ubiquinone undergoes oxidation-reduction; the isoprenoid side chain remains unchanged. What is
the function of this chain?
to diffuse in the semifluid membrane. This is important because ubiquinone transfers electrons
from Complexes I and II to Complex III, all of which are embedded in the inner mitochondrial
become more reduced and those beyond the block become more oxidized (see Fig. 19-6). For each of the
conditions below, predict the state of oxidation of ubiquinone and cytochromes b, c1, c, and a a3.
(a) Abundant NADH and O2, but cyanide added
(b) Abundant NADH, but O2 exhausted
(c) Abundant O2, but NADH exhausted
(d) Abundant NADH and O2
(b) In the absence of O2, no terminal electron acceptor is present; all carriers become
(c) In the absence of NADH, no carrier can be reduced; all carriers become oxidized.
(d) These are the usual conditions for an aerobic, actively metabolizing cell; the early carriers
(e.g., Q) are somewhat reduced, while the late ones (e.g., cytochrome c) are oxidized.
from plants, strongly inhibits NADH dehydrogenase of insect and fish mitochondria. Antimycin A, a
toxic antibiotic, strongly inhibits the oxidation of ubiquinol.
(a) Explain why rotenone ingestion is lethal to some insect and fish species.
(b) Explain why antimycin A is a poison.
(c) Given that rotenone and antimycin A are equally effective in blocking their respective sites in the
electron-transfer chain, which would be a more potent poison? Explain.
through the respiratory chain, which in turn decreases the rate of ATP production. If this
reduced rate is unable to meet its ATP requirements, the organism dies.
(b) Antimycin A strongly inhibits the oxidation of reduced Q in the respiratory chain, severely
limiting the rate of electron transfer and ATP production.
(c) Electrons flow into the system at Complex I from the NAD-linked reactions and at
Complex II from succinate and fatty acyl-CoA through FAD (see Figs. 19-8 and 19-16).
Antimycin A inhibits electron flow (through Q) from all these sources, whereas rotenone
inhibits flow only through Complex I. Thus, antimycin A is a more potent poison.
tightly coupled to the demand for ATP. When the rate of use of ATP is relatively low, the rate of
electron transfer is low; when demand for ATP increases, electron-transfer rate increases. Under these
conditions of tight coupling, the number of ATP molecules produced per atom of oxygen consumed
when NADH is the electron donor—the P/O ratio-is about
the rate of electron transfer and the P/O ratio.
can compensate for this by increasing the rate of electron flow; ATP levels can be kept
relatively normal. At high levels of uncoupler, P/O ratios approach zero and the cell
cannot maintain ATP levels.
consumption increases, heat is released, and the pH gradient across the inner mitochondrial membrane
increases. Does valinomycin act as an uncoupler or an inhibitor of oxidative phosphorylation?
agent that causes the free energy released in electron transfer to appear as heat rather than
in ATP. In respiring mitochondria, H ions are translocated out of the matrix during electron
transfer, creating a proton gradient and an electrical potential across the membrane. A significant
portion of the free energy used to synthesize ATP originates from this electric potential.
Valinomycin combines with K ions to form a complex that passes through the inner mitochondrial
membrane. So, as a proton is translocated out by electron transfer, a K ion moves
in, and the potential across the membrane is lost. This reduces the yield of ATP per mole of
protons flowing through ATP synthase (FoF1). In other words, electron transfer and phosphorylation
become uncoupled. In response to the decreased efficiency of ATP synthesis, the rate
of electron transfer increases markedly. This results in an increase in the H gradient, in oxygen
consumption, and in the amount of heat released.
and the rate of ATP production dramatically decrease
What process in electron transfer or oxidative phosphorylation is affected by DCCD?
of electron transfer also
now added to the preparation, O2 consumption returns to normal but ATP production remains inhibited
Why does DCCD affect the O2 consumption of mitochondria? Explain the effect of 2,4-dinitrophenol
on the inhibited mitochondrial preparation.
uncouples electron transfer from ATP synthesis, allowing respiration to increase. No ATP
is synthesized and the P/O ratio decreases.
in the mitochondrion, but malate dehydrogenase is found in both the cytosol and mitochondrion. What
is the role of cytosolic malate dehydrogenase?
NADH produced in the cytosol cannot cross the inner mitochondrial membrane, but
must be oxidized if glycolysis is to continue. Reducing equivalents from NADH enter the mitochondrion
by way of the malate-aspartate shuttle. NADH reduces oxaloacetate to form malate
and NAD, and the malate is transported into the mitochondrion. Cytosolic oxidation of glucose
can continue, and the malate is converted back to oxaloacetate and NADH in the mitochondrion
Predict the effect of this inhibitor on (a) glycolysis, (b) oxygen consumption,
(c) lactate formation, and (d) ATP synthesis.
cells, NADH accumulates in the cytosol. This forces glycolysis to operate anaerobically,
with reoxidation of NADH in the lactate dehydrogenase reaction.
(b) Because reducing equivalents from the oxidation reactions of glycolysis do not enter the
mitochondrion, oxygen consumption slows and eventually ceases.
(c) The end product of anaerobic glycolysis, lactate, accumulates.
(d) ATP is not formed aerobically because the cells have converted to anaerobic glycolysis.
Overall, ATP synthesis decreases drastically, to 2 ATP per glucose molecule.
lactate ceases. This effect, first observed by Louis Pasteur in the 1860s, is characteristic of most cells
capable of aerobic and anaerobic glucose catabolism.
(a) Why does the accumulation of lactate cease after O2 is added?
(b) Why does the presence of O2 decrease the rate of glucose consumption?
(c) How does the onset of O2 consumption slow down the rate of glucose consumption? Explain in
terms of specific enzymes.
transfer and oxidative phosphorylation as the mechanism for NADH oxidation.
(b) Cells produce much more ATP per glucose molecule oxidized aerobically, so less glucose
(c) As [ATP] rises, phosphofructokinase-1 is inhibited, thus slowing the rate of glucose entry
into the glycolytic pathway.
of fermentation (ethanol, acetate, lactate, or glycerol) via the normal respiratory route.
These mutants do not have a working citric acid cycle because they cannot reoxidize NADH
through the O2-dependent electron-transfer chain. Thus, catabolism of glucose stops at the
ethanol stage, even in the presence of oxygen. The ability to carry out these fermentations in
the presence of oxygen is a major practical advantage because completely anaerobic conditions
are difficult to maintain. The Pasteur effect—the decrease in glucose consumption that
occurs when oxygen is introduced—is not observed in the absence of an active citric acid
cycle and electron-transfer chain.
3-phosphate dehydrogenase use NAD as their electron acceptor, the two enzymes do not compete
for the same cellular NAD pool. Why?
3-phosphate dehydrogenase in the cytosol. Because the mitochondrial and cytosolic pools of
NAD are separated by the inner mitochondrial membrane, the enzymes do not compete for
the same NAD pool. However, reducing equivalents are transferred from one nicotinamide
coenzyme pool to the other via shuttle mechanism
the mitochondrion. Neither NAD nor NADH passes through the inner membrane, thus the labeled
NAD moiety of [7-14C]NADH remains in the cytosol. The 3
H on [4-3
H]NADH enters the
mitochondrion via the malate-aspartate shuttle (see Fig. 19-29). In the cytosol, [4-3
transfers its 3
H to oxaloacetate to form [3
H]malate, which enters the mitochondrion via the
malate- -ketoglutarate transporter, then donates the 3
H to NAD to form [4-3
H]NADH in the
an antiporter that brings ATP out of the matrix into the ims and ADP into the matrix from the ims
this phenomenon in molecular terms. What happens to the P/O ratio in the presence of uncouplers?
make sufficient ATP by oxidizing more fuel. The heat produced by this increased rate of
oxidation raises the body temperature
agent, in principle, serve as a weight-reducing aid? Uncoupling agents are no longer prescribed
because some deaths occurred following their use. Why might the ingestion of uncouplers lead to
degradation of additional energy stores (glycogen and fat). By oxidizing more fuel in an
attempt to produce the same amount of ATP, the organism loses weight
dehydrogenase (Complex II) are associated with midgut carcinoid tumors
mutations that lead to unregulated cell division (cancer)
cancer cells are in an altered metabolic state; mutations in mitochondrial DNA encoded genes can contribute to the development of cancer. It is possible that such mutations provide metabolic adaptivity to the cancer cell
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