Structure of methemerythrin at 2.8-.Å resolution: computer graphics fit of an averaged electron density map

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Abstract
The crystal structure of methemerythrin from Themiste dyscritum was determined at 2.8 .ANG. resolution by single isomorphous replacement techniques combined with anomalous scattering from a K2HgI4 derivative. Noncrystallographic symmetry relating the 4 subunits in the asymmetric unit was used to obtain an average electron density map of the hemerythrin monomer and a computer graphics system was used to fit a polypeptide model to the electron density. The average map was of sufficient quality to locate most of the amino acid side chains and to confirm the assignment of His-25, His-54, Glu-58, His-73, His-77, His-101, Asp-106 and Tyr-109 as the iron ligands. Of the mercury sites in the heavy atom derivative, 1 is located between 2 Cys-9 residues related by a noncrystallographic 2-fold axis, although no intersubunit disulfide bond is present in the native structure. The residues responsible for the binding of the subunits to form the octamer are identified.