Getting Smaller by Denaturation: Acid-Induced Compaction of Antibodies

Protein denaturation is a ubiquitous process that occurs
both in vitro and in vivo. While
our 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 a neutral pH. Scattering
profiles showed a compact globular shape, supported by analytical
ultracentrifugation. The acid denaturation of proteins with a decrease
in size is energetically costly, and acid-induced compaction requires
an attractive force for domain reorientation. Such intramolecular
aggregation may be widespread in immunoglobulin proteins as noncanonical
structures. Herein, we discuss the potential biological significance
of these noncanonical structures of antibodies.