Crystal structures of ferrous horse heart myoglobin complexed with nitric oxide and nitrosoethane

The interactions of nitric oxide (NO) and organic nitroso compounds with heme proteins are biologically important, and adduct formation between NO-containing compounds and myoglobin Mb) have served as prototypical systems for studies of these interactions. We have prepared crystals of horse heart (hh) MbNO from nitrosylation of aqua-metMb crystals, and we have determined the crystal structure of hh MbNO at a resolution of 1.9 Angstrom. The Fe-N-O angle of 147degrees in hh MbNO is larger than the corresponding 112degrees angle previously determined from the crystal structure of sperm whale MbNO (Brucker et al., Proteins 1998;30:352356) but is similar to the 150degrees angle determined from a MS XAFS study of a frozen solution of hh MbNO (Rich et al., J Am Chem Soc 1998;120:10827-10836). The Fe-N(O) bond length of 2.0 Angstrom (this work) is longer than the 1.75 Angstrom distance determined from the XAFS study and suggests distal pocket influences on FeNO geometry. The nitrosyl N atom is located 3.0 Angstrom from the imidazole N-is an element of atom of the distal His64 residue, suggesting electrostatic stabilization of the FeNO moiety by His64. The crystal structure of the nitrosoethane adduct of ferrous hh M was determined at a resolution of 1.7 Angstrom. The nitroso O atom of the EtNO ligand is located 2.7 Angstrom from the imidazole N-is an element of atom of His64, suggesting a hydrogen bond interaction between these groups. To the best of our knowledge, the crystal structure of hh Mb(EtNO) is the first such determination of a nitrosoalkane adduct of a heme protein. (C) 2003 Wiley-Liss, Inc.