carbaminohemoglobin

Carbaminohemoglobin is the form of hemoglobin in which carbon dioxide is bound to the protein portion of the molecule, through carbamylation of the amino groups at the N-termini of the globin chains. The primary binding sites are the N-terminal valine residues on the α- and β-globin chains, where CO2 forms carbamate linkages (–NH–COO−) and releases a proton. The reaction is reversible and its extent depends on the oxygenation state of hemoglobin.

In terms of transport, carbaminohemoglobin contributes to the circulation of CO2 from tissues to the lungs along with dissolved CO2 and bicarbonate. The fraction bound as carbaminohemoglobin is relatively modest under physiological conditions, often cited as about 5–15%, and is larger when hemoglobin is deoxygenated, since deoxyhemoglobin has a higher affinity for CO2. The proton released during carbamylation also contributes to the Bohr effect, promoting oxygen release in tissues by lowering local pH.

In the lungs, oxygenation of hemoglobin reduces carbamino formation and CO2 is released as it dissociates from the protein and is expelled in exhalation. The major CO2 carrier in blood is bicarbonate, formed from CO2 catalyzed by carbonic anhydrase within red blood cells; however, carbaminohemoglobin provides a significant, reversible route for CO2 transport and influences acid-base balance during respiration.