α-Hemoglobin stabilizing protein (AHSP), a kinetic scheme of the action of a human mutant, AHSPV56G

Abstract

A kinetic analysis has been made of the interaction of α-Hb chains with a mutant α-hemoglobin stabilizing protein, AHSPV56G, 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 α chains until final binding to the partner β-Hb. Rather than protecting α chains, the mutant chaperone is partially unfolded but recovers its secondary structure via interaction with α-Hb. For both AHSPWT and AHSPV56G, the binding to α-Hb is quite rapid relative to the α-β reaction, as expected because the chaperone binding must be quite competitive to complete the α chain folding process before α-Hb binds irreversibly to β-Hb. The main kinetic difference is a dissociation rate of AHSPV56G·α-Hb some four times faster relative to AHSP·α-Hb. Considering a role of protein folding, the AHSPV56G apparently does not bind long enough (0.5 s versus 2 s for the WT) to complete the structural modifications. The overall replacement reaction (AHSP·α-Hb +β-Hb 3 AHSP + αβ) can be quite long, especially if there is an excess of AHSP relative to β-Hb monomers. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.