THE PROXIMAL RESIDUE LARGELY DETERMINES THE CO DISTORTION IN CARBON MONOXY GLOBIN PROTEINS - AN AB INITO STUDY OF A HEME PROSTHETIC UNIT

An ab initio investigation of a model heme prosthetic group based on t he carbon monoxy myoglobin (MbCO) 1MBC X-ray structure reproduces the large off-perpendicular distortions of the Fe-C-O unit reported for th e protein. The distortion is mainly caused by the nonequilibrium orien tation of the proximal residue and not by the distal residue: inclusio n of the distal residue in a supermolecule calculation has a smaller e ffect on the Fe-C-O geometry. If such a mechanism primarily determines the Fe-C-O distortion in the protein itself, then the large strain en ergies implied by the Fe-C-O geometries in the X-ray structures are de livered by the protein tertiary structure, via the proximal residue, a nd not by the mobile distal side chain, as had been previously propose d. The structure-function relationship, as revealed by the X-ray struc ture, would then be clarified. Distortion of the Fe-C-O geometry is la rgely determined by the proximal residue, and so Fe-C-O is nonperpendi cular even in the His64Gly mutant. The distal residue is not subject t o a large repulsive interaction with the carbonyl ligand; thus, its or ientation in the solvated protein can be determined by weaker attracti ve electrostatic interactions, as inferred from recent experimental. s tudies of distal residue mutant myoglobins. This result removes the ne ed to invoke a large stabilization of the distal side chain orientatio n 'by a rigid hydrogen-bonding network, an interpretation of the physi ological structure-function relationship that was at odds with the X-r ay B factors and the mobility of surface residue side chains expected under physiological conditions.