Conformational Isomers of Nitrosyl‐haemoglobin

Abstract

The electron spin resonance spectra of the nitric oxide derivatives of horse and human haemoglobin indicate the existence of two Conformational states, the distribution of which depends on pH. The p K ‐value of this allosteric transformation is 5.35. This p K is shifted by 0.3 units to 5.65 in the presence of a 50‐fold molar excess of 2,3‐bisphosphoglycerate. The ESR spectrum of nitrosyl‐myoglobin corresponds exactly to that of the alkaline species of nitrosyl‐haemoglobin, but no pH dependent change was observed. These findings might suggest a relation between the observed Conformational transition and the acid Bohr effect. At 77 K the spectrum of the acid as well as that of the alkaline species of NO‐haemoglobin indicates rhombic symmetry of the inner molecular electric field. Raising the temperature to 298 K the ESR spectrum of the alkaline species shows a transition to axial symmetry. The spectrum of the acid species exhibits hyperfine structures in the direction of the x ‐ and z ‐axis, that of the alkaline species in the direction of the z ‐axis only. Substitution of 14 NO by 15 NO reveals that these hyperfine lines are caused by an interaction of the unpaired electron with the N‐nucleus of the 6th ligand. The hyperfine coupling tensor of the acid species ( 15 N: ΔH 3, 3 = 23 G; 14 N: ΔH 3,3,3 = 17 G) is smaller than in case of the alkaline species ( 15 N:Δ H 3,3 = 32 G; 14 N:ΔH 3,3,3 = 23 G).