Ligand orientation of human neuroglobin obtained from solution NMR and molecular dynamics simulation as compared with X-ray crystallography

Neuroglobin, a new member of hemoprotein family, can reversibly bind oxygen and take part in many biological processes such as enzymatic reaction, signal transduction and the mitochondria function. Different from myoglobin and hemoglobin, it has a hexacoordinated heme environment, with histidyl imidazole of proximal His⁹⁶(F8) and distal His⁶⁴(E7) directly bound to the metal ion. In the present work, solution ¹H NMR spectroscopy was employed to investigate the electronic structure of heme center of wild-type met-human neuroglobin. The resonances of heme protons and key residues in the heme pocket were assigned. Two heme orientations resulting from a 180° rotation about the α-γ-meso axis with a population ratio about 2:1 were observed. Then the ¹H NMR chemical shifts of the ferriheme methyl groups were used to predict orientations of the axial ligand. The obtained axial ligand plane angle φ is consistent with that from the molecular dynamics simulation but not with those from the crystal data. Compared with mouse neuroglobin, the obtained average ligand orientation of human neuroglobin reflects the changeability of heme environment for the Ngb family.