Crystal structures of myoglobin-ligand complexes at near-atomic resolution

We have used x-ray crystallography to determine the structures of sperm wha le myoglobin (Mb) in four different ligation states (unligated, ferric aquo met, oxygenated, and carbonmonoxygenated) to a resolution of better than 1. 2 Angstrom. Data collection and analysis were performed in as much the same way as possible to reduce model bias in differences between structures. Th e structural differences among the ligation states are much smaller than pr eviously estimated, with differences of <0.25 Angstrom root-mean-square dev iation among all atoms. One structural parameter previously thought to vary among the ligation states, the proximal histidine (His-93) azimuthal angle , is nearly identical in all the ferrous complexes, although the tilt of th e proximal histidine is different in the unligated form. There are signific ant differences, however, in the heme geometry, in the position of the heme in the pocket, and in the distal histidine (His-64) conformations. In the CO complex the majority conformation of ligand is at an angle of 18 +/- 3 d egrees with respect to the heme plane, with a geometry similar to that seen in encumbered model compounds; this angle is significantly smaller than re ported previously by crystallographic studies on monoclinic Mb crystals, bu t still significantly larger than observed by photoselection. The distal hi stidine in unligated Mb and in the dioxygenated complex is best described a s having two conformations. Two similar conformations are observed in MbCO, in addition to another conformation that has been seen previously in low-p H structures where His-64 is doubly protonated. We suggest that these confo rmations of the distal histidine correspond to the different conformational substates of MbCO and MbO(2) seen in vibrational spectra, Full-matrix refi nement provides uncertainty estimates of important structural parameters. A nisotropic refinement yields information about correlated disorder of atoms ; we find that the proximal (F) helix and heme move approximately as rigid bodies, but that the distal (E) helix does not.