Ligand‐Dependent Bohr Effect of Chironomus Hemoglobins

Abstract

The O2 and CO Bohr effects of monomeric and dimeric Hb of the insect C. thummi thummi were determined as proton releases upon ligation. For the O2 Bohr effect of the monomeric Hb III a maximum value of 0.20 H+/heme was obtained at pH 7.5. Upon ligation with CO, only 0.04 H+/heme were released at the same pH. In agreement with this finding isoelectric focusing experiments revealed different isoelectric points for O2-liganded and CO-liganded states of Hb III. Analogous results were obtained in the cases of the monomeric Hb IV and the dimeric Hb of C. t. thummi: here O2 Bohr effects of 0.43 and 0.86 H+/heme were observed. For the corresponding CO Bohr effects values of 0.08 and 0.31 H+/heme were obtained, respectively. On the basis of the available structural data the reduced CO Bohr effect in Hb III was discussed as arising from a steric hindrance of the CO ligand by the side chain of isoleucine-E11, obstructing the movement of the heme-Fe upon reaction with CO. It should be noted that ligands, according to their different electron donor and acceptor properties, may generally induce different conformational changes and different Bohr effects, in those HB in which distinct tertiary and/or quaternary constraints have not evolved. The general utilization of CO instead of O2 as allosteric effector is ruled out by the results reported.