Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

774 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
Summary
HIGH
H + affinity
CRISTA SPACE
MATRIX
H + unidirectional
conformational
changes driven
by electron
transport H +
LOW
H + affinity
The respiratory chain embedded in the inner mitochondrial membrane contains
three respiratory enzyme complexes, through which electrons pass on their way
MBoC6 m14.31/14.28
from NADH to O 2 . In these complexes, electrons are transferred along a series of protein-bound
electron carriers, including hemes and iron–sulfur clusters. The energy
released as the electrons move to lower and lower energy levels is used to pump
protons by different mechanisms in the three respiratory enzyme complexes, each
coupling lateral electron transport to vectorial proton transport across the membrane.
Electrons are shuttled between enzyme complexes by the mobile electron carriers
ubiquinone and cytochrome c to complete the electron-transport chain. The
path of electron flow is NADH → NADH dehydrogenase complex → ubiquinone
→ cytochrome c reductase → cytochrome c → cytochrome c oxidase complex →
molecular oxygen (O 2 ).
H +
Figure 14–28 A general model for H +
pumping coupled to electron transport.
This mechanism for H + pumping by a
transmembrane protein is thought to
be used by NADH dehydrogenase and
cytochrome c oxidase, and by many
other proton pumps. The protein is driven
through a cycle of three conformations. In
one of these conformations, the protein has
a high affinity for H + , causing it to pick up
an H + on the inside of the membrane. In
another conformation, the protein has a low
affinity for H + , causing it to release an H + on
the outside of the membrane. As indicated,
the transitions from one conformation
to another occur only in one direction,
because they are being driven by being
allosterically coupled to the energetically
favorable process of electron transport
(discussed in Chapter 11).
ATP PRODUCTION IN MITOCHONDRIA
As we have just discussed, the three proton pumps of the respiratory chain each
contribute to the formation of an electrochemical proton gradient across the inner
mitochondrial membrane. This gradient drives ATP synthesis by ATP synthase, a
large membrane-bound protein complex that performs the extraordinary feat of
converting the energy contained in this electrochemical gradient into biologically
useful, chemical-bond energy in the form of ATP (see Figure 14–10). Protons flow
down their electrochemical gradient through the membrane part of this proton
turbine, thereby driving the synthesis of ATP from ADP and P i in the extramembranous
part of the complex. As discussed in Chapter 2, the formation of ATP from
ADP and inorganic phosphate is highly unfavorable energetically. As we shall see,
ATP synthase can produce ATP only because of allosteric shape changes in this
protein complex that directly couple ATP synthesis to the energetically favorable
flow of protons across its membrane.
The Large Negative Value of ∆G for ATP Hydrolysis Makes ATP
Useful to the Cell
An average person turns over roughly 50 kg of ATP per day. In athletes running a
marathon, this figure can go up to several hundred kilograms. The ATP produced
in mitochondria is derived from the energy available in the intermediates NADH,
FADH 2 , and GTP. These three energy-rich compounds are produced both by the
oxidation of glucose (Table 14–1A), and by the oxidation of fats (Table 14–1B; see
also Figure 2–56).
Glycolysis alone can produce only two molecules of ATP for every molecule of
glucose that is metabolized, and this is the total energy yield for the fermentation
processes that occur in the absence of O 2 (discussed in Chapter 2). In oxidative