Comparative Structural and Immunochemical Properties of Leghaemoglobins

Abstract

Circular dichroism studies on leghemoglobins from snake bean [Vigna sinensis], lupin [Lupinus luteus], serradella [Ornithopus sativus] and other plants show that, in common with soybean, they have a similar overall polypeptide chain conformation and heme environment and orientation. Immunochemical studies suggest that the antigenic determinants on the surface of the leghemoglobins vary considerably. The .alpha.-helix content of the leghemoglobins as a class is very similar (60-65%) and approaches that of the myoglobins; the sign, magnitude and shape of their circular dichroism spectra in the near UV, Soret and visible regions suggest close similarities in the environment and orientation of a structurally important tryptophan residue and of the heme moiety; and, there is comparatively weak heme-protein interaction. The extent of immuno cross-reactivity was found to be best demonstrated using the Farr radioimmunoassay procedure. Five leghemoglobins from 1 plant (soybean) cross-reacted completely but with varying affinities. The extent of cross reactivity between leghemoglobins from different plants was compared to that within a single plant; the reaction of antiserum to a soybean leghemoglobin with a serradella leghemoglobin was weak, with a snake bean leghemoglobin still weaker (and incomplete) while lupin leghemoglobins showed no cross reactivity at all. The "rapid" attenuation of cross reactivity among different plant leghemoglobins is explicable in terms of the extensive amino acid substitutions which have been demonstrated in the literature and in the present studies. In view of this rapid divergence it is not surprising that sperm whale and horse heart myoglobins showed no cross reactivity with soybean leghemoglobins. Amino acid substitutions in the leghaemoglobin family are conformationally but not immunochemically conservative.