TABLE 14-3
Energy-Yielding Reactions in the Complete Oxidation
of Glucose
Net Moles of ATP
Generated per
Mole of Glucose
(phosphoglycerate kinase, pyruvate
kinase; two ATPs are expended)
NADH shuttle
glycerol-phosphate shuttle (or malate
aspartate shuttle)
Pyruvate dehydrogenase (NADH)
Succinyl CoA synthetase
(GTP is equivalent to ATP)
Succinate dehydrogenase
(succinate —>fumarate + FADH2)
Other TCA cycle reactions
£-ketoglutarate —> succinyl CoA,
malate—>oxaloacetate; total of 3 NADH
next. The energy is ultimately stored as a phosphoric
acid anhydride bond in ATP. The formulation, in general
terms, is
AH2 +
+ C ^ A ~
+ BH2
+ ADP + P; ^
A +
+ B
ADP + P; ^ A +
where A and B represent the known redox pair, C is a hypo-
thetical ligand, and A ~ C is a hypothetical high-energy
intermediate. The above mechanism can be modified to
include other phosphorylated intermediates.
A model reaction that supports the above mechanism
is the glycolytic substrate-linked phosphorylation, which
proceeds via a thiol ester prior to the formation of the
phosphorylated intermediate (Chapter 13). Although the
chemical hypothesis is consistent with the substrate-linked
phosphorylation mechanism, it is deficient in explaining
the oxidative phosphorylation in mitochondria for two
1. The postulated high-energy chemical intermediates,
either phosphorylated or nonphosphorylated, have
never been identified despite many attempts to find
them, and
2. The chemical-coupling mechanism does not explain
why the inner mitochondrial membrane must be
present as a completely closed vesicle for oxidative
phosphorylation to occur.
conformational hypothesis
proposes that the
energy-yielding steps generate protein conformational
changes that are used in ATP synthesis. The conforma-
tional changes that occur in the redox catalysts are trans-
mitted to the energy-transducing units via protein-protein
interactions, the formation of covalent intermediates, or
the proton-motive force. Current opinion holds that the
conformational changes are linked with a proton-motive
force (see below).
Morphological changes do occur in the inner mem-
branes of the mitochondria when active respiration is
stimulated by ADP. Fluorescent probes, such as
-sulfonate (ANS) and the antibiotic
aurovertin, bind either to the inner membrane (ANS) or di-
rectly to ATP synthase (aurovertin). The binding enhances
or diminishes fluorescence in response to changes in con-
formation or hydrophobicity of the inner membrane. Re-
sults support the hypothesis that ATP synthase undergoes
conformational changes during respiration and oxidative
phosphorylation (discussed later).
According to the
chemiosmotic hypothesis,
by Peter Mitchell, an electrochemical gradient (pH gradi-
ent), generated across the inner mitochondrial membrane
by the passage of reducing equivalents along the respira-
tory chain provides the driving force for the synthesis of
ATP. There are three prerequisites for achieving oxidative
phosphorylation according to this hypothesis:
1. An anisotropic (direction-oriented)
proton-translocating respiratory chain capable of
vectorial transport of protons across the membrane;
2. A coupling membrane impermeable to ions except via
specific transport systems; and
3. An anisotropic ATP synthase whose catalytic activity
is driven by an electrochemical potential.
The transport of reducing equivalents in the respiratory
chain generates a proton gradient across the membrane
by virtue of the specific vectorial arrangement of the
redox components within the inner mitochondrial mem-
brane. The proton gradient is generated by ejection of
protons from the matrix into the intermembrane space
during proton-absorbing reactions, which occur on the
M side of the inner membrane, and the proton-yielding
reactions, which occur on the C side, to form redox loops
(Figure 14-13).
According to the chemiosmotic hypothesis, ejection of
two or more protons occurs at each of three sites in com-
plexes I, III, and IV. Thus, in the transfer of two reducing
equivalents from NADH to oxygen, at least six protons
chapter 14
Electron Transport and Oxidative Phosphorylation
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