Electron Transport and Oxidative Phosphorylation

Oxidative phosphorylation - process in which NADH and QH2 are oxidized and ATP is
produced.
Enzymes are found in inner mitochondrial membrane in eukaryotes.
In prokaryotes, enzymes are found in cell membrane.
Process consists of 2 separate, but coupled processes:
1) respiratory electron-transport chain
Responsible for NADH and QH2 oxidation
Final e- acceptor is molecular oxygen
Energy generated from electron transfer is used to pump H+ into
intermembrane space from matrix ---> matrix becomes more alkaline
and negatively charged.
2) ATP synthesis
Proton concentration gradients represents stored energy
When H
+
are moved back across inner mitochondrial membrane
through ATP synthase ---> ADP is phosphorylated to form ATP
Chemiosmotic Theory of ATP Production
Proposed by Peter Mitchell in 1961 (won Nobel Prize for this work).
Tenet: Proton concentration gradient serves as energy reservoir for ATP synthesis.
Proton concentration gradient also known as proton motive force (PMF).
Components of Electron Transport System
There are 5 protein complexes:
I) NADH-ubiquinone oxidoreductase
II) succinate-ubiquinone oxidoreductase
III) ubiquinol-cytochrome c oxidoreductase
IV) cytochrome c oxidase
V) ATP synthase
Electrons flow through ETS in direction of increasing reduction potential.
Two mobile electron carriers also involved: ubiquinone (Q) between complexes I or II and
III, and cytochrome c between complexes III and IV.
Electrons enter ETS 2 at a time from either NADH or succinate.
I - NADH-ubiquinone oxidoreductase
Transfers 2e
-
from NADH to Q as hydride ion (H-)
First electron transferred to FMN --> FMNH2 ---> Fe-S cluster ---> Q
Also pumps 4H
+
/2e
-
into intermembrane space
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II - succinate-ubiquinone oxidoreductase
Transfers e
-
from succinate to Q
First transferred to FAD ---> FADH2 ---> 3 Fe-S clusters ---> Q
Not enough energy to contribute to proton gradient via proton pumping
III - ubiquinol-cytochrome c oxidoreductase
Rransfers e
-
from QH2 to cytochrome c facing intermembrane space
Composed of 9-10 subunits including 2 Fe-S clusters, cytochrome b560, cytochrome b566,
and cytochrome c1.
Transports 2H
+ from matrix into intermembrane space
IV - cytochrome c oxidase
Contains cytochromes a and a3
Contributes to proton gradient in two ways:
1) pumps 2H
+
for each pair of e
-
transferred (per O2 reduced)
2) consumes 2H
+
when oxygen is reduced to H2O ---> lowers [H
+
]matrix
Carbon monoxide (CO) and cyanide (HCN) bind here
V - ATP synthase
Does not contribute to H
+
gradient, but helps relieve it
Also called FOF1 ATP synthase
F1 component contains catalytic subunits
FO component is proton channel that is transmembrane
Per ATP synthesized, 3H
+
move through ATP synthase
oligomycin - antibiotic that binds to channel and prevents proton entry --> no ATP
synthesized
TRANSPORT OF MOLECULES ACROSS MITOCHONDRIAL MEMBRANE
Inner mitochondrial membrane is impermeable to NADH and NAD+.
Must use a shuttle to regenerate NAD+ for glycolysis; solution is to shuttle electrons across
membrane, rather than NADH itself.
There are two shuttles in operation:
1) glycerol phosphate shuttle
_ Found in insect flight muscles and mammalian cells in which high rates of
oxidative phosphorylation must occur
_ Cytosolic glycerol 3-phosphate dehydrogenase converts DHAP to glycerol 3-
phosphate
_ Converted back to DHAP by membrane-bound glycerol 3-phosphate
dehydrogenase
_ Result is transfer to 2e
-
to FAD --> Q ---> complex III
_ Produces fewer ATP molecules (1.5 vs. 2) because complex I is bypassed
2) malate-aspartate shuttle
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Found in liver and heart
Cytosolic NADH reduces oxaloacetate --> malate --> transported via
dicarboxylate translocase into matrix
In matrix, malate --> oxaloacetate --> aspartate ---> transported out via
glutamate-aspartate translocase
Converted back to oxaloacetate.......
No reduction in ATP yield
Must also be able to transport other metabolites into and out of matrix:
1) ADP/ATP carrier or ADP/ATP translocase
Adenine nucleotide translocase which exchanges ADP and ATP (antiporter)
2) Pi/H
+
carrier
Couples inward movement of Pi with symport of H
+
from gradient