Structure of myoglobin refined at 2-0 A resolution. I. Crystallographic refinement of metmyoglobin from sperm whale.

The structure of sperm whale metmyoglobin has been refined using new intensity data to 2·0 A collected on a four-circle diffractometer. Starting with the original phase angles determined by isomorphous replacement with heavy atoms ( Kendrew et al. , 1960 ) an electron density map was calculated, and atomic parameters were subjected to real-space refinement. Phases derived from these improved atomic parameters were used to calculate a new electron density map, which served as a basis for the next real-space refinement. Several cycles of this procedure led to an R factor of 0·235. The C-terminal residues and many atoms in side chains which had not been clearly defined in the original map were located, as were 82 solvent molecules, including the two sulphate ions originally described. The iron atom is displaced by 0·40 A from the mean plane of the haem and lies at a distance of 2·13 A from N (e) of the proximal histidine. At least two of the four pyrrole rings are tilted out of the haem plane, while the exact inclination of the other two could not be determined. The haem is in van der Waals' contact with 83 atoms of the globin (excluding hydrogens). These include three large clusters of hydrophobic side chains, each with spare room for a foreign molecule. The tertiary structure of the globin is stabilized by many hydrogen bonds between the various helical and non-helical segments, many of which have been described before ( Watson, 1969 ). Metmyoglobin differs from the α-and β-chains of horse methaemoglobin in the orientation of the haem and in several details of the tertiary structure of the globin, which is not surprising seeing that the number of homologies between myoglobin and the α- and the β-chains are only 32 and 30, respectively ( Dayhoff, 1972 ).