CHAPTER
14
Electron Transport and
Oxidative Phosphorylation
The energy requirements of aerobic cells are met by
the energy released in the oxidation of carbohydrates,
fatty acids, and amino acids by molecular oxygen. In
these oxidation processes, the reducing equivalents from
substrates are transferred to NAD+, flavin mononu-
cleotide (FMN), or flavin adenine dinucleotide (FAD). The
hydrogens, electrons, or hydride (FI- ) ions are removed
catalytically from NADH and reduced flavin nucleotides
and transferred through a series of coupled reduction-
oxidation (redox) reactions to oxygen, which serves as
the ultimate electron acceptor. The reduced oxygen is
then converted to water. The entire process is known
as
cell respiration.
The numerous coupled redox reac-
tions are tightly linked and take place in the inner mi-
tochondrial membrane. This reaction sequence is called
the respiratory chain
electron transport.
Electrons in sub-
strate or cofactor begin with a high potential energy and
end at oxygen with a lower potential energy. During this
electron flow, a portion of the free energy liberated is
conserved by an energy-transducing system (by which
electrical energy is changed to chemical energy). Since
the energy is conserved in the terminal phosphoanhy-
dride bond of ATP through phosphorylation of ADP to
ATP, the overall coupled process is known as
oxidative
phosphorylation.
The following reactions yield NADH or FADH
2
dur-
ing the breakdown of glucose by glycolysis and in the
TCA cycle (Chapter 13). The NADH-producing reactions
are
Glyceraldehyde 3-phosphate
Pyruvate
Isocitrate
a-Ketoglutarate
Malate
1,3-bisphosphoglycerate
acetyl-CoA
a-ketoglutarate
succinyl-CoA
oxaloacetate
The FADH
2
-producing reaction is
Succinate —»■
fumarate
Except for the conversion of glyceraldehyde 3-phosphate
to 1,3-bisphosphoglycerate occurring in the cytoplasm, all
of the above reactions take place in the mitochondria. Since
mitochondria are not permeable to NADH, two shuttle
pathways transport the reducing equivalents of cytoplas-
mic NADH into the mitochondria.
The TCA cycle functions for both the oxidation and
the production of reducing equivalents from a variety of
metabolites such as carbohydrates, amino acids, and fatty
acids. In the mitochondria fatty acids undergo succes-
sive removal of two carbon units in the form of acetyl-
CoA, which is converted in the TCA cycle to CO
2
and
reducing equivalents found in NADH and FADH2. The
latter two substances are then fed into the electron trans-
port chain. The formation of each acetyl-CoA molecule
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