STRUCTURE DETERMINATION OF THE BILIVERDIN APOMYOGLOBIN COMPLEX - CRYSTAL-STRUCTURE ANALYSIS OF 2 CRYSTAL FORMS AT 1.4 AND 1.5 ANGSTROM RESOLUTION

Crystal structure determinations of two orthorhombic (P2(1)2(1)2(1)) c rystal modifications of the biliverdin apomyoglobin complex are descri bed. The two structures were determined by X-ray diffraction at 100 K to a resolution of 1.5 Angstrom and 1.4 Angstrom. Both crystal forms w ere grown by hanging-drop techniques, using phosphate as precipitant. The structures were solved by molecular replacement and refined to fin al X-values of 19.4% and 21.2%. Both structures are very similar with respect to the binding site and the conformation of the biliverdin chr omophore, which occurs in a (P) helical conformation. It is located wi thin the heme pocket, very close in position and orientation to the he me binding site in myoglobin. Two water molecules not present in the c rystal structure of myoglobin are sequestered within the heme pocket i n the biliverdin-apomyoglobin complex, and they are engaged in hydroge n bonding to the biliverdin and to the protein. Comparison with struct ural results from an earlier NMR study of the same complex shows good agreement.