?-Hemoglobin-stabilizing protein (AHSP) perturbs the proximal heme pocket of oxy-?-hemoglobin and weakens the iron-oxygen bond.

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

SUBMITTER: Dickson CF 

PROVIDER: S-EPMC3707698 | biostudies-literature | 2013 Jul

REPOSITORIES: biostudies-literature