chain). During this transfer, energy is maintained particularly
in the form of ATP produced by oxidative phosphorylation.
The reducing power* or reducing equivalent sum that is
produced during the oxidation of organic substrates is essentially reserved for energy production (Fig. 3.12). However, the
reducing power operates in all cases where a reduction process
is necessary: enzymatic reactions, biosynthesis, etc.
The oxidation reactions that take place in several steps are
often reactions to dehydrogenation involving electrons and
protons. The resulting reducing power is transferred to
coenzymes (NAD
+
, NADP
+
, and FAD). There are many metabolic pathways of oxidation. For carbohydrates, the main
pathway is glycolysis or the Embden–Meyerhof pathway
associated with the tricarboxylic acid cycle (TAC), also called
citric acid cycle, organic acid cycle to four carbon atoms (C4
cycle: succinic, fumaric, and malic acids), or the Krebs cycle.
Carbohydrate Oxidation
During glycolysis (Fig. 3.13a), glucose is oxidized to pyruvate in several enzymatic steps with concomitant formation
of ATP by phosphorylation at the substrate level and reduced
coenzymes:
Glucose þ 2ADP þ 2 Pi þ 2 NAD
þ
! 2 pyruvate þ 2 ATP þ 2NADH, H
þ
Pyruvate is then decarboxylated to acetyl-CoA
and releases a molecule of CO 2 and reducing power
(NADH, H
+
):
2 pyruvate þ 2 CoA þ 2 NAD
þ
! 2 acetyl À CoA þ 2 CO 2 þ 2 NADH, H
þ
The two molecules of acetyl-CoA are then oxidized via
the Krebs cycle (Fig. 3.13b), by combining the acetyl group
to a molecule of oxaloacetate to form a compound to six
carbon atoms (citrate) that regenerates C4 compound (oxaloacetate) via a series of oxidation, decarboxylation, dehydration, and hydration reactions. During one cycle, the
acetyl-CoA is oxidized to two molecules of CO 2 and allows
the production of one molecule of ATP by phosphorylation
at the substrate level, two molecules of NADH, H
+
, one of
NADP H, H
+ , and one of FADH 2 .
Thus, the total oxidation of glucose by glycolysis
(Embden–Meyerhof pathway) and the Krebs cycle
(Fig. 3.13c) produces six molecules of CO 2 , four of ATP
by phosphorylation at the substrate level, and 24 reducing
equivalents [H] in the form of eight NADH, H
+ , 2 NADP H,
H
+
, and two FADH 2 . In prokaryotic microorganisms,
glycolysis and the Krebs cycle take place in the cytoplasm,
whereas in eukaryotes, glycolysis occurs in the cytoplasm
and the Krebs cycle in the matrix of mitochondria.
Glucose can be degraded by other metabolic pathways such
as the pentose phosphate methylglyoxal, phosphoketolase, and
Entner–Doudoroff pathways.
Lipid Oxidation
Under the action of lipase, triglycerides are hydrolyzed to
glycerol and fatty acids (Fig. 3.14a).
Glycerol enters into the Embden–Meyerhof pathway via
the phosphoglycerate (Fig. 3.14b). The fatty acids are
degraded by β-oxidation (Fig. 3.14c). During the β-oxidation, the fatty acid is esterified to acyl-CoA in the presence of
coenzyme A. The acyl-CoA is then oxidized in position β in
three steps (two dehydrogenations and one hydration). A
new esterification in β releases a molecule of acetyl-CoA
and a molecule of acyl-CoA that has lost two carbon atoms.
A new round of β-oxidation begins. Finally, the fatty acid is
cut into a succession of acetyl-CoA. These are metabolized
through the glyoxylate cycle (Fig. 3.40).
ATP
Oxidation
CATABOLISM
(carbon flow)
Organic compound
(substrate : electron
donor)
CO 2
Terminal
electron
acceptor
ELECTRON FLOW
via the respiratory chain
Δ p
O 2
Fig. 3.12 Carbon flux and
electron flow in aerobic
respiration. Δp ¼ Proton-motive
force (Drawing: M.-J. Bodiou)
38
R. Matheron and P. Caumette
in the form of ATP produced by oxidative phosphorylation.
The reducing power* or reducing equivalent sum that is
produced during the oxidation of organic substrates is essentially reserved for energy production (Fig. 3.12). However, the
reducing power operates in all cases where a reduction process
is necessary: enzymatic reactions, biosynthesis, etc.
The oxidation reactions that take place in several steps are
often reactions to dehydrogenation involving electrons and
protons. The resulting reducing power is transferred to
coenzymes (NAD
+
, NADP
+
, and FAD). There are many metabolic pathways of oxidation. For carbohydrates, the main
pathway is glycolysis or the Embden–Meyerhof pathway
associated with the tricarboxylic acid cycle (TAC), also called
citric acid cycle, organic acid cycle to four carbon atoms (C4
cycle: succinic, fumaric, and malic acids), or the Krebs cycle.
Carbohydrate Oxidation
During glycolysis (Fig. 3.13a), glucose is oxidized to pyruvate in several enzymatic steps with concomitant formation
of ATP by phosphorylation at the substrate level and reduced
coenzymes:
Glucose þ 2ADP þ 2 Pi þ 2 NAD
þ
! 2 pyruvate þ 2 ATP þ 2NADH, H
þ
Pyruvate is then decarboxylated to acetyl-CoA
and releases a molecule of CO 2 and reducing power
(NADH, H
+
):
2 pyruvate þ 2 CoA þ 2 NAD
þ
! 2 acetyl À CoA þ 2 CO 2 þ 2 NADH, H
þ
The two molecules of acetyl-CoA are then oxidized via
the Krebs cycle (Fig. 3.13b), by combining the acetyl group
to a molecule of oxaloacetate to form a compound to six
carbon atoms (citrate) that regenerates C4 compound (oxaloacetate) via a series of oxidation, decarboxylation, dehydration, and hydration reactions. During one cycle, the
acetyl-CoA is oxidized to two molecules of CO 2 and allows
the production of one molecule of ATP by phosphorylation
at the substrate level, two molecules of NADH, H
+
, one of
NADP H, H
+ , and one of FADH 2 .
Thus, the total oxidation of glucose by glycolysis
(Embden–Meyerhof pathway) and the Krebs cycle
(Fig. 3.13c) produces six molecules of CO 2 , four of ATP
by phosphorylation at the substrate level, and 24 reducing
equivalents [H] in the form of eight NADH, H
+ , 2 NADP H,
H
+
, and two FADH 2 . In prokaryotic microorganisms,
glycolysis and the Krebs cycle take place in the cytoplasm,
whereas in eukaryotes, glycolysis occurs in the cytoplasm
and the Krebs cycle in the matrix of mitochondria.
Glucose can be degraded by other metabolic pathways such
as the pentose phosphate methylglyoxal, phosphoketolase, and
Entner–Doudoroff pathways.
Lipid Oxidation
Under the action of lipase, triglycerides are hydrolyzed to
glycerol and fatty acids (Fig. 3.14a).
Glycerol enters into the Embden–Meyerhof pathway via
the phosphoglycerate (Fig. 3.14b). The fatty acids are
degraded by β-oxidation (Fig. 3.14c). During the β-oxidation, the fatty acid is esterified to acyl-CoA in the presence of
coenzyme A. The acyl-CoA is then oxidized in position β in
three steps (two dehydrogenations and one hydration). A
new esterification in β releases a molecule of acetyl-CoA
and a molecule of acyl-CoA that has lost two carbon atoms.
A new round of β-oxidation begins. Finally, the fatty acid is
cut into a succession of acetyl-CoA. These are metabolized
through the glyoxylate cycle (Fig. 3.40).
ATP
Oxidation
CATABOLISM
(carbon flow)
Organic compound
(substrate : electron
donor)
CO 2
Terminal
electron
acceptor
ELECTRON FLOW
via the respiratory chain
Δ p
O 2
Fig. 3.12 Carbon flux and
electron flow in aerobic
respiration. Δp ¼ Proton-motive
force (Drawing: M.-J. Bodiou)
38
R. Matheron and P. Caumette