Neutron diffraction study of carbonmonoxymyoglobin

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 ε atom of the distal histidine residue is deprotonated (not deuterated), and a water molecule is bound to the Nδ 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 Å 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)°. 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.