Zur Wirkung von Liganden auf das Assoziations‐Dissoziations‐Gleichgewicht des Methämoglobins der Flußneunaugen (Lampetra fluviatilis L.)

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Abstract
The dissociation‐association equilibrium of lamprey methemoglobin as a function of protein concentration, pH, and ligandation of the heme Fe3+ has been studied using sedimentation analysis.The sedimentation constant s°20,w of MetHb in the pH range 4.5 < pH < 10 amounts to 1.94 S leading to a calculated molecular weight of 17000 which corresponds to the Hb protomer.At pH 7.5 the MetHb · N3 complex is monomeric. Below pH 5.7 and at high protein concentration the dimer of 34000, s°20,w= 3.13 S, is formed, the dimerization being accompanied by an uptake of two protons per protomer: (a) 2 MetHb + 2 N3−+ 2 H+⇌ (H+· MetHb · N3)2. At high concentrations of protein and azide the association of MetHb · N3 becomes a function of pH only: (b) 2 MetHb · N3+ 2 H+⇌ (H+· MetHb · N3)2. The pK value of equilibrium (b) is found to be 6.7 pointing to a heme‐linked imidazole determining the equilibrium.As compared to the azide complex, other ligands, such as F− > OCN− > CN− > SCN−, favour the dimerization of lamprey MetHb to a lesser degree.Dimerization of the heme polypeptide chains in the MetHb‐ligand complexes and their association in DeoxyHb supposedly involves different aminoacids in the interface between the protomers.

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