Myohemerythrin (Mhr) is a nonheme iron oxygen carrier found in the retractor muscles of marine 'peanut' worms. The X-ray crystal structures of two recombinant Themiste zostericola Mhrs are reported to a resolution of 1.8 Å. Surprisingly, the met wild-type structure (R = 17.8%) was found to contain chloride bound to Fe2, while coordinated hydroxide was found in the met L103N structure (R = 18.3%). An internal water molecule was also found distal to the Fe-O-Fe center of the mutant protein, forming hydrogen bonds with the coordinated hydroxide and the OD1 atom of Asn-103. This finding is consistent with the kinetic and spectroscopic results reported for the L103N mutant Mhr [Raner, G. M., Martins, L. J., and Ellis, W. R., Jr. (1997) Biochemistry 36, 7037-7043]. Possible roles for the side chain of residue 103 (Leu in wild-type Mhr) in gating ligand binding are also discussed.
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