AHSP-induced alterations to the heme pocket and iron coordination of αHb. (A) The heme pocket of normal globin subunits, including oxy αHb. Helices E and F, which line the distal and proximal aspects of the planar heme structure, respectively, are indicated. Residues within helix G, located posteriorly in the diagram, participate in heme binding. Iron is indicated as a small sphere in the center of heme. The proximal HisF8 is shown bound to iron. The distal histidine E7 is not bound to iron and not shown. Oxygen, not shown, binds iron at the distal surface of heme, which is opposed by helix E. (B) The bis-histidyl globin, which is generated in holo α globin-AHSP complex. Helices E and F are distorted and both proximal His F8 and distal His E7 bind the heme iron. (C) Spectrophotometric changes reflecting conversion of AHSP-bound αHb from the oxygenated ferrous form to the ferric bis-histidyl form shown in panel B. The reaction was performed at 37°C. Tracings were recorded every 2 minutes with the direction of change over time indicated by. Adapted from Weiss et al.¹⁰⁸