Spectral changes upon subunit association in valency hybrid hemoglobins

Abstract

The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of valency hybrid hemoglobins and their constituents α + and β chains for α 2 + β 2, α and β + chains for α 2 β 2 +: + denotes ferric heme) were measured in the Soret region for F −, H 2O, N 3 − and CN − derivatives. Absorption and MCD spectra of valency hybrid hemoglobins were very similar to the arithmetic mean of respective spectra of their corresponding component chains in all derivatives. The Soret MCD intensity around 408 nm for various complexes of valency hybrid hemoglobins seems to reflect the spin state of ferric chains. Upon ferric and deoxy ferrous subunit association to make the deoxy valency hybrid hemoglobins, only the high-spin forms bound with F − and H 2O of α 2 + β 2 displayed a blue shift in the peak position around 430 nm and those of α 2 β 2 + an increase in intensity around 430 nm. The blue shift and the increase in intensity were considered to be caused by the structural changes in deoxy β chains of α 2 + β 2 and deoxy α chains of αβ 2 +, respectively. These spectral changes were interpreted on the basis of their oxygen-equilibrium properties. In contrast to absorption and MCD spectra, the CD spectra of valency hybrid hemoglobins were markedly different frm the simple addition of those of their component chains in all derivatives examined. The large part of CD spectral changes upon subunit association were interpreted as changes in the heme vicinity accompanied by formation of the α 1 β 1 subunit contact.