Reverse Bohr effect on the oxygen-binding affinity of heme embedded in a bilayer of liposome as a hemoglobin model: pH-induced oxygen uptake and evolution by aqueous synthetic lipid-heme solution

Abstract

The oxygen-binding affinity (p_½; oxygen pressure at 50% binding for the heme) of a heme derivative embedded in a bilayer of natural phospholipid liposomes was influenced by the solution pH: the p_½ value of the liposome–lipid-heme increased with pH {Bohr coefficient, r=d[log(p_½)]d(pH)=+0.09 to +0.32}, which is the reverse of the Bohr effect for hemoglobin. This pH dependence was affected by the composition of the liposome–lipid-heme. Its mechanism is discussed in relation to the surrounding lipid bilayer structure and protonation equilibrium of the imidazole ligand. pH-induced oxygen-uptake and -evolution was observed, in a process which was sensitive to ±1 unit of pH change at pH 7.