Download Chapter 14 (Part 1)

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 limit
Inner 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, ribosomes
Intermembrane 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’ = -0.32
½ O2 + 2e- + 2H+  H2O
Eo’ = 0.816
Go‘= -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+
NAD+
FMN
FMNH2
Fe2+S
Fe3+S
CoQ
CoQH2
Complex II
•
•
•
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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
Succinate
Fumarate
FAD
FADH2
Fe2+S
Fe3+S
CoQ
CoQH2
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 oneelectron transfer agents
• cyt c is a water-soluble electron carrier
• 4 protons pumped out of mitochondria (2 from UQH2)
CoQH2
CoQ
cyt b ox
cyt b red
Fe2+S
Fe3+S
cyt c1 ox
cyt c1 red
cyt c red
cyt c ox
Complex IV
• Cytochrome c Oxidase
• Electrons from cyt c are used in a fourelectron 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)
cyt c red
cyt c ox
cyt a ox
cyt a red
cyt a3 red
cyt a3 ox
O2
2 H2O
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
H
NO2
O2N
OH
NO2
O 2N
O