)] + 2H+

• This reactions of this complex results in a decrease in free energy that is sufficient to drive the phosphorylation of ADP to ATP

• The flow of electrons from reduced CoQ, a quinone that can exist in 3 forms, is known as the __________________________

Oxidized and Reduced Forms of CoQ

Electron Transport Complexes

Complex IV: Cytochrome c oxidase• Catalyzes the final step in electron transport

2Cyt c[Fe(II)] + 2H+ + ½O2 2 Cyt c[Fe(III)] + H2O

• Complex IV contains cytochrome a, cytochrome a3, and Cu(II), which are also involved in the electron transport

• Complex IV is the link to __________ ____________________ __________

Electron Flow

The Energetics of Electron Transport Reactions

The Heme Group of Cytochromes

• All cytochromes contain a _____________ _____________ group• Side chain differences depending on the heme

Connection between Electron Transport & Phosphorylation

• The energy-releasing oxidations give rise to proton pumping and a pH gradient across the _____________ _____________ _____________ _____________ membrane

• Differences in the concentration of ions across the membrane generates a _____________ __________________________ _____________

• A coupling process converts the electrochemical potential to the chemical energy of ATP

• The coupling factor is ATP __________________________, a complex protein oligomer, separate from the electron transport complexes

• Uncouplers inhibit the phosphorylation of ADP without affecting electron transport; examples are ________________, _____________, & __________________________, & _____________

Uncouplers

P/O Ratio

• P/O ratio:P/O ratio: the number of moles of Pi consumed in

_____________ _____________ to the number of moles of oxygen atoms consumed in __________________________

• Phosphorylation: ADP + Pi ATP + H2O

• Oxidation: 1/2O2 + 2H+ + 2e- H2O

• P/O = ______ when NADH is oxidized

• P/O = ______ when FADH2 is oxidized

Summary

• The coupling of electron transport to oxidative phosphorylation requires a multisubunit membrane-bound enzyme, ATP synthase.

• This enzyme has a channel for protons to flow from the intermembrane space into the mitochondrial matrix.

• The proton flow is coupled to ATP production in a process that appears to involve a conformational change of the enzyme.

Chemiosmotic Coupling

• Chemiosmotic coupling• based on a ________ ________ concentration gradient between

the intermembrane space and the matrix• a proton gradient exists because the various proteins

that serve as electron carriers are not symmetrically oriented with respect to the two sides of the inner mitochondrial membrane

• these proteins take up protons from the matrix when they are reduced and release them to the intermembrane space when they are reoxidized

• the reactions of NADH, CoQ, and O2 all require _______________________

Chemiosmotic Coupling

Chemiosmotic Coupling

Evidence for chemiosmotic coupling suggested (Mitchell 1961):• A system with definite inside and outside _____________ _____________

(closed vesicles) is essential.• Submitochondrial vesicles can be prepared, which carry

out oxidative phosphorylation and have an asymmetric orientation of respiratory complexes.

• A model system for oxidative phosphorylation can be constructed with proton pumping in the absence of electron transport; the model system consists of reconstituted membrane vesicles, mitochondrial ATP synthase, and a proton pump.

• The existence of the pH gradient has been demonstrated and _____________ __________________________ _____________

Chemiosmotic Coupling

• The mechanism by which the proton gradient leads to the production of ATP depends on ion channels through the inner mitochondrial membrane• Protons flow back into the matrix through channels in

the F0 unit of ATP __________________________

• The flow of protons is accompanied by formation of ATP in the F1 unit of ATP __________________________

• The details of how phosphorylation takes place as a result of the linkage to the proton gradient are not explicitly specified by this mechanism

Chemiosmotic Coupling

Conformational Coupling

• The proton gradient leads to changes in conformation in a number of proteins, including ATP ______________________________

• There are 3 sites for substrate on ATP __________________________, and 3 possible conformations:- Open (O); a low affinity for substrate

- Loose-binding (L); not catalytically active, binds ADP & Pi

- Tight-binding (T); catalytically active, binds ATP

• These sites interconvert as a result of proton flux through ATP synthase

• Proton flux converts L to T, which produces ATP• Proton flux converts T to O, releasing ATP

Release of ATP from ATP Synthase

Summary

• In chemiosmotic coupling, the proton gradient is the crux of the matter. The flow of protons through pores in the synthase drives ATP production.

• In conformational coupling, a change in the shape of the synthase releases bound ATP that has already been formed.

Respirator Inhibitors & Electron Transport

• Respiratory inhibitors can be used to determine the order of reactions in the electron transport chain• Isolate intact _____________ _____________ from cells• Provide an oxidizable substrate so that electron

transport can occur• Add a respiratory inhibitor• Measure relative amounts of oxidized and reduced

forms of various electron carriers• Inhibitors have an effect on three sites in the electron

transport chain

The Effect of Respiratory Inhibitors

Sites of Action of Some Respiratory Inhibitors

Shuttle Mechanisms

• Shuttle mechanisms:Shuttle mechanisms: transport metabolites between _____________ _____________ and __________________________

• Glycerol phosphate shuttle:Glycerol phosphate shuttle:• We know glycolysis in the _____________ _____________ produces

NADH• NADH does not cross the _____________________ _____________________

membrane, but glycerol phosphate and dihydroxyacetone phosphate do

• Through the glycerol phosphate shuttle, ____ ATP are produced in the mitochondria for each cytosolic NADH

The Glycerol-Phosphate Shuttle

The Malate-Aspartate Shuttle

• The Malate-Aspartate Shuttle:• Has been found in mammalian kidney, liver, and heart• Malate crosses the mitochondrial membrane, while

oxaloacetate cannot• The transfer of electrons from NADH in the cytosol

produces NADH in the mitochondria• In the malate-aspartate shuttle, _____ mitochondrial

ATP are produced for each cytosolic NADH

The Malate-Aspartate Shuttle

Summary

• Shuttle mechanisms transfer ______________________, but not __________________________, from the cytosol across the mitochondrial membrane

• In the malate-aspartate shuttle, 2.5 molecules of ATP are produced for each molecule of cytosolic NADH, rather than 1.5 ATP in the glycerol-phosphate shuttle…

• This affects the overall yield of ATP in these tissues

ATP Yield from Complete Oxidation of Glucose

• In the complete oxidation of glucose, a total of ______ or ______ molecules of ATP are produced for each molecule of glucose, depending on the shuttle mechanism

The ATP Yield from Complete Oxidation of Glucose