Crystal structures of the nitrite and nitric oxide complexes of horse heart myoglobin

Nitrite is an important species in the global nitrogen cycle, and the nitrite reductase enzymes convert nitrite to nitric oxide (NO). Recently, it has been shown that hemoglobin and myoglobin catalyze the reduction of nitrite to NO under hypoxic conditions. We have determined the 1.20 Å resolution crystal structure of the nitrite adduct of ferric horse heart myoglobin (hh Mb). The ligand is bound to iron in the nitrito form, and the complex is formulated as MbIII(ONO−). The Fe–ONO bond length is 1.94 Å, and the O–N–O angle is 113°. In addition, the nitrite ligand is stabilized by hydrogen bonding with the distal His64 residue. We have also determined the 1.30 Å resolution crystal structures of hh MbIINO. When hh MbIINO is prepared from the reaction of metMbIII with nitrite/dithionite, the FeNO angle is 144° with a Fe–NO bond length of 1.87 Å. However, when prepared from the reaction of NO with reduced MbII, the FeNO angle is 120° with a Fe–NO bond length of 2.13 Å. This difference in FeNO conformations as a function of preparative method is reproducible, and suggests a role of the distal pocket in hh MbIINO in stabilizing local FeNO conformational minima.