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

The crystal structure of wild-type human brain neuroglobin reveals flexibility of the disulfide bond that regulates oxygen affinity

Neuroglobin plays an important function in the supply of oxygen in nervous tissues. In human neuroglobin, a cysteine at position 46 in the loop connecting the C and D helices of the globin fold is presumed to form an intramolecular disulfide bond with Cys55. Rupture of this disulfide bridge stabilizes bi-histidyl haem hexacoordination, causing an overall decrease in the affinity for oxygen. Here, the first X-ray structure of wild-type human neuroglobin is reported at 1.74 Å resolution. This structure provides a direct observation of two distinct conformations of the CD region containing the intramolecular disulfide link and highlights internal cavities that could be involved in ligand migration and/or are necessary to enable the conformational transition between the low and high oxygen-affinity states following S-S bond formation.

Heme orientation modulates histidine dissociation and ligand binding kinetics in the hexacoordinated human neuroglobin

Neuroglobin (Ngb) is a globin present in the brain and retina of mammals. This hexacoordinated hemoprotein binds small diatomic molecules, albeit with lower affinity compared with other globins. Another distinctive feature of most mammalian Ngb is their ability to form an internal disulfide bridge that increases ligand affinity. As often seen for prosthetic heme b containing proteins, human Ngb exhibits heme heterogeneity with two alternative heme orientations within the heme pocket. To date, no details are available on the impact of heme orientation on the binding properties of human Ngb and its interplay with the cysteine oxidation state. In this work, we used ¹H NMR spectroscopy to probe the cyanide binding properties of different Ngb species in solution, including wild-type Ngb and the single (C120S) and triple (C46G/C55S/C120S) mutants. We demonstrate that in the disulfide-containing wild-type protein cyanide ligation is fivefold faster for one of the two heme orientations (the A isomer) compared with the other isomer, which is attributed to the lower stability of the distal His64–iron bond and reduced steric hindrance at the bottom of the cavity for heme sliding in the A conformer. We also attribute the slower cyanide reactivity in the absence of a disulfide bridge to the tighter histidine–iron bond. More generally, enhanced internal mobility in the CD loop bearing the disulfide bridge hinders access of the ligand to heme iron by stabilizing the histidine–iron bond. The functional impact of heme disorder and cysteine oxidation state on the properties of the Ngb ligand is discussed.

Human neuroglobin functions as a redox-regulated nitrite reductase

Neuroglobin is a highly conserved hemoprotein of uncertain physiological function that evolved from a common ancestor to hemoglobin and myoglobin. It possesses a six-coordinate heme geometry with proximal and distal histidines directly bound to the heme iron, although coordination of the sixth ligand is reversible. We show that deoxygenated human neuroglobin reacts with nitrite to form nitric oxide (NO). This reaction is regulated by redox-sensitive surface thiols, cysteine 55 and 46, which regulate the fraction of the five-coordinated heme, nitrite binding, and NO formation. Replacement of the distal histidine by leucine or glutamine leads to a stable five-coordinated geometry; these neuroglobin mutants reduce nitrite to NO ∼2000 times faster than the wild type, whereas mutation of either Cys-55 or Cys-46 to alanine stabilizes the six-coordinate structure and slows the reaction. Using lentivirus expression systems, we show that the nitrite reductase activity of neuroglobin inhibits cellular respiration via NO binding to cytochrome c oxidase and confirm that the six-to-five-coordinate status of neuroglobin regulates intracellular hypoxic NO-signaling pathways. These studies suggest that neuroglobin may function as a physiological oxidative stress sensor and a post-translationally redox-regulated nitrite reductase that generates NO under six-to-five-coordinate heme pocket control. We hypothesize that the six-coordinate heme globin superfamily may subserve a function as primordial hypoxic and redox-regulated NO-signaling proteins.

Neuroglobin genetic polymorphisms and their relationship to functional outcomes after traumatic brain injury

Neuroglobin has shown rich neuroprotective effects against cerebral hypoxia, and therefore has the potential to impact outcomes after traumatic brain injury (TBI). However, to date an association between genetic variation within the human neuroglobin (NGB) gene and recovery post-TBI has not been reported. The purpose of this study was to explore the relationship between NGB genotypes and outcomes (as assessed by the Glasgow Outcome Scale [GOS], the Disability Rating Scale [DRS], and the Neurobehavioral Rating Scale-Revised [NRS-R]) after severe TBI. Genotyping using TaqMan allele discrimination for two tagging single nucleotide polymorphisms (tSNPs) that represent the two haplotype blocks for NGB (rs3783988 and rs10133981) was completed on DNA obtained from 196 Caucasian patients recovering from severe TBI. Patients were dichotomized based on the presence of the variant allele for each tSNP. Chi-square and Fisher's exact tests were used to compare characteristics between groups. Multivariate linear regression was used to examine NGB tSNPs and recovery from severe TBI. Subjects with the TT genotype (wild-type) for rs3783988 were more likely to have better GOS and DRS scores at 3, 6, 12, and 24 months, while rs10133981 genotype was not significantly related to functional outcome. After controlling for age, gender, and Glasgow Coma Scale (GCS) score, those subjects with the rs3783988 TT genotype had more than a 2.65-times greater likelihood of better functional outcomes than individuals with genotypes harboring a variant allele. Data suggest that the haplotype block represented by rs3783988 in NGB appears to influence recovery after severe TBI. Represented within this haplotype block of NGB is the region that codes for the oxygen-binding portion of NGB.