Has been suggested to be involved in cytochrome c peroxidase or P450-like oxygen chemistry or cyanide detoxification. The high oxygen affinity of this protein suggests that it probably does not function as an oxygen transporter. Binds 1 heme group per subunit; upon expression in E.coli it purifies predominantly in the Fe(2+)-oxy form, binding O(2) tightly even under anaerobic conditions. Monomer. Constitutively expressed, it is further induced by S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) (at protein level). Cells lacking this gene grow less well than wild-type under conditions of high aeration. Belongs to the truncated hemoglobin family. Group III subfamily.