916
UNIT 4
Maintenance of the Body
24
based on
niacin
, and
flavin adenine dinucleotide (FAD)
, derived
from
riboflavin
. Te oxidation of succinic acid to fumaric acid and
the simultaneous reduction of FAD to FADH
2
, an example of a
coupled redox reaction, is
fumaric
acid
succinic
acid
oxidation [
-
2H]
FAD
(oxidized)
FADH
2
(reduced)
C
C
COOH
COOH
H
H
C
C
COOH
COOH
H
H
H
H
ATP Synthesis
How do our cells capture some of the energy liberated during
cellular respiration to make A±P molecules? Tere appear to be
two mechanisms—substrate-level phosphorylation and oxida-
tive phosphorylation.
Substrate-level phosphorylation
occurs when high-energy
phosphate groups are transferred directly from phosphorylated
substrates (metabolic intermediates such as glyceraldehyde
3-phosphate) to ADP
(Figure 24.4a)
. Essentially, this process
occurs because the high-energy bonds attaching the phosphate
groups to the substrates are even more unstable than those in
A±P. A±P is synthesized by this route twice during glycolysis, and
once during each turn of the Krebs cycle. Te enzymes catalyzing
As you will soon see, essentially all oxidation of food fuels in-
volves the step-by-step removal of pairs of hydrogen atoms (and
also pairs of electrons) from the substrate molecules, eventually
leaving only carbon dioxide (CO
2
). Molecular oxygen (O
2
) is the
final electron acceptor. It combines with the removed hydrogen
atoms at the very end of the process, to form water (H
2
O).
Whenever one substance loses electrons (is oxidized), an-
other substance gains them (is reduced). For this reason, oxi-
dation and reduction are coupled reactions and we speak of
oxidation-reduction (redox) reactions
. Te key understand-
ing about redox reactions is that “oxidized” substances
lose
energy and “reduced” substances
gain
energy as energy-rich
electrons are transferred from one substance to the next. Con-
sequently, as food fuels are oxidized, their energy is transferred
to a “bucket brigade” of other molecules and ultimately to ADP
to form energy-rich A±P.
Like all other chemical reactions in the body, redox reactions
are catalyzed by enzymes. Tose that catalyze redox reactions in
which hydrogen atoms are removed are called
dehydrogenases
(de-hi
9
dro-jen-ās
0
ez), while enzymes catalyzing the transfer of
oxygen are
oxidases
.
Most of these enzymes require the help of a specific coenzyme,
typically derived from one of the B vitamins. Although the en-
zymes catalyze the removal of hydrogen atoms to oxidize a sub-
stance, they cannot
accept
the hydrogen (hold on or bond to it).
Teir
coenzymes
, however, can act as hydrogen (or electron) ac-
ceptors, becoming reduced each time a substrate is oxidized.
±wo very important coenzymes of the oxidative pathways are
nicotinamide adenine dinucleotide (NAD
1
)
(nik
0
o-tin
9
ah-mīd),
H
+
H
+
P
i
P
ATP
ATP
ADP
Substrate
Product
Enzyme
Catalysis
Enzyme
ADP
+
Membrane
High H
+
concentration in
intermembrane space
Low H
+
concentration
in mitochondrial matrix
Energy
from food
Proton
pumps
(electron
transport
chain)
ATP
synthase
(a) Substrate-level phosphorylation
(b) Oxidative phosphorylation
Figure 24.4
Mechanisms of
phosphorylation.
(a)
Substrate-level
phosphorylation occurs when a high-
energy phosphate group is transferred
directly from a substrate to ADP to form
ATP. This reaction occurs both in the
cytosol and in the mitochondrial matrix.
(b)
Oxidative phosphorylation, which occurs
in mitochondria, is carried out by electron
transport proteins that act as proton
“pumps” to create a proton gradient across
the inner mitochondrial membranes. The
source of energy for this pumping is energy
released when food molecules are oxidized.
As the protons flow passively back into the
mitochondrial matrix through ATP synthase,
some of this gradient energy is captured and
used to bind phosphate groups to ADP.
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