Biomedical Engineering Reference
In-Depth Information
Tabl e 5. 2.
Electron transport chain and ATP synthase (CytC: cytochrome-C; IMS: inner
mitochondrial membrane; MM: mitochondrial matrix; O
2
: superoxide anion; P
i
: inorganic
phosphate). The inner mitochondrial membrane is impermeable to most metabolites and ions,
thereby creating a transmembrane potential and ionic gradient. Reactive oxygen species are formed
at complex-I and -III of the respiratory chain. Superoxide anion produced from the transfer of an
electron from the partially reduced ubiquinone to oxygen is then reduced to hydrogen peroxide
(H
2
O
2
) by superoxide dismutase. Hydrogen peroxide is eliminated by glutathione peroxidase or
reacts with ferrous ions and other transition metals to form hydroxyl radical.
Complex
Alias
Reaction
I
NADH-coenzyme-Q oxidoreductase, NADH dehydrogenase
NADH
5H
MM
→
NAD
+
+
4H
IMS
+
UQ
+
UQH
2
+
e
−
−→
O
2
O
2
+
II
Succinate-coenzyme-Q oxidoreductase
(made of heme, iron-sulfur clusters, and
flavin adenine dinucleotide)
Succinate
+
UQ
→
fumarate
+
UQ
2
III
Coenzyme-Q-Cytochrome-C oxidoreductase, cytochrome-BC1 complex,
cytochrome-C reductase
(made of iron-sulfur cluster, cytochrome-C1, and 2 cytochrome-B)
UQH
2
2H
MM
→
4H
IMS
+
2CytC
ox
+
UQ + 2 CytC
red
+
O
2
e
−
−→
O
2
+
IV
Cytochrome-C oxidase
4CytC
red
8H
MM
→
4H
IMS
+
O
2
+
4CytC
ox
+
2H
2
O
+
V
ATP synthase, F
1
F
0
AT P a s e
(F
O
proton channel and F
1
synthase domain)
ADP
+
P
i
+
4H
IMS
AT P
+
H
2
O
+
4H
MM
5.2.3.2
Electron Transport Chain
The electron transport chain, composed of 4 main proteic complexes (Table
5.2
)
in the mitochondrion inner membrane, enables oxidative phosphorylation. Reduced
nicotinamide adenine dinucleotide (NADH)
6
and succinate generated in the citric
acid cycle, also called tricarboxylic acid and Krebs cycle, are oxidized to release
protons. In redox reactions, electrons are transferred from electron donors such as
NADH to electron acceptors such as oxygen. Redox reactions release energy that
powers ATP synthase. Electrons carried through the electron transport chain are
used to transport protons across the inner mitochondrial membrane. This process
creates a potential energy in the form of a proton concentration (pH) gradient and
electrical potential across the inner mitochondrial membrane. Protons move back
6
Oxidized form of nicotinamide adenine dinucleotide (NAD
+
) is an electron acceptor (oxidizing
agent, oxidant, or oxidizer). Reduced NADH form is a reducing agent (reductants or reducers) that
donates electron(s).
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