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. Hell’s Gate globin I (HGbI), a heme-containing protein structurally homologous to mammalian neuroglobins, has been identified from an acidophilic and thermophilic obligate methanotroph, Methylacidiphilum infernorum. HGbI has very high affinity for O2 and shows barely detectable autoxidation in the pH range of 5.2–8.6 and temperature range of 25–50°C. Examination of the heme pocket by X-ray crystallography and molecular dynamics showed that conformational movements of Tyr29(B10) and Gln50(E7), as well as structural flexibility of the GH loop and H-helix, may play a role in modulating its ligand binding behavior. Bacterial HGbI’s unique resistance to the sort of extreme acidity that would extract heme from any other hemoglobin makes it an ideal candidate for comparative structure–function studies of the expanding globin superfamily.