A kinetic analysis has been made of the interaction of ␣-Hb chains with a mutant ␣-hemoglobin stabilizing protein, AHSP V56G , which is the first case of an AHSP mutation associated with clinical symptoms of mild thalassemia syndrome. The chaperone AHSP is thought to protect nascent ␣ chains until final binding to the partner ␤-Hb. Rather than protecting ␣ chains, the mutant chaperone is partially unfolded but recovers its secondary structure via interaction with ␣-Hb. For both AHSP WT and AHSP V56G , the binding to ␣-Hb is quite rapid relative to the ␣-␤ reaction, as expected because the chaperone binding must be quite competitive to complete the ␣ chain folding process before ␣-Hb binds irreversibly to ␤-Hb. The main kinetic difference is a dissociation rate of AHSP V56G ⅐␣-Hb some four times faster relative to AHSP⅐␣-Hb. Considering a role of protein folding, the AHSP V56G apparently does not bind long enough (0.5 s versus 2 s for the WT) to complete the structural modifications. The overall replacement reaction (AHSP⅐␣-Hb ؉ ␤-Hb 3 AHSP ؉ ␣␤) can be quite long, especially if there is an excess of AHSP relative to ␤-Hb monomers.During the investigation of a child presenting with a mild thalassemia syndrome, normal globin gene pattern was observed while a structural abnormality of the ␣-hemoglobin stabilizing protein (AHSP) 2 gene (AHSP Val 56 3 Gly or AHSP V56G ) was found at the homozygous state (1). The child suffered from a microcytic, hypochromic anemia, which required a blood transfusion at the age of 1 month. This clinical observation was the first evidence that a homozygous defect of AHSP may be the etiology of thalassemia syndrome in human. AHSP is the ␣-hemoglobin (␣-Hb) chaperone that binds the holo or apo␣-Hb chain but not ␤-Hb or Hb tetramers (2). The steady state red blood cell showed values below the normal range for total Hb (11 g/decaliter), mean corpuscular volume (60 femtoliter), and mean corpuscular hemoglobin (20 pg).AHSP is a small 102-residue protein expressed only in erythroid cells, which adopts a three helix bundle (3). Helices 1 and 2 and the intervening segment of AHSP specifically recognize the G and H helices of ␣-Hb, which also are involved in the ␣1␤1 interface of Hb dimers (4). dos Santos et al. (5) have shown that, in human erythroid cells, the AHSP gene expression is related to that of the ␣-globin gene. AHSP concentration is estimated to be ϳ0.1 mM at the end of erythropoiesis (2), whereas that of Hb is ϳ5 mM. Nascent ␣ chains are protected by the chaperone before binding to the partner ␤ chains. Within the developing red blood cell precursors, the chaperone must first bind the ␣ chain to complete its correct folding. Evidence that AHSP facilitates folding of the ␣-globin subunits for Hb synthesis has been brought by Yu et al. (6), which adds a key element to the reaction scheme of Hb synthesis.If the chaperone simply acts as a buffer for an excess of ␣ chains, an appropriate affinity is necessary, but there would be freedom in the protein-protein interaction rates. This rol...

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