Hemoglobin (Hb) is in equilibrium between low affinity Tense (T) and high affinity Relaxed (R) states associated with its unliganded and liganded forms, respectively. Mammalian species can be classified into two groups on the basis of whether they express 'high' and 'low' oxygen affinity Hbs. Although Hbs from former group have been studied extensively, a limited number of structural studies have been performed for the low oxygen affinity Hbs. Here, the crystal structure of low oxygen affinity sheep methemoglobin (metHb) has been determined to 2.7 angstrom resolution. Even though sheep metHb adopts classical R state like quaternary structure, it shows localized quaternary and tertiary structural differences compared with other liganded Hb. The critical group of residues in the "joint region", shown as a major source of quaternary constraint on deoxyHb, formed unique interactions in the alpha 1 beta 2/alpha 2 beta 1 interfaces of sheep metHb structure. In addition, the constrained beta subunits heme environment and the contraction of N-termini and A-helices of beta subunits towards the molecular dyad are observed for sheep metHb structure. These observations provide the structural basis for a low oxygen affinity and blunt response to allosteric effector of sheep Hb. (C) 2014 Elsevier Inc. All rights reserved.