A hexaPEGylated hemoglobin (Hb), (Propyl-PEG5K)6-Hb, is essentially in αβ dimers (Hu et al. (2007) Biochem. J. 402, 143-151). In order to provide a biochemical insight into the tetramer-dimer dissociation of this PEGylated Hb, we prepared and characterized two PEGylated Hbs site-specifically modified at Val-1(R) and at Val-1(β), respectively. PEGylation at Val-1(α) and at Val-1(β) increase the tetramer-dimer dissociation constant (Kd) of Hb by 2 and 1 order of magnitude, respectively. Accordingly, the sites of PEGylation can determine the tetramer stability of the PEGylated Hb. In order to determine the role of the polyethylene glycol (PEG) chains on the tetramer stability of Hb, we prepared a propylated Hb site-specifically modified at Val-1(α). Interestingly, site-specific propylation of Hb at Val-l(α) stabilizes the Hb tetramer by 1 order of magnitude. Therefore, conjugation of the PEG chains at Val-1(α) can greatly destabilize the tetramer stability of Hb. On the structural aspects, the PEG chains conjugated at Va-1(α) unfavorably alter the heme environment and quaternary structure and destabilize the α1β2 interface of Hb. On the functional aspects, the PEG chains conjugated at Val-1(α) decrease the Hill coefficient, the Bohr effect of Hb and the sensitization to the presence of the allosteric effectors. In contrast, PEGylation of Hb at Val-1(β) gives rise to less pronounced structural alteration and different functional change.
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