Reduced Bohr Effect in NO‐Ligated <i>Chironomus</i> Haemoglobin

Abstract

The low‐temperature electron‐spin‐resonance spectra of the NO‐derivatives of the monomeric Chironomus haemoglobins I, III, and IV being nearly identical to each other, indicate rhombic symmetry of the innermolecular electric field. At g z = 2.003 nine hyperfine lines (Δ H = 7 G) of nearly identical intensity are observed in the presence of 14 NO, while 15 NO produces two hyperfine lines (Δ H = 30 G) both showing a further splitting into three superhyperfine lines (Δ H = 7 G). These hyperfine structures represent the interaction of the unpaired electron with the N‐nuclei of the 5th and 6th ligand. No changes of the electron‐spin‐resonance spectra were found in the range of pH 5 to 10, although haemoglobin III and IV are known to possess an alkaline Bohr effect. Thus in case of NO‐binding the Bohr effect is apparently reduced. With the aid of proton‐magnetic‐resonance studies of the Bohr proton binding site, His‐G2, in NO‐haemoglobin III it was demonstrated that the p K value of this group is shifted by only 0.15 units from that of deoxy‐haemoglobin. This shift is considerably lower as compared with that induced by CO‐ligation.