Abstract
The structures of the deoxy, oxy, and aquomet forms of native sperm whale myoglobin reconstituted with cobalt protoporphyrin IX have been determined by x-ray crystallography. As expected, cobalt myoglobin closely resembles native iron myoglobin in overall structure, especially in their respective aquomet forms. In the cobalt oxymyoglobin structure, the Nepsilon of distal histidine 64 lies within hydrogen bonding distance to both the oxygen atom directly bonded to the cobalt and the terminal oxygen atom, in agreement with previous EPR and resonance Raman studies. The metal atom in cobaltous myoglobin does show a small 0.06-A out-of-porphyrin plane displacement when moving from the oxy to deoxy state. In the case of the native iron-containing myoglobin, the oxy to deoxy transition results in a larger 0.16-A displacement of the metal farther out of the porphyrin plane, attributed to an increase in spin from S = 0 to S = 2. The small displacement in cobalt myoglobin is due to a change in coordination geometry, not spin state (S = 1/2 for both cobalt deoxy- and oxymyoglobin). The small out-of-porphyrin plane movement of cobalt which accompanies deoxygenation of myoglobin also occurs in cobalt hemoglobin and serves to explain why cooperativity, although reduced, is still preserved when iron is replaced by cobalt in human hemoglobin.
Highlights
Cobalt myoglobin and cobalt hemoglobin, in which the heme prosthetic group is replaced by cobalt protoporphyrin IX, are capable of reversible oxygen binding with affinities 50 to 100 times weaker than those of the native iron counterparts [1, 2]
The EPR and resonance Raman spectroscopic features associated with the dioxygen complex of cobaltous myoglobin are altered in deuterated buffers, suggesting formation of a hydrogen bond between the bound oxygen and the distal histidine residue [3,4,5]
These interpretations, rely on an assumption of identical oxygen binding geometry in cobalt and native myoglobins and quantitative analyses of spectroscopic properties have been hampered by the lack of a high resolution crystal structure
Summary
Unique reflections Resolution (Å) Completeness (%) R-merge (%) Space group Unit cell a (Å) b (Å) c (Å)  (°) R cryst (%) RMS deviation Bonds (Å) Angles (°) Dihedrals (°) Impropers (°). 0.02 1.78 20.0 1.89 the high resolution crystal structures of the deoxy, oxy, and aquomet forms of sperm whale cobalt myoglobin. The results have been compared in detail to the corresponding structures of the native iron-containing protein
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