Question

The toxic effect of carbon monoxide is due to its greater affinity for hemoglobin as compared to oxygen, approximately by
A. 2 times
B. 20 times
C. 200 times
D. 1000 times

Answer 1

Carbon monoxide combines with Hb far more readily than $O_2$ forming a relatively stable compound carboxyhemoglobin.

Complete answer: Carbon monoxide is produced endogenously in small amounts as a byproduct of haem catabolism. Together with nitric oxide it affects cellular function and acts as a neurotransmitter.
1. Environmental carbon monoxide is produced by incomplete combustion of any carbon-containing fuel (coal, petroleum, peat, natural gas). In Britain, most accidents arise through central heating faults by contrast, in the USA most deaths are caused by inhalation of exhaust fumes.
2. In the United Kingdom car exhaust emissions of carbon monoxide have been reduced by catalytic converters in all new cars. Surprisingly, when deaths occur in garages there have usually been open doors and windows. A small environmental concentration will cause toxic levels of carboxyhemoglobin. 3. After the carbon monoxide has selectively bound to haemoglobin the oxygen-hemoglobin dissociation curve of the remaining oxyhaemoglobin shifts to the left, reducing oxygen release.
4. Cellular uptake of oxygen is blocked by the binding of carbon monoxide with mitochondrial cytochrome aa3. The hypoxia precipitates endothelial cell and platelet release of nitric acid, which forms the free radical peroxynitrite.
5. The molecule of hemoglobin is mostly spherical and, as its name suggests, it is made of two main parts – globin molecules (polypeptide) with embedded heme prosthetic groups. The whole molecule is a tetramer, consisting of four subunits joined together by H- bridges and ion bonds.
6. Metal ions used in catalysis have to be firmly attached to a specific site of the molecule.
It is achieved via so-called coordination (or dative) bonds, which form an ion complex.
7. Ion complexes possess a central atom of metal surrounded by a different number of ligands.
Ligands have free electron pairs, which they donate to the metal ion and thus form a bond.
8. The H+ cation formed during dissociation of carbonic acid binds to hemoglobin that acts as a buffer. Deoxygenated hemoglobin is more basic than oxygenated and that is why it binds protons more readily.

So, the correct answer is “Option C ”.

Note: Normally when a person breathes fresh air into the lungs, the oxygen in the air binds with a molecule called hemoglobin (Hb) that is found in red blood cells. Inhaling carbon monoxide gas interferes with this oxygen transport system. In the lungs, CO competes with oxygen to bind with the hemoglobin molecule.