CHARACTERIZATION OF HEMOPEXIN AND ITS INTERACTION WITH HEME BY DIFFERENTIAL SCANNING CALORIMETRY AND CIRCULAR-DICHROISM

Hemopexin is a plasma glycoprotein that has two structural domains (I and II) and binds and transports heme particularly to liver cells. Differential scanning calorimetry (DSC) studies show that hemopexin is largely stabilized by heme, which binds exclusively to domain I. The melting temperature (T(m)) of heme-hemopexin is 66.4 +/- 0.7-degrees-C as compared with 53.9 +/- 0.3-degrees-C for apohemopexin, and this T(m) increase is accompanied by a 100 kcal increase in molar enthalpy. Heme stabilizes hemopexin by stabilizing domain I. This is demonstrated by the 26-degrees-C increase in T(m) from 51.9 +/- 0.3 to 77.6 +/- 0.6-degrees-C and the over 3-fold increase in molar enthalpy when domain I associates with heme. A moderate change in domain I secondary structure is indicated by an increase in negative molar ellipticity at 206 nm. However, there is no net effect on the secondary structure of holo-hemopexin caused by heme binding as indicated by both far-UV circular dichroism (CD) and Fourier-transform infrareds spectra. The characteristic positive ellipticity of hemopexin at 233 nm, ascribed to tryptophan residues in domain II, is dramatically increased, suggesting a change in tertiary structure for domain II of hemopexin. DSC and CD results show that isolated domain I and domain II interact both in the presence and absence of heme. Moreover, domain II destabilizes heme-domain I, which may be an important factor in facilitating heme release to the hemopexin receptor. Examination of a variety of heme analogs revealed three classes of effects on both the thermodynamic stability and the positive ellipticity of hemopexin at 233 nm: heme and cobalt-protoporphyrin increase both; iron-meso-tetrakis(4-sulfonatophenyl)porphine does not affect either; protoporphyrin and nickel-protoporphyrin decrease both.