Structural and functional properties of a single domain hemoglobin from the food-borne pathogen Campylobactor jejuni

Campylobacter jejuni contains two globins, a truncated hemoglobin, Ctb, and a single domain hemoglobin, Cgb. The physiological function of Ctb remains unclear, whereas Cgb has been linked to NO detoxification. With resonance Raman scattering, the iron-histidine stretching mode of Cgb was identified at 251 cm^-1. This frequency is unusually high, suggesting an imidazolate character of the proximal histidine as a result of the H-bonding network linking the catalytic triad involving the F8His, H23Glu, and G5Tyr residues. In the CO-complex, two conformers were identified with the ν_C-O/ ν_Fe-CO at 529/1914 cm^-1 and 492/1963 cm^-1. The former is assigned to a "closed" conformation, in which the heme-bound CO is stabilized by the H-bond(s) donated from the B10Tyr-E7Gln residues, whereas the latter is assigned to an "open" conformer, in which the H-bonding interaction is absent. The presence of the two alternative conformations demonstrates the plasticity of the protein matrix. In the O ₂-complex, the iron-O₂ stretching frequency was identified at 554 cm^-1, which is unusually low, indicating that the heme-bound O₂ is stabilized by strong H-bond(s) donated by the B10Tyr-E7Gln residues. This scenario is consistent with its low O₂ off-rate (0.87 s^-1). Taken together the data suggest that the NO-detoxifying activity of Cgb is facilitated by the imidazolate character of the proximal F8His and the distal positive polar environment provided by the B10Tyr-E7Gln. They may offer electronic "push" and "pull," respectively, for the O-O bond cleavage reaction required for the isomerization of the presumed peroxynitrite intermediate to the product, nitrate.