chapter 14 (part 1)
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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 - PowerPoint PPT PresentationTRANSCRIPT
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