Chapter 14 (Part 1)

Electron transport

Chemiosmotic Theory• Electron Transport: Electrons carried by

reduced coenzymes are passed through a chain of proteins and coenzymes to drive the generation of a proton gradient across the inner mitochondrial membrane

• Oxidative Phosphorylation: The proton gradient runs downhill to drive the synthesis of ATP

• Electron transport is coupled with oxidative phosphorylation

• It all happens in or at the inner mitochondrial membrane

Outer Membrane – Freely permeable to small molecules and ions. Contains porins with 10,000 dalton limitInner membrane – Protein rich (4:1 protein:lipid). Impermeable. Contains ETR, ATP synthase, transporters.Cristae – Highly folded inner membrane structure. Increase surface area.Matrix- “cytosol” of the mitochondria. Protein rich (500 mg/ml) Contains TCA cycle enzymes, pyruvate dehydrogenase, fatty and amino acid oxidation pathway, DNA, ribosomesIntermembrane Space – composition similar to cytosol

Reduction Potentials• High Eo' indicates a strong tendency to be

reduced • Crucial equation: Go' = -nF Eo'

• Eo' = Eo'(acceptor) - Eo'(donor)

• NADH + ½ O2 + H+ NAD++ H+ + H2O

NAD++ H+ + 2e- NADH Eo’ = -0.32½ O2 + 2e- + 2H+ H2O Eo’ = 0.816Go‘= -nF(Eo'(O2) - Eo'(NADH)) Go‘= -nF(0.82 –(-0.32)) = -nF(1.14) = -2(96.5 kJ mol-1V-1)(1.136) = -220 kJ mol-1

Electron Transport• Four protein complexes in the

inner mitochondrial membrane • A lipid soluble coenzyme (UQ,

CoQ) and a water soluble protein (cyt c) shuttle between protein complexes

• Electrons generally fall in energy through the chain - from complexes I and II to complex IV

Standard reduction potentials of the major

respiratory electron carriers.

Complex I• NADH-CoQ Reductase • Electron transfer from NADH to CoQ • More than 30 protein subunits - mass of

850 kD• 1st step is 2 e- transfer from NADH to FMN• FMNH2 converts 2 e- to 1 e- transfer • Four H+ transported out per 2 e-

NADH + H+

         FMN

         Fe2+S

         CoQ

NAD+ FMNH2 Fe3+S CoQH2

Succinate          

FAD          

Fe2+S          

CoQFumarate FADH2 Fe3+S CoQH2

Complex II• Succinate-CoQ Reductase • aka succinate dehydrogenase (from TCA

cycle!) • four subunits• Two largest subunits contain 2 Fe-S proteins• Other subunits involved in binding succinate

dehydrogenase to membrane and passing e- to Ubiquinone

• FAD accepts 2 e- and then passes 1 e- at a time to Fe-S protein

• No protons pumped from this step

Q-Cycle• Transfer from the 2 e- carrier

ubiquinone (QH2) to Complex III must occur 1 e- at a time.

• Works by two single electron transfer steps taking advantage of the stable semiquinone intermediate

• Also allows for the pumping of 4 protons out of mitochondria at Complex III

• Myxothiazol (antifungal agent) inhibits electron transfer from UQH2 and Complex III.

UQ

UQ.-

UQH2

Complex III• CoQ-Cytochrome c Reductase • CoQ passes electrons to cyt c (and pumps H+) in

a unique redox cycle known as the Q cycle • Cytochromes, like Fe in Fe-S clusters, are one-

electron transfer agents • cyt c is a water-soluble electron carrier• 4 protons pumped out of mitochondria (2 from

UQH2)

                             CoQH2 cyt b ox Fe2+S cyt c1 ox cyt c red

CoQ cyt b red Fe3+S cyt c1 red cyt c ox

cyt c red          

cyt a ox          

cyt a3 red          

O2

cyt c ox cyt a red cyt a3 ox 2 H2O

Complex IV• Cytochrome c Oxidase • Electrons from cyt c are used in a four-

electron reduction of O2 to produce 2H2O • Oxygen is thus the terminal acceptor of

electrons in the electron transport pathway - the end!

• Cytochrome c oxidase utilizes 2 hemes (a and a3) and 2 copper sites

• Complex IV also transports H+ (2 protons)

Inhibitors of Oxidative Phosphorylation

• Rotenone inhibits Complex I - and helps natives of the Amazon rain forest catch fish!

• Cyanide, azide and CO inhibit Complex IV, binding tightly to the ferric form (Fe3+) of a3

• Oligomycin and DCCD are ATP synthase inhibitors

Shuttling Electron Carriers into the Mitochondrion

• The inner mitochondrial membrane is impermeable to NADH.

• Electrons carried by NADH that are created in the cytoplasm (such as in glycolysis) must be shuttled into the mitochondrial matrix before they can enter the ETS

Glycerol phosphate shuttle

malate/aspartate shuttle system

Electron transport is coupled to oxidative phosphorylation

Uncouplers• Uncouplers disrupt the tight coupling

between electron transport and oxidative phosphorylation by dissipating the proton gradient

• Uncouplers are hydrophobic molecules with a dissociable proton

• They shuttle back and forth across the membrane, carrying protons to dissipate the gradient

• w/o oxidative-phosphorylation energy lost as heat

• Dinitrophenol once used as diet drug, people ran 107oF temperatures

O2N OH

NO2 H

O2N O

NO2