When expressed in E.coli and M.smegmatis, HbO increases oxygen uptake. Membrane vesicles of E.coli carrying HbO show a respiration activity about twice that of membranes without HbO. HbO seems to interact with a terminal oxidase. Therefore, HbO could participate in oxygen/electron-transfer process, suggesting a function related to the facilitation of oxygen transfer during aerobic metabolism of M.tuberculosis. Binds 1 heme group per subunit. Homododecamer. Contains L-DOPA (3',4'-dihydroxyphenylalanine). Belongs to the truncated hemoglobin family. Group II subfamily.