Structural rearrangements due to ligand binding and haem replacement in myoglobin and leghaemoglobins

Structural rearrangements in sperm whale myoglobin and leghaemoglobins caused by changes in the spin or oxidation state of the iron as a consequence of ligand binding have been measured by difference spectroscopy in the ultraviolet. When compared with the high-spin acetate complex, ligands which cause a transition to the low-spin state also cause large perturbations of tyrosine(s) remote from the haem pocket in myoglobin but only minor perturbations of tryptophan (s) in leghaem calobin. This may indicate a weaker coupling between events at the haem site and conformational changes in the protein in leghaemoglobins. The absorption spectra of various haem-liganded forms of the two proteins as well as the binding of the dye rose Bengal to the two apoproteins are consistent with weaker interactions between the haem ano apoprotein and a more solvent-exposed haem pocket in leghaemoglobin compared with myoglobin.