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CASE FILES: BIOCHEMISTRY
In the electron transport chain (see Figure 16-1 in Case 16) only complex
IV (cytochrome oxidase) interacts directly with O2. As with hemoglobin,
CO has a higher affinity for cytochrome oxidase than O2. Thus CO binds
tightly to cytochrome oxidase and inhibits the transfer to O2. In tightly
coupled mitochondria the binding of CO to cytochrome oxidase also results in
the inhibition of the phosphorylation of adenosine diphosphate (ADP),
reducing the production of adenosine triphosphate (ATP). This becomes
more profound as additional molecules of cytochrome oxidase are bound by
CO. Similarly, myoglobin also binds CO more avidly than O2, and the trans-
fer of O2 to enzymes requiring O2 is inhibited.
The binding of CO to hemoglobin is fully dissociable, and dissociation
requires ventilation. After removal from exposure to CO, administration of
O2 reverses CO binding to hemoglobin. Utilization of 100 percent O2 accel-
erates the washout of CO. Use of hyperbaric chambers with pressures up
to 2 atmospheres speeds up the CO washout process even more. Addition of 5
to 7 percent CO2 to the O2 is sometimes used as a prompt to ventilatory
exchange. One disadvantage of the addition of CO2 is the serious acidosis that
results when the respiratory acidosis produced by CO2 inhalation is added to
the metabolic acidosis produced by O2 deprivation in the tissues because of
CO poisoning.
C O M P R E H E N SIO N Q U E ST IO N S
An elderly couple living in the suburbs of Hanover, New Hampshire, was
snowbound for several days in their home during a blizzard. During periods of
electrical outage they relied on an unvented gas heater to warm one room in
their home where they stayed throughout the blizzard. As soon as the roads
cleared, their granddaughter came to check on them and found them disori-
ented, complaining of headache, fatigue, and nausea and breathing and walk-
ing hesitantly and with a stumbling gait.
[17.1] Laboratory data show a remarkably increased carboxyhemoglobin
level in the blood. The best explanation for this finding is that CO has
which of the following effects?
A. It increases the hydrogen ion concentration causing oxyhemoglo-
bin to precipitate
B. It changes the valence state of iron in hemoglobin
C. It competitively displaces O2 from oxyhemoglobin
D. It converts myoglobin to carboxyhemoglobin at a rapid rate
E. It prevents transfer of O2 across the alveolar membranes
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