Coupling of the heme and an internal disulfide bond in human neuroglobin

Neuroglobin displays a hexacoordination His-Fe-His in the absence of external ligands such as oxygen. The observed oxygen affinity therefore depends on the binding rates of both oxygen and the competing distal histidine. Furthermore, the binding properties depend on the presence of an internal disulfide bond. In the case of human neuroglobin, cysteines at positions CD7 and D5 are sufficiently close to form an internal disulfide bond. For cytoglobin, the cysteine residues at positions A7 and GH4 may also form a disulfide bond. Mass spectrometry, ligand binding, and thiol accessibility studies were used to study the role influence of these disulfide bonds. Mutation of specific cysteines, or reduction to break the S-S bond, led to a large decrease in the observed oxygen affinity of human neuroglobin, mainly due to a decrease in the histidine dissociation rate. This suggests a novel mechanism for the oxygen binding; reduction of the disulfide bond would provoke the release of oxygen.