Biophysical characterization of the alpha-globin binding protein alpha-hemoglobin stabilizing protein

α-Hemoglobin stabilizing protein (AHSP) is a small (12 kDa) and abundant erythroid-specific protein that binds specifically to free α-(hemo)globin and prevents its precipitation. When present in excess over β-globin, its normal binding partner, α-globin can have severe cytotoxic effects that contribute to important human diseases such as β-thalassemia. Because AHSP might act as a chaperone to prevent the harmful aggregation of α-globin during normal erythroid cell development and in diseases of globin chain imbalance, it is important to characterize the biochemical properties of the AHSP·α-globin complex. Here we provide the first structural information about AHSP and its interaction with α-globin. We find that AHSP is a predominantly α-helical globular protein with a somewhat asymmetric shape. AHSP and α-globin are both monomeric in solution as determined by analytical ultracentrifugation and bind each other to form a complex with 1:1 subunit stoichiometry, as judged by gel filtration and amino acid analysis. We have used isothermal titration calorimetry to show that the interaction is of moderate affinity with an association constant of 1 × 107 M-1 and is thus likely to be biologically significant given the concentration of AHSP (∼0.1 mM) and hemoglobin (∼4 mm) in the late pro-erythroblast.