section 14.2
Oxidative Phosphorylation
257
4. Complex IV spans the membrane, with cytochrome a
oriented toward the C side; copper ions and
cytochrome
a^
are oriented toward the M side.
5. Nicotinamide nucleotide transhydrogenase, which
catalyzes the reaction NADPH + NAD+ ^ NADH +
NADP+, spans the membrane, but its catalytic site
faces the M side.
The anisotropic organization of electron carriers
across the membrane accounts for the vectorial
transport of protons from the inside to the outside of
the membrane, which occurs with the passage of
electrons. The coupling of this proton gradient to a
proton-translocating ATP synthase (also known as
ATP synthetase) accounts for the chemiosmotic
coupling in oxidative phosphorylation.
14.2 Oxidative Phosphorylation
ATP is synthesized from ADP and phosphate during elec-
tron transport in the respiratory chain. This type of phos-
phorylation is distinguished from substrate-level phos-
phorylation, which occurs as an integral part of specific
reactions in glycolysis and the TCA cycle. The free en-
ergy available for the synthesis of ATP during electron
transfer from NADH to oxygen can be calculated from
the difference in the value of the standard potential of
the electron donor system and that of the electron accep-
tor system. The standard potential of the NADH/NAD+
redox component is —0.32 V and that of H
2 0
/ | 0 2
is
+0.82 V; therefore, the standard potential difference
between them is
A
E° =
(electron acceptor —
electron donor)
= 0 .8 2 -(-0 .3 2 ) = + 1 .1 4 V
The standard free energy is calculated from the expression
A G 0' =
-riF
A
E0'
where
n
is the number of electrons transferred, and F is the
Faraday constant,
which is equal to 23,062 cal/eV. Thus,
the standard free energy for a two-electron transfer from
NADH to molecular oxygen is
AG 0' = -n F
A
E°'
= - 2 x 23, 062 x 1.14
= —52.6 kcal/mol (—220 kJ/mol)
Although 52.6 kcal of free energy is available from the
reaction, only 21.9 kcal is conserved in the formation
of phosphoanhydride bonds at ATP. Formation of each
phosphoanhydride bond requires 7.3 kcal (30.5 kJ), and
21.9 kcal accounts for three ATPs synthesized. The re-
mainder of the energy is assumed to be dissipated as heat.
F I G U R E 1 4 -1 2
Flow of reducing equivalents in the respiratory chain and their relationship
to energy availability to drive ATP synthesis. The largest free energy
changes occur between NADH and FMN, between cytochromes b and ci,
and between cytochromes (a +
8 3
) and molecular oxygen; ATP formation
is coupled to these three sites.
In the mitochondria of brown adipose tissue, very little
ATP is synthesized, and most of the energy liberated in
the electron transport system is converted to heat.
The conservation of energy in the electron transport
chain occurs at three sites where there is a large decrease
in free energy (Figure 14-12): between NADH and FMN,
between cytochromes b and ci, and between cytochromes
(a +
8 3
) and molecular oxygen. Because electrons from
succinate bypass complex I and enter at the CoQ level,
only two moles of ATP are synthesized per mole of succi-
nate. The number of ATP molecules synthesized depends
on where the reducing equivalents enter the respiratory
chain and is indicated by the ratio (or quotient) of phos-
phate esterified (or ATP produced) to oxygen consumed
(P/O or ATP/O) per two-electron transport. The NAD+-
linked substrates (e.g., malate, pyruvate, a-ketoglutarate,
isocitrate, (
6
-hydroxybutyrate, and /3-hydroxyacyl-C.oA)
have P/O values of 3, and some flavin-linked substrates
(e.g., succinate, glycerol phosphate, and fatty acyl-CoA)
have P/O ratios of 2. The complete oxidation of 1 mol of
glucose yields either 36 or 38 mol of ATP, depending on
which shuttle pathway is used in the transport of cytoplas-
mic NADH to mitochondria. A list of the energy-yielding
reactions in glucose oxidation is shown in Table 14-3.
Mechanisms of Oxidative Phosphorylation
Three hypotheses have been proposed to explain how the
mechanism of energy conservation is coupled to electron
transport (the energy transduction system): the chemical,
conformational, and chemiosmotic hypotheses.
The
chemical hypothesis
proposes that the energy is
conserved by the formation of high-energy intermediates
as reducing equivalents pass from one carrier to the
