Hell's Gate globin I: An acid and thermostable bacterial hemoglobin resembling mammalian neuroglobin

► HGbI is a bacterial globin that resists oxidation and functions at extremely acidic pH. ► Heme-iron coordination of ferrous HGbI shifts from 5- to 6-coordinate with increasing pH. ► HGbI is structurally similar to mammalian neuroglobins. ► HGbI has strikingly greater flexibility in the GH loop and H-helix upon O2 binding. ► Flexibility of Tyr29(B10) and rotation of Gln50(E7) may modulate ligand binding.