Ligand‐Specific Bohr Effect in Haemoglobins

Abstract

The CO binding properties of monomerie (Hb III and IV), dimeric, and tetrameric Chironomus haemoglobins as well as of the monomer‐analogue valence hybrid (αMmet2βmetβdeoxy) of Hb M Iwate were studied and compared with the respective O2 binding properties. All these haemoglobins show hyperbolic O2 binding curves and a Bohr effect. The amplitudes of the O2 Bohr effect curves range from Δlog (PO2(1/2), = 0.3 (Hb III) to 2.4 (tetrameric Chironomus haemoglobin).The ligands CO and O2 differ not only in affinity but also with regard to the Bohr effect. The CO affinities of the various Chironomus haemoglobins are 60 to 490 times larger than the O2 affinities and the amplitudes of the CO Bohr effect curves are 1.25 to 8 times smaller than those of the respective O2 Bohr effect curves. In the case of Hb M Iwate, however, the CO Bohr effect is larger than the O2 Bohr effect by the factor 1.25 (2,3‐bisphosphoglycerate‐reacted Hb M) and 1.4 (stripped Hb M).The ligand‐specific differences in the magnitude of the Bohr effect cannot be explained solely by the allosteric interaction connected with the trans‐effect of the complex. The differences are discussed in terms of steric influences on the binding geometry of the 6th ligand by protein side chains at the distal side of the haem group.