Carbon Dioxide and Hemoglobin

Release of carbon dioxide from respiring tissues lowers the oxygen affinity of hemoglobin in two ways.

> 1. Some of the carbon dioxide becomes bicarbonate,

> CO 2 + H 2O <-> HCO 3- + H+

> releasing protons that contribute to the Bohr effect.

> 2. Some of this bicarbonate is transported out of the erythrocytes and is carried dissolved in the blood serum. A portion reacts directly with hemoglobin, binding to the N-terminal amino groups of the chains to form carbamates, as follows:

> The formation of carbamates allows hemoglobin to aid in the transport of CO 2 from tissues to lungs or gills. It has two additional effects, too. First, protons released on binding of HCO 3- contribute to the Bohr effect. Second, a negatively charged group is introduced at the N-terminus of the chains. The negatively charged group stabilizes salt bridge formation between and chains, which is characteristic of the deoxy state. Thus, both the latter effect and the lower pH promote oxygen release when CO 2 is abundant.

> The reverse of the carbamation reaction, which occurs in lungs or gills (where O 2 concentration is high, is equally important. Here, the high O 2 concentration favors oxygenation and hence the oxy form of the molecule. When this switch occurs, stabilization of the carbamated N-termini is decreased, and CO 2 is expelled and expired.

Thus, from Figure 7.1, in the lungs or gills of an animal, O 2 is abundant. Oxygenation favors the oxy conformation of hemoglobin, which stimulates the release of CO 2. As the blood then travels via arteries into the tissue capillaries, the lower pH and high CO 2 concentration favor the deoxy form, promoting O 2 release and binding of CO 2. Carbon dioxide, both in forming bicarbonate and in reacting with hemoglobin, causes the release of more protons, further stimulating O 2 release and CO 2 binding.

See also: The Bohr Effect, Models of Allosteric Activity