Changes of Near-UV Circular Dichroism Spectra of Human Hemoglobin upon the R→T Quaternary Structure Transition

Abstract

Human adult hemoglobin (Hb A) exhibits a distinct negative CD band at 287 nm in the deoxy-form (T, tense), but this band disappears in the oxy-form (R, relaxed). It was suggested that the environmental alteration of aromatic amino acids, Tyr-α42 and/or Trp-β37, at the α 1 β 2 subunit contact contributed to the negative CD band in the deoxy-form. However, precise assignment of aromatic amino acids responsible for the negative CD band still remains unsettled. To evaluate contribution of the aromatic amino acid residues to the negative CD band upon the R→ T structure transition, we examined near-UV CD spectra of four mutant hemoglobins, two at the α 1 β 2 subunit contact, recombinant hemoglobin (rHb) (Tyr-α42→Ser) and rHb (Trp-β37→His), and the other two at the penultimate Tyr, rHb (Tyr-β145→Thr) and Hb Rouen (Tyr-α140→His). Environmental alteration in the penultimate tyrosine of both Tyr-α140 and Tyr-β145 primarily contributed to the negative CD band in the deoxy-form. Contributions of Tyr-a42 and Trp-β37 to the negative CD band were relatively small. Comparison of the negative CD bands of arithmetic mean of the isolated subunits with that of the recombined Hb A revealed contribution due to their tertiary structure change. The negative CD band of deoxyHb A was attributed to the sum of environmental alterations of aromatic amino acid residues induced by both tertiary structure change (longer wavelength region) and quaternary structure transition (shorter wavelength region).