α-Hemoglobin-stabilizing Protein (AHSP) Perturbs the Proximal Heme Pocket of Oxy-α-hemoglobin and Weakens the Iron-Oxygen Bond *

摘要: α-Hemoglobin (αHb)-stabilizing protein (AHSP) is a molecular chaperone that assists hemoglobin assembly. AHSP induces changes in αHb heme coordination, but how these are facilitated by interactions at the αHb·AHSP interface not well understood. To address this question we have used NMR, x-ray absorption spectroscopy, and ligand binding measurements to probe conformational induced binding. NMR chemical shift analyses of free CO-αHb CO-αHb·AHSP indicated seven helical elements native structure retained Fe(II) remains coordinated proximal His-87 side chain. However, differences revealed alterations F, G, H helices pocket bound AHSP. Comparisons iron-ligand geometry using extended fine spectroscopy showed small 0.03 Å lengthening Fe-O2 bond, explaining previous reports decreases O2 affinity roughly 4-fold promotes autooxidation due primarily 3–4-fold increase rate dissociation. Pro-30 mutations diminished pocket, restored normal dissociation equilibrium constants, reduced O2-αHb rates. Thus, contacts mediated wild-type promote introducing strain into pocket. As chaperone, facilitates rapid assembly Hb when βHb abundant diverts redox resistant holding state limiting.