The extracellular giant hemoglobin is able to bind and transport oxygen and hydrosulfide simultaneously and reversibly at two different sites. The 400 kDa hemoglobin consists of a spherical 24-mer arranged as a double layer of dome-shaped dodecamers. Each dodecamer is composed of the 3-fold trimer of the tetramer A1-A2-B1-B2 having one intra-tetramer (A1-B2) disulfide bond and one inter-tetramer (B1-B2) disulfide bond per tetramer. Belongs to the globin family.