The heme environment of leghemoglobins. Absorption and circular dichroism spectra of artificial leghemoglobins and myoglobins

Abstract

Artificial leghemoglobins were reconstituted from apoleghemoglobin and meso-, deutero- and diacetyldeuteroheme. Absorption and circular dichroism spectra of their high-spin and low-spin derivatives in the ferrous and ferric forms were recorded in the ultraviolet and visible wavelength regions. The substitution of the 2,4-side-chains of heme induced changes in the optical activity, reflecting alterations in the heme environment. The effect on the conformation of aromatic amino acid residues around heme obviously correlates with the sixth axial ligand and the spin state of iron. Absorption and CD spectra of the aquoferric derivatives of artificial myoglobins were recorded in comparison. Strongly electron-withdrawing acetyl side-chains at the 2,4-positions of diacetyldeuteroheme caused a change in the absorption spectra of aquoferric leghemoglobin and myoglobin towards low spin. On the basis of the spectra it was suggested that the displacement of the ferric iron from the pyrrole plane in leghemoglobin derivatives would be smaller than in the corresponding myoglobin derivatives.