X-ray Studies of the Interaction of CO₂ with Human Deoxyhaemoglobin

Abstract

IN addition to transporting CO₂ indirectly by way of the alkaline Bohr effect, haemoglobin can interact directly with CO₂ to form carbamate residues : RNH₂ + CO₂⇌RNHCOO⁻ + H⁺ This reaction of CO₂ with haemoglobin is oxygen-linked so that at constant pH more CO₂ is bound to deoxy- than to oxyhaemoglobin, and conversely the oxygen affinity of haemoglobin is decreased in the presence of CO₂ (refs 1 and 2). The studies of Kilmartin and Rossi-Bernardi^3,4 on horse haemoglobin specifically modified at the N terminal amino groups by reaction with cyanate showed that at constant pH CO₂ has no effect on the oxygen affinity of haemoglobin when all four α-amino groups are carbamylated, but that only part of the oxygen-linked effect is inhibited when the α-amino groups of only the α or only the β chains are blocked. This implies that the N-terminal amino groups are solely responsible for the oxygen-linked CO₂ interactions and that this effect is shared between the α and β chains. More recent experiments have confirmed these results for human haemoglobin⁵.