Structure of human R-state aquomethemoglobin at 2.0 Å resolution.

The crystal structure of tetrameric (αβ)(2) R-state human adult aquomethemoglobin is reported at 2.0 Å resolution. The asymmetric unit contained one αβ subunit pair. The R-state crystal belonged to space group P4(1)2(1)2, with unit-cell parameters a = b = 53.6, c = 192.8 Å. An Fe-bound water molecule was modeled into the heme distal pockets of each of the α and β subunits. In the α subunit, a highly ordered liganded water was modeled with an Fe-O(water) distance of 2.2 Å and appears to be protected against escape from the distal pocket by the conformation of the heme propionate groups, which point upwards towards the distal His58 residue aided by a hydrogen-bonding network involving the solvent. In the β subunit, the liganded water exhibited greater motion and was modeled with a longer Fe-O(water) distance of 2.5 Å; in this subunit both propionate groups point downwards away from the distal His63 residue, presumably allowing greater motion of the liganded water in and out of the distal pocket.