Ligand induced conformational changes in various normal and modified hemoglobins as indicated by changes in optical rotatory dispersion.

Abstract

A decrease of the optical rotation in the wavelength region from 200 to 250 mμ is observed when ligands bind to hemoglobin. The change at 233 mμ amounts to about 8% of the total rotation at this wavelength, independently of the nature of the ligand (O2, CO, NO or ethyl‐isocyanide). The effect is interpreted as being caused by a small conformational change, which influences the electronic transitions of a number of peptide bonds or (and) a few aromatic side chains. The change is observed with αβ‐dimers, but not with the isolated single chains. The full effect is present however, in chemically modified hemoglobins which no longer show cooperative interactions in the ligand equilibrium curve. It is speculated that the conformational change takes place within the protomers but only when these are joined in αβ‐dimers (or tetramers). The chemical modifications might not prevent the initial ligand‐induced conformational change, but only abolish the transmission of the conformational change from one subunit to the others.