Studies on erythrocruorin: IV. Circular dichroism of earthworm erythrocruorin

Abstract

Circular dichroism spectra of Lumbricus erythrocruorin in the absence and in the presence of heme ligands have been analyzed under a variety of experimental conditions in view of the peculiarities in ligand binding displayed by this high molecular weight heme protein ( M r = 3 × 10 6). The undisaociated molecule exists in a “metastable” form with high cooperativity in oxygen binding, which can be converted into a stable form with low co-operativity either by changes in pH or temperature; circular dichroism spectra of oxyerythrocruorin in the Soret region give direct evidence of a local alteration in the heme environment under the conditions which affect co-operativity in oxygen binding of the undissociated molecule. Similar, although more pronounced changes in the same spectral region are observed in the dissociated molecule of M r = 270,000, which displays low co-operativity in oxygen binding. Deoxygenation is accompanied by an inversion in the double Soret-Cotton effect, which indicates a substantial rearrangement in the heme environment upon removal of the ligand. The double peak in the Soret region found in all erythrocruorin derivatives can be taken as an indication of a distinctive distribution of the aromatic side-chains interacting with the heme chromophore.