110
Chapter 4
Cytosol
Outer membrane
NAD
Succinate
I
y?
Mitochondrial matrix ADP
Figure
4.1
The electron transport chain and oxidative phosphorylation in eukaryotes.
Electrons are
transported from NADH or succinate, through the electron transport chain, to oxygen. The elements of
the electron transport chain are organized in large complexes located in the inner mitochondrial
membrane. Electrons from NADH are first donated to complex I, the NADH dehydrogenase. Electrons
from succinate are first donated to FAD, which is integrated in complex II, the succinate dehydrogenase
(one of the TCA cycle enzymes). Electrons from complexes I and II (or other flavoproteins located in
the inner mitochondrial membrane) are transferred to ubiquinone (UQ), which diffuses freely in the lipid
membrane.
From UQ, electrons are passed on to the cytochrome system. First, the electrons are
transferred to complex III, which consists of two 6-type cytochromes
(b566
and
b562)
and cytochrome
cl.
Electrons are then transferred to complex IV via cytochrome
c,
which is only loosely attached to the
outside face of the membrane. Complex IV (or cytochrome oxidase), which contains cytochromes
a
and
a3,
finally delivers the electrons to oxygen. Complexes I, III, and IV span the inner mitochondrial
membrane, and when two electrons are transported through these complexes, protons (four at each
complex) are released into the intermembrane space. These electrons may be transported back into the
mitochondrial matrix by a proton-conducting ATP-synthase (or the F|F0-ATPase complex).
In this
complex, one ATP is generated when three protons pass through the ATPase. One additional proton is
transported into the mitochondrial matrix in connection with the uptake of ADP and -P and the export of
ATP, so that a total of four protons are required per ATP generated by this mechanism. Thus, for each
electron pair transferred from complex I all the way to complex IV, 12 protons (four protons for each of
the three complexes) are pumped from the mitochondrial matrix into the intermembrane space. Upon re-
entry of the protons into the mitochondrial matrix through the ATP synthase, 3 moles of ATP are
generated
(12
protons/4
protons
per
ATP).
The
theoretical
stoichiometry
of the
oxidative
phosphorylation is, therefore, 3 mol of ATP synthesized per mole NADH oxidized and 2 mol of ATP
synthesized per mole of succinate oxidized.
Some organisms have a more or less defective respiration system. Lactic bacteria lack the transfer
agent cytochrome c between complexes III and IV, and cannot gain any energy by respiration,
although other advantages, especially the possibility to oxidize NADH by means of NADH
oxidases, are obtained in aerobic lactic fermentations.
Saccharomyces cerevisiae
has a defective
stage 1 of the chain, which means that NADH and FADH
2
are energetically equivalent in this
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