Characterization of Heme Orientational Disorder in a Myoglobin Reconstituted with a Trifluoromethyl-Group-Substituted Heme Cofactor

The orientation of a CF3-substituted heme in sperm whale myoglobin and L29F, H64L, L29F/H64Q and H64Q variant proteins has been investigated using F-19 NMR spectroscopy to elucidate structural factors responsible for the thermodynamic stability of the heme orientational disorder, i.e., the presence of two heme orientations differing by a 180 degrees rotation about the 5-15 meso axis, with respect to the protein moiety. Crystal structure of the metaquo form of the wild-type myoglobin reconstituted with 13,17-bis (2-carboxylatoethyl)-3,8-diethy1-2,12,18-trimethyl-7-trifluoromethylporphyrinatoiron(III), determined at resolution of 1.25 angstrom, revealed the presence of the heme orientational disorder. Alterations of the salt bridge between the heme 13-propionate and Arg45(CD3) side chains due to the mutations resulted in equilibrium constants of the heme orientational disorder ranging between 0.42 and 1.4. Thus, the heme orientational disorder is affected by the salt bridge associated with the heme 13-propionate side chain, confirming the importance of the salt bridge in the heme binding to the protein.