Lecture Notes for Chapter 15

Oxidative Phosphorylation

Essential BiochemistryThird Edition

Charlotte W. Pratt | Kathleen Cornely

KEY CONCEPTS: Section 15-1

• The standard reduction potential indicates a substance’s tendency to become reduced; the actual reduction potential depends on the concentrations of reactants.

• Electrons are transferred from a substance with a lower reduction potential to a substance with a higher reduction potential.

• The free energy change for an oxidation-reduction reaction depends on the change in reduction potential.

Overview in Context

Recap on Oxidation-Reduction

• One reactant is in its oxidized state while the other is in its reduced state.

• Loss of electrons = oxidation• Gain of electrons = reduction

Reduction potential

indicates a substance’s tendency to

accept electrons.

The actual reduction potential depends on the actual concentrations

of oxidized and reduced species.

The Nernst EquationR = Gas Constant = 8.3145 J mol-1 K-1 T = temperature in Kelvinn = # of electrons F = Faraday’s constant = 96,485 J V-1 K-1

The free energy change can be calculated from the change in reduction

potential.

Overview of Mitochondrial

Electron Transport

KEY CONCEPTS: Section 15-2

• The inner mitochondrial membrane encloses the matrix and includes specific transport proteins.

• Complex I transfers electrons from NADH to ubiquinone.

• The citric acid cycle, fatty acid oxidation, and other processes also generate mitochondrial ubiquinol.

• The Q cycle mediated by Complex III reduces cytochrome c.

• Complex IV uses electrons from cytochrome c to reduce O2 to H2O.

Electron transport takes place in the mitochondrion.

Experimental imaging helps us know what mitochondria look like.

Electron Micrograph3D Reconstruction by Electron Tomography

Electron Micrograph of a Fibroblast

Mitochondria in Green

The malate-aspartate shuttle transports reducing agents across the inner mitochondrial membrane.

A different transport system is used to move ATP from the matrix to the cytosol.

ATP translocase protein imports ADP and exports ATP.

A symport protein permitssimultaneous movement of Pi

and H+.

Complex I binds ubiquinone.

• Structure of a bacterial complex– FMN and Fe-S clusters are

cofactors in space-filling representation.

– Q binding site is highlighted.– Part of the protein has membrane-

spanning a helices.

Complex I transfers electrons from NADH to Q.

• Electrons transfer from NADH to FMN, then from FMN to Q.

• As electrons are transferred from NADH to ubiquinone, Complex I transfers four protons from the matrix to the intermembrane space.

FMN can pick up two electrons from NADH.

Iron-sulfur clusters undergo

one-electron transfer reactions.

Other oxidation reactions contribute to the ubiquinol pool.

• Succinate dehydrogenase produces QH2 during the citric acid cycle.

• QH2 is also produced during fatty acid oxidation.

• Electrons from cytosolic NADH can enter the mitochondrial ubiquinol pool through the actions of a cytosolic and a mitochondrial glycerol-3-phosphate dehydrogenase.

Summary of Q Pool Reactions

Complex III transfers electrons from ubiquinol to cytochrome c.

• Cytochromes are proteins with heme prosthetic groups.

• Unlike the heme groups in hemoglobin and myoglobin, heme in cytochrome c undergoes reversible one-electron transfers.

• The central iron atom is either oxidized (Fe3+) or reduced (Fe2+).

Structure of Mammalian Complex III

Arrangement of Prosthetic Groups

Cyt b Cyt c1

Fe-S clusters

Heme groups of Cyt b

The circuitous route of electrons from ubiquinol to

cytochrome c is described by

the Q cycle.

Results of the Q Cycle

• Two electrons from QH2 reduce two molecules of cytochrome c.

• Four protons are pumped into the intermembrane space.– Two from QH2 in the first round

– Two from QH2 in the second round

Complex IV oxidizes cytochrome c and reduces O2.

• Cytochrome c is a small membrane-soluble protein.

• Cytochrome c transfers one electron at a time from Complex III to Complex IV.

• Complex IV (cytochrome oxidase) catalyzes this reaction:

Complex IV (Cytochrome c Oxidase) Structure

A Proposed Model for

Cytochrome c Oxidase Activity

• O2 is reduced to H2O via the Fe-S clusters in

cytochrome c oxidase.

More on Complex IV Function

• For every two electrons donated by cytochrome c, two protons are translocated to the intermembrane space

• Two protons from the matrix are also consumed in the reaction: ½ O2 H2O

KEY CONCEPTS: Section 15-3

• The formation of a transmembrane proton gradient during electron transport provides the free energy to synthesize ATP.

• Both concentration and charge contribute to the free energy of the proton gradient.

How much energy is available from electron transport?

• This is enough energy to drive the endergonic phosphorylation of ADP to form ATP (ΔG°' = +30.5 kJ•mol–1)!

The imbalance of protons represents a source of free energy, also called a

protonmotive force, that can drive the activity of an ATP synthase.

[H+] = high

[H+] = low

Computing the free energy change for the imbalance of protons.

With respect to the chemical imbalance

of protons:

With respect to the electrical imbalance

of protons:

Z = Ion’s ChargeΔy = Membrane Potential

Combining these effects:

KEY CONCEPTS: Section 15-4

• Proton translocation drives the rotation of a portion of ATP synthase.

• Rotation-induced conformational changes allow ATP synthase to bind ADP and Pi to phosphorylate ADP, and to release ATP.

• Because ATP synthesis is indirectly linked to electron transport, the P:O ratio is not a whole number.

• The supply of reduced cofactors determines the rate of oxidative phosphorylation.

The protein that taps the electrochemical proton gradient to

phosphorylate ADP is known as ATP synthase (Complex V).

ATP Synthase FunctionForm ATP

• Use theelectrochemicalgradient to drivephosphorylation.

ATP synthase rotates as it translocates protons.

• H+ binds to a c subunit.

• The c subunit moves away from the a subunit.

• As a new c subunit reaches the a subunit a proton is released.

• One rotation of the ring translocates 8 protons

The 3 ab pairs interact asymmetrically with the γ subunit.

a = blueb = greeng = purple

The binding change mechanism explains how

ATP is made.• The ab subunits form three different

conformations.– O = open– T = tight– L = loose

• ADP and Pi bind to the open conformation.

• ATP is formed in the tight conformation.

• ATP is released in the open conformation.

Quantifying Oxidative Phosphorylation

• The P:O ratio describes the stoichiometry of oxidative phosphorylation.– P:O ratio = # phosphorylations of ADP per # of oxygen

atoms reduced– P:O ratios will not be integral because chemical energy

becomes a protonmotive force, then mechanical movement of ATP synthase and back to chemical energy

• The rate of oxidative phosphorylation depends on the rate of fuel catabolism.– Oxidative phosphorylation is regulated by the

availability of reduced cofactors (NADH and QH2) produced by other metabolic processes.