Biochemical characterization and ligand binding properties of neuroglobin, a novel member of the globin family

Neuroglobin is a recently discovered member of the globin superfamily that is suggested to enhance the O-2 supply of the vertebrate brain. Spectral measurements with human and mouse recombinant neuroglobin provide evidence for a hexacoordinated deoxy ferrous (Fe2+) form, indicating a His-Fe2+-His binding scheme. O-2 or CO can displace the endogenous protein ligand, which is identified as the distal histidine by mutagenesis. The ferric (Fe3+) form of neuroglobin is also hexacoordinated with the protein ligand E7-His and does not exhibit pH dependence. Flash photolysis studies show a high recombination rate (k(on)) and a slow dissociation rate (k(off)) for both O-2 and CO, indicating a high intrinsic affinity for these ligands. However, because the rate-limiting step in ligand combination with the deoxy hexacoordinated form involves the dissociation of the protein ligand, O-2 and CO binding is suggested to be slow in vivo. Because of this competition, the observed O-2 affinity of recombinant human neuroglobin is average (1 torr at 37 degrees C). Neuroglobin has a high autoxidation rate, resulting in an oxidation at 37 degrees C by air within a few minutes. The oxidation/reduction potential of mouse neuroglobin (E'(o) = -129 mV) lies within the physiological range. Under natural conditions, recombinant mouse neuroglobin occurs as a monomer with disulfide-dependent formation of dimers. The biochemical and kinetic characteristics are discussed in view of the possible functions of neuroglobin in the vertebrate brain.