Biomedical Engineering Reference
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2 H
+
2 H
+
FAD
FADH
2e
-
2
2e
-
NADH
2e
-
Proton-motive force
2 H
+
2 H
+
2e
-
O
2
H
2
O
ADP + Pi
ATP synthesis
ATPase
ATP
Membrane
FIGURE 10.29
Electron transport and electron transport phosphorylation. To p : Oxidation of NADH and the
flow of electrons through the electron transport system, leading to the transfer of protons (II) from the inside to
the outside of the membrane. The tendency of protons to return to the inside is called proton-motive force. Bottom:
ATP synthesis occurs as protons reenter the cell. An ATPase enzyme uses the proton-motive force for the
synthesis of ATP. The proton-motive force is discussed in
Section 10.6
.
normally referred to as the P/O ratio, and the value of this stoichiometric coefficient indi-
cates the overall thermodynamic efficiency of the process. If NADH was the only coen-
zyme formed in the catabolic reactions, the theoretical P/O ratio would be exactly 3,
but since some FADH
2
is also formed the P/O ratio is always less than 3. Furthermore,
the proton and electrochemical gradient are also used for solute transport and the overall
stoichiometry in the process is therefore substantially smaller than the upper value of 3.
As the different reactions in the oxidative phosphorylation are not directly coupled the
P/O ratio varies with the growth conditions, and the overall stoichiometry is therefore
written as:
þ
2
H
þ
þ
NAD
þ
þð1 þ
O ATP
(10.41)
In many microorganisms, one or more of the sites of proton pumping are lacking, and this of
course results in a substantially lower P/O ratio.
Since the electron transport chain is located in the inner mitochondrial membrane in
eukaryotes and since NADH cannot be transported from the cytosol into the mitochondrial
matrix NADH formed in the cytosol needs to be oxidized by another route. Strain-specific
NADH dehydrogenases face the cytosol, and these proteins donate the electrons to the
NADH
O
2
þ
P
=
O ADP
þ
P
=
OH
PO
4
¼
P
=
O
Þ
H
O
þ
P
=
3
2
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