Wednesday, November 03, 2010

Why we can survive carbon monoxide poisoning..... ( read to the end)

My students have been drawing Lewis dot diagrams and predicting shapes of molecules based on their Lewis dot structures. This is a very detailed process and requires them to be familiar with the concepts of valence electrons, types of bonds and bond orders. It is so easy to get lost in all the details. This is why I wanted to stress the importance of WHY. Why does anybody really care about the shapes of molecules. Does it really matter?

Biochemical processes depend largely on the shapes of molecules. Receptors in our tongues and noses deliver taste and smell information to our brains based on the shapes of molecules. Many drugs bind in a way that is completely dependent on the shape of one molecule fitting with the shape of another (like two puzzle pieces fitting together).

The binding of receptors in the body drives many everyday processes like the production of ATP to get energy from your food. If the shape of any molecule involved in the process is modified, the entire process is out-of-whack and the body does not function.

Oxygen is a wonderful example to highlight the importance of molecular shape. Oxygen is essential for us to stay alive (as we all know). It travels through the body attached to the heme molecule in the blood. Oxygen binds to heme at a slight angle due to its valence structure including two groups of lone-pair electrons. In the presence of carbon monoxide, the heme binds to it rather than oxygen gas. (Carbon monoxide is more reactive.) There is also a bulky histidine group near the oxygen/heme or carbon monoxide/heme binding site. With it, nature built in a structural protection against CO poisoning: the histidine group prohibits the carbon monoxide from binding at a 180 degree angle. It must bind at a bent angle and is, therefore, not as strong of a bond as it otherwise would be. This is why carbon monoxide poisoning can be reversed if the subject is brought into the presence of pure oxygen again. The histidine prevents the heme/carbon monoxide bond from being irreversibly lethal.

You can see in the figure that the carbon monoxide binds with the iron in a straight line (180 degree angle) without the presence of the histidine to hinder it. In the presence of the histidine, the VSEPR shape cannot be fulfilled because the histidine is in the way. Carbon monoxide cannot bind in a straight line, and must bond at an angle. At an angle, the bond is not nearly as strong and can be broken by  moving into the presence of oxygen gas.