Purification and spectroscopic characterization of Ctb, a group III truncated hemoglobin implicated in oxygen metabolism in the food-borne pathogen Campylobacter jejuni

Campylobacter jejuni is a food-borne bacterial pathogen that possesses two distinct hemoglobins, encoded by the ctb and cgb genes. The former codes for a truncated hemoglobin (Ctb) in group III, an assemblage of uncharacterized globins in diverse clinically and technologically significant bacteria. Here, we show that Ctb purifies as a monomeric, predominantly oxygenated species. Optical spectra of ferric, ferrous, O₂- and CO-bound forms resemble those of other hemoglobins. However, resonance Raman analysis shows Ctb to have an atypical ν_Fe-CO stretching mode at 514 cm^-1, compared to those of the other truncated hemoglobins that have been characterized so far. This implies unique roles in ligand stabilization for TyrB10, HisE7, and TrpG8, residues highly conserved within group III truncated hemoglobins. Because C. jejuni is a microaerophile, and a ctb mutant exhibits O₂-dependent growth defects, one of the hypothesized roles of Ctb is in the detoxification, sequestration, or transfer of O₂. The midpoint potential (Eh) of Ctb was found to be -33 mV, but no evidence was obtained in vitro to support the hypothesis that Ctb is reducible by NADH or NADPH. This truncated hemoglobin may function in the facilitation of O₂ transfer to one of the terminal oxidases of C. jejuni or, instead, facilitate O ₂ transfer to Cgb for NO detoxification.