Cyanide binding to Lucina pectinata hemoglobin I and to sperm whale myoglobin: An x-ray crystallographic study

The x-ray crystal structures of the cyanide derivative of Lucina pectinata monomeric hemoglobin I (L. pectinata Hbl) and sperm whale (Physeter catodon ) myoglobin (Mb), generally taken as reference models for monomeric hemopro teins carrying hydrogen sulfide and oxygen, respectively, have been determi ned at 1.9 Angstrom (R-factor = 0.184), and 1.8 Angstrom\ (R-factor = 0.181 ) resolution, respectively, at room temperature (lambda = 1.542 Angstrom). Moreover, the x-ray crystal structure of the L. pectinata Hbl:cyanide deriv ative has been studied at 1.4-Angstrom resolution (R-factor = 0.118) and 10 0 K (on a synchrotron source lambda = 0.998 Angstrom). At room temperature, the cyanide ligand is roughly parallel to the heme plane of L. pectinata H bl, being located similar to 2.5 Angstrom from the iron atom. On the other hand, the crystal structure of the L. pectinata Hbl:cyanide derivative at 1 00 K shows that the diatomic ligand is coordinated to the iron atom in an o rientation almost perpendicular to the heme (the Fe-C distance being 1.95 A ngstrom), adopting a coordination geometry strictly reminescent of that obs erved in sperm whale Mb, at room temperature. The unusual cyanide distal si te orientation observed in L. pectinata Hbl, at room temperature, may refle ct reduction of the heme Fe(lll) atom induced by free radical species durin g x-ray data collection using Cu K alpha radiation.