Re: Why does hemoglobin accept carbon monoxide better

Blake,

Because the affinity of carbon monoxide (CO) for hemoglobin is over 200 times greater than oxygen, carbon monoxide binds to hemoglobin even when carbon monoxide is at very low partial pressures. With carbon monoxide attached, hemoglobin can no longer transport oxygen, rendering the hemoglobin useless until the carbon monoxide dissociates. The effects of carbon monoxide poisoning are thus identical to those of low levels of oxygen.

The hemoglobin protein contains four heme units. Each heme molecule contains a central iron atom, and it is this atom that binds either carbon monoxide, or oxygen, depending on which is present. All four iron sites are involved in binding oxygen, but each site binds a single oxygen molecule. The oxygen molecules bind cooperatively, which means that the first binds weakly, but induces a conformational change that causes the second to bind more tightly, and so on, so that the fourth oxygen is bound several hundred times more strongly than the first. Thus, a high partial pressure of oxygen is required to bind the first oxygen, which is desirable, because the oxygen transport system depends on hemoglobin binding oxygen well in the lungs, where it is more abundant, but not in the oxygen-poor tissues elsewhere, where it needs to be released. Upon release of the first oxygen molecule, the second one is more easily released, and so on, with the effect that oxygen release is facilitated where it is needed the most.

Carbon monoxide is dangerous not so much because of its relatively stronger bond, but because once a hemoglobin molecule has even one bound CO molecule, it can't adopt the configuration necessary to start binding oxygen. Thus, one or two CO molecules effectively inhibit all four of the oxygen binding sites in hemoglobin.

Here is a URL with some additional descriptions of the binding of oxygen by hemoglobin, with graphics:

URL: http://www.aw-bc.com/ mathews/ch07/c07cobh.htm

Moderator's note

The affinity of CO for the heme molecule (the iron-bearing porphyrin co-factor for hemoglobin) is 25,000 times that of the affinity of O2 for heme. The affintiy of CO for heme is reduced 125 fold in hemoglobin because of the presence of Histidine E7, which is coordinated over the iron in the heme group. The presence of this histidine forces the CO to bind to the heme iron at an angle in hemoglobin, where it binds in a straight line to the heme iron in heme alone. This steric hinderance greatly reduces the afinity of CO for heme in hemoglobin. O2 normally binds to the iron in heme at an angle anyway, so its binding is not affected by the presence of Histidine E7.

You can find more information about heme, hemoglobin, and CO and O2 binding in any college- level biochemistry book, such as Biochemistry by Lupert Stryer.