Abstract
The crystal structure of sperm whale myoglobin (Mb):azide complex has been determined and refined at 1.8 A resolution. The ligand is coordinated to the heme iron, and deeply buried in the inner part of the distal site. The structural organization of this complex differs substantially from that observed inA. limacina Mb: azide complex, in which the iron coordinated ligand is directed towards the outer solvent region of the protein. In the case of sperm whale Mb the bound ligand is stabilized by a hydrogen bond to residue HisE7; in the case ofA. limacina Mb the ligand is directly hydrogen bonded to ArgE10. The structures of the complexes described underline different mechanisms of ligand recognition and stabilization in the two Mb’s, and allow a rational interpretation of the different kinetic and thermodynamic properties observed in globins lacking a polar residue at the distal E7 site.
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