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The N-terminal Sequence Affects Distant Helix Interactions in Hemoglobin

The N-terminal 18-amino acid sequence of the β-chain of hemoglobin, as far as the end of the A helix, has been replaced by the corresponding sequence of the γ-chain of fetal hemoglobin with the remaining sequence of the β-chain retained (helices B through H). The γ-β-chain had the correct mass, and its entire sequence was established by mass spectrometric analysis of its tryptic peptides; the α-chain also had the correct mass. This recombinant hemoglobin (named Hb Felix) retains cooperativity and has an oxygen affinity like that of HbA both in the presence and absence of the allosteric regulators, 2,3-diphosphoglycerate or chloride but differs from HbF in its 2,3-diphosphoglycerate response. However, Hb Felix has some features that resemble fetal hemoglobin,i.e. its significantly decreased tetramer-dimer dissociation and its circular dichroism spectrum, which measure the strength of the tetramer-dimer interface in the oxy conformation and its rearrangement to the deoxy conformation, respectively. Even though Hb Felix contains the HbA amino acids at its tetramer-dimer interface, which is located at a distance from the substitution sites, its interface properties resemble those of HbF. Therefore, the N-terminal sequence and not just those amino acids directly involved at the subunit interface contacts with α-chains must have a strong influence on this region of the molecule. The results reinforce the concept of fluid long range relationships among various parts of the hemoglobin tetramer (Dumoulin, A., Manning, L. R., Jenkins, W. T., Winslow, R. M., and Manning, J. M. (1997) J. Biol. Chem. 272, 31326–31332) and demonstrate the importance of the N-terminal sequence, especially in some mutant hemoglobins, in influencing its overall structure by affecting the relationship between helices.