The molecular properties of the methaemoglobin of Chironomus plumosus L. in solution.

Abstract

By means of an analytical ultracentrifuge the molecular weight of purified methaemoglobin from Chironomus plumosus has been determined at different pH values. Under weak acidic conditions (pH 5.2 to 7.0) the sedimentation constant amounts to 2.73 ± 0.02 S and the diffusion constant varies from 7.5 to 8.5 F. The molecular weight was calculated as 32,000 ± 1,500. Outside of this pH‐range the molecular weight decreases. In alkaline solution (pH 9.2–10.0) the dimer molecule splits off reversibly into monomer subunits, with an average molecular weight of 14,600 ± 300. In the pH‐range 7.5 to 9.0 and below 5.0 there exists a mixture of dimer and monomer molecules. The monomer‐dimer‐equilibrium is pH‐dependent. pH1/2 of this equilibrium coincides with the pH1/2 of the spectrophotometrically estimated main ionization constant (pH1/2= 8.3). The deprotonation of a haemin‐bound water molecule induces a change of the quarternary structure of the Chironomus‐methaemoglobin‐molecule. According to the frictional ratio and the Stoke‐radius the monomer and the dimer MetHb‐molecules have an asymmetrical shape. Assuming a rotation ellipsoid (prolate type) for the monomer form (pH > 9.2) the axes are about 24 and 12.6 Å. The relatively high frictional ratio of the dimer molecule may be interpreted by an association of the α‐ and β‐monomers in direction of their longer axes.