The rate of respiration (O
2 consumption) in
mitochondria is under tight regulation; it is generally limited by the
availability of ADP as a substrate for phosphorylation. As we saw in Figure
18-13b, the respiration rate in isolated mitochondria is low in the absence of
ADP and increases strikingly with the addition of ADP; this phenomenon is part
of the definition of coupling of oxidation and phosphorylation. The dependence
of the rate of O
2 consumption on the concentration of
the P
i acceptor ADP, called
acceptor control of
respiration, can be dramatic. In some animal tissues the
acceptor
control ratio, the ratio of the maximal rate of ADP-induced
O
2 consumption to the basal rate in the absence of
ADP, is at least 10.
The intracellular concentration of ADP is one measure of the energy status of
cells. Another, related measure is the
mass-action ratio of the
ATP-ADP system: [ATP]/([ADP][P
i]). Normally this ratio is very
high, so that the ATP-ADP system is almost fully phosphorylated. When the rate
of some energy-requiring process in cells (protein synthesis, for example)
increases, there is an increased rate of breakdown of ATP to ADP and
P
i, lowering the mass-action ratio. With more ADP available for
oxidative phosphorylation, the rate of respiration increases, causing
regeneration of ATP. This continues until the massaction ratio returns to its
normal high level, at which point respiration slows again. The rate of oxidation
of cell fuels is regulated with such sensitivity and precision that the ratio
[ATP]/([ADP][P
i] fluctuates only slightly in most tissues, even
during extreme variations in energy demand. In short, ATP is formed only as fast
as it is used in energyrequiring cell activities.
Uncoupled Mitochondria in Brown Fat Produce Heat
There is a remarkable and instructive exception to the general rule that
respiration slows when the ATP supply is adequate. In most mammals, including
humans, newborns have a type of adipose tissue called
brown fat
in which fuel oxidation serves not to produce ATP, but to generate heat to keep
the newborn warm. Hibernating mammals (grizzly bears, for example) also have
large amounts of brown fat. This specialized adipose tissue, located at the back
of the neck of human infants, is brown because of the presence of large numbers
of mitochondria and thus large amounts of cytochromes, whose heme groups are
strong absorbers of visible light.
The mitochondria of brown fat oxidize fuels (particularly fatty
acids) normally, passing electrons through the respiratory chain to
O2. This electron transfer is accompanied by proton
pumping out of the matrix, as in other mitochondria. The mitochondria of brown
fat, however, have a unique protein in their inner membrane: thermogenin, also
called the uncoupling protein (UCP) (Table 18-4). This protein, an integral
membrane protein, provides a path for protons to return to the matrix without
passing through the F0F1
complex (Fig. 18-27). As a result of this short-circuiting of protons, the
energy of oxidation is not conserved by ATP formation but is dissipated as heat,
which contributes to maintaining the body temperature. For the hairless newborn
infant, maintaining body heat is an important use of metabolic energy.
Hibernating animals depend on uncoupled mitochondria of brown fat to generate
heat during their long winter period of dormancy
ATP-Producing Pathways Are Coordinately Regulated
The major catabolic pathways (glycolysis, the citric acid cycle,
fatty acid and amino acid oxidation, and oxidative phosphorylation) have
interlocking and concerted regulatory mechanisms that allow them to function
together in an economical and self regulating manner to produce ATP and
biosynthetic precursors. The relative concentrations of ATP and ADP control not
only the rates of electron transfer and oxidative phosphorylation but also the
rates of the citric acid cycle, pyruvate oxidation, and glycolysis (Fig. 18-28).
Whenever there is an increased drain on ATP, the rate of electron transfer and
oxidative phosphorylation increases. Simultaneously, the rate of pyruvate
oxidation via the citric acid cycle increases, thus increasing the flow of
electrons into the respiratory chain. These events can in turn evoke an increase
in the rate of glycolysis, increasing the rate of pyruvate formation. When the
conversion of ADP to ATP lowers the ADP concentration, acceptor control will
slow electron transfer and thus oxidative phosphorylation. Glycolysis and the
citric acid cycle will also slow, because ATP is an allosteric inhibitor of
phosphofructokinase-1 (see Fig. 14-20) and of pyruvate dehydrogenase (see Fig.
15-14).
Phosphofructokinase-1 is inhibited not only by ATP but by citrate, the first
intermediate of the citric acid cycle. When both ATP and citrate are elevated
they produce a concerted allosteric inhibition that is greater than the sum of
their individual effects.
In many types of tumor cells, this interlocking coordination appears to be
defective: glycolysis proceeds at a higher rate than required by the citric acid
cycle. As a result, cancer cells use far more blood glucose than normal cells,
but cannot oxidize the excess pyruvate formed by rapid glycolysis, even in the
presence of O2. To reoxidize cytoplasmic NADH, most of
the pyruvate is reduced to lactate (Chapter 14), which passes from the cells
into the blood. The high glycolytic rate may result in part from smaller numbers
of mitochondria in tumor cells. In addition, some tumor cells overproduce an
isozyme of hexokinase that associates with the cytosolic face of the
mitochondrial inner membrane and is insensitive to feedback inhibition by
glucose-6phosphate (p. 432). This enzyme may monopolize the ATP produced in
mitochondria, using it to produce glucose-6-phosphate and committing the cell to
continue glycolysis.
Mitochondria contain their own genome, a circular, double-stranded DNA
molecule. There are 37 genes (16,569 base pairs) in the human mitochondrial
chromosome (Fig. 18-29), including 13 that encode proteins of the respiratory
chain (Table 18-7); the remaining genes code for ribosomal and transfer RNA
molecules essential to the proteinsynthesizing machinery of mitochondria. Many
of the mitochondrial proteins are encoded in nuclear genes, synthesized on
cytoplasmic ribosomes, then imported and assembled within mitochondria.
Mutations in the mitochondrial DNA are known to cause the human disease
called Leber's hereditary optic neuropathy (LHON). This rare
genetic disease affects the central nervous system, including the optic nerves,
causing bilateral loss of vision, of rapid onset, in early adulthood. The
disease is invariably inherited from the female parent, a consequence of the
fact that all of the mitochondria of the developing embryo are derived from the
mother. The unfertilized egg contains many mitochondria, and the few
mitochondria in the much smaller sperm cell do not enter the egg at the time of
fertilization. Mitochondria arise only from the division of preexisting
mitochondria.
A single base change in the mitochondrial gene ND4
(Fig. 18-29a) changes an Arg residue to a His residue in one of the proteins of
Complex I, and the result is mitochondria partially defective in electron
transfer from NADH to ubiquinone. Although these mitochondria are able to
produce some ATP by electron transfer from succinate, they apparently cannot
supply ATP in sufficient quantity to support the very active metabolism of
neurons. One result is damage to the optic nerve, leading to blindness.
A single base change in the mitochondrial gene for cytochrome b, a component
of Complex III, also produces LHON, demonstrating that the pathology of the
disease results from a general reduction of mitochondrial function, not just
from a defect in electron transfer through Complex I.
Another serious human genetic disease, myoclonic epilepsy and
ragged-red fiber disease (MERRF) is caused by a mutation in the
mitochondrial gene that encodes a transfer RNA (Fig. 18-29). This disease,
characterized by uncontrollable muscular jerking, apparently results from
defective production of several of the proteins synthesized using mitochondrial
transfer RNAs. Skeletal muscle fibers of individuals with MERRF have abnormally
shaped mitochondria that sometimes contain paracrystalline structures (Fig.
18-29b).
Mitochondria Probably Evolved from Endosymbiotic Bacteria
The fact that mitochondria contain their own DNA, ribosomes, and transfer
RNAs supports the theory of the endosymbiotic origin of mitochondria (see Fig.
2-17). This theory supposes that the first organisms capable of aerobic
metabolism, including respiration-linked ATP production, were prokaryotes.
Primitive eukaryotes that lived anaerobically (by fermentation) acquired the
ability to carry out oxidative phosphorylation when they established a symbiotic
relationship with bacteria living in their cytosol. After much evolution and the
movement of many bacterial genes into the nucleus of the "host" eukaryote, the
endosymbiotic bacteria eventually became mitochondria.
This theory presumes that early free-living prokaryotes had the enzymatic
machinery for oxidative phosphorylation, and it predicts that their modern
prokaryotic descendents have respiratory chains closely similar to those of
modern eukaryotes. They do.
Aerobic bacteria carry out NAD-linked electron transfer from substrates to
O2, coupled to the phosphorylation of cytosolic ADP.
The dehydrogenases are located in the bacterial cytosol, but the electron
carriers of the respiratory chain are in the plasma membrane. The electron
carriers are similar to those of mitochondria, act in the same sequence (Fig.
18-30), and translocate protons outward across the plasma membrane concomitantly
with electron transfer to O2. We noted earlier that
bacteria such as E. coli have
F0F1 complexes in their
plasma membranes; the F1 portions protrude into the
cytosol and catalyze ATP synthesis from ADP and Pi as protons
flow back into the cell through proton channels formed by
F0.
Figure 18-30 Respiratory chain in the inner
membrane of E. coli. Electrons from NADH pass to menaquinone (MQ), at
which point the chain branches. The upper path is dominant in cells grown under
normal aerobic conditions, but when O2 is the limiting
factor in growth, the lower route predominates.
Certain bacterial transport systems bring about uptake of
extracellular nutrients (lactose, for example) against a concentration gradient,
in symport with protons (see Fig. 10-25). The respiration-linked transmembrane
proton extrusion provides the driving force for this uptake. The rotary motion
of bacterial flagella, which move cells through their surroundings, is provided
by "proton turbines," molecular rotary motors driven not by ATP but directly by
the transmembrane electrochemical potential generated by respiration-linked
proton pumping (Fig. 18-31). It appears likely that the chemiosmotic mechanism
evolved early (before the emergence of eukaryotes). The protonmotive force can
clearly be used to power processes other than ATP synthesis.
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