title: Neutron diffraction study of carbonmonoxymyoglobin.
abstract: Neutron diffraction data from a crystal of carbonmonoxymyoglobin were refined by PROLSQ, a modern restrained least-squares procedure in reciprocal space, in conjunction with a solvent analysis technique, to a final R-factor of 11.3%. The ligand CO occupies two sites and its binding conformations are distorted from the linear conformation. The N epsilon atom of the distal histidine residue is deprotonated (not deuterated), and a water molecule is bound to the N delta atom of the distal histidine. The side-chain of Lys56 (D6) exists in two alternative charge-binding sites. His24 (B5) and His119 (GH1) share a hydrogen atom. His12 (A10) and His36 (C1) are deprotonated. The deprotonated imidazole ring of His12 (A10) may act as a hydrogen-bond acceptor. The heme group is planar within 0.09 A root-mean-square (r.m.s.) deviation from planarity. The solvent environments for the two propionic acid groups are different. The side-chain of Arg45 (CD3) forms hydrogen bonds with the side-chain of Asp60 (E3) and one of the two propionic acid groups. An average N-2H . . . O angle in helical regions is 147 (+/- 11) degrees. Eleven main-chain amide hydrogen atoms from hydrophobic residues do not exchange with deuterium. The overall atomic occupancy factors for the main-chain and side-chain atoms are quite uniform, at 0.97 (+/- 0.07) and 0.93 (+/- 0.10), respectively, as shown by an occupancy analysis made at the end of the refinement procedure.
authors.affiliation: Department of Biology, Brookhaven National Laboratory, Upton, NY 11973.
authors.forename: X D
authors.initials: XD
authors.lastname: Cheng
authors.forename: B P
authors.initials: BP
authors.lastname: Schoenborn
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002248:Carbon Monoxide / Q000378:metabolism; D003460:Crystallization; D005583:Fourier Analysis; D006418:Heme / Q000378:metabolism; D008433:Mathematics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009211:Myoglobin / Q000737:chemistry* / Q000378:metabolism; D009502:Neutrons; D011487:Protein Conformation; D012542:Scattering, Radiation
chemical_list: D009211:Myoglobin; C018013:carboxymyoglobin; D006418:Heme; D002248:Carbon Monoxide