Kinetics of ?-globin binding to ?-hemoglobin stabilizing protein (AHSP) indicate preferential stabilization of hemichrome folding intermediate.

ABSTRACT: Human ?-hemoglobin stabilizing protein (AHSP) is a conserved mammalian erythroid protein that facilitates the production of Hemoglobin A by stabilizing free ?-globin. AHSP rapidly binds to ferrous ? with association (k'(AHSP)) and dissociation (k(AHSP)) rate constants of ?10 ?m(-1) s(-1) and 0.2 s(-1), respectively, at pH 7.4 at 22 °C. A small slow phase was observed when AHSP binds to excess ferrous ?CO. This slow phase appears to be due to cis to trans prolyl isomerization of the Asp(29)-Pro(30) peptide bond in wild-type AHSP because it was absent when ?CO was mixed with P30A and P30W AHSP, which are fixed in the trans conformation. This slow phase was also absent when met(Fe(3+))-? reacted with wild-type AHSP, suggesting that met-? is capable of rapidly binding to either Pro(30) conformer. Both wild-type and Pro(30)-substituted AHSPs drive the formation of a met-? hemichrome conformation following binding to either met- or oxy(Fe(2+))-?. The dissociation rate of the met-?·AHSP complex (k(AHSP) ? 0.002 s(-1)) is ?100-fold slower than that for ferrous ?·AHSP complexes, resulting in a much higher affinity of AHSP for met-?. Thus, in vivo, AHSP acts as a molecular chaperone by rapidly binding and stabilizing met-? hemichrome folding intermediates. The low rate of met-? dissociation also allows AHSP to have a quality control function by kinetically trapping ferric ? and preventing its incorporation into less stable mixed valence Hemoglobin A tetramers. Reduction of AHSP-bound met-? allows more rapid release to ? subunits to form stable fully, reduced hemoglobin dimers and tetramers.

SUBMITTER: Mollan TL 

PROVIDER: S-EPMC3322864 | biostudies-literature | 2012 Mar

REPOSITORIES: biostudies-literature