Role of tertiary structures on the Root effect in fish hemoglobins

► The Root effect in fish Hbs causes a change in cooperativity and oxygen affinity. ► Structures and function of T state Antarctic fish Hb at pH 6.2 and 8.4 are compared. ► At low pH ligation causes no structural effects, and cooperativity is abolished. ► At high pH, ligation causes a displacement of the switch region CD corner. ► A significant degree of cooperativity is associated with tertiary changes.