The shapes of biantennary and tri/tetraantennary alpha 1 acid glycoprotein by small-angle neutron and X-ray scattering.

Two forms of alpha 1 acid glycoprotein (orosomucoid) have been studied using small-angle neutron and X-ray scattering techniques; in one form all the five glycan chains were biantennary, while in the other they were either triantennary or tetraantennary. The radius of gyration RG was found to be sensitive to salt for the biantennary form, but to be unchanged up to an ionic strength of 3 M for the triantennary and tetraantennary forms. Conformational heterogeneity is thus associated with carbohydrate heterogeneity. Hydrodynamic frictional coefficients <f> confirm these findings. Simple models of alpha 1 acid glycoprotein were developed to account for the RG and <f> values. These show that the compact conformation is slightly more elongated than a globular protein and that the expanded biantennary conformation has a most extended carbohydrate structure. Up to half of the surface of the compact shape can be covered by carbohydrate residues.