Getting Smaller by Denaturation: Acid-Induced Compaction of Antibodies

Protein denaturation is a ubiquitous process that occurs both in vitro and in vivo. While the molecular understanding of the denatured structures of proteins is limited, it is commonly accepted that the loss of unique intramolecular contacts makes proteins larger. Herein, we report compaction of the immunoglobulin G1 (IgG1) protein upon acid denaturation. Small-angle X-ray scattering coupled with size-exclusion chromatography revealed that IgG1 radii of gyration at pH 2 were ~75% of those at neutral pH. Scattering profiles showed a globular shape with approximately triaxial ellipsoids, supported by analytical ultracentrifugation. A theoretical model indicated that the acid denaturation of proteins with size reduction was energetically costly, and thus acid-induced compaction required an attractive force for domain reorientation. Such intramolecular aggregation may be widespread in multidomain proteins as non-canonical structures. Herein, we discuss the potential biological significance of these non-canonical structures of immunoglobulins.