Influence of the Internal Disulfide Bridge on the Folding Pathway of the C_L Antibody Domain

Disulfide bridges are one of the most important factors stabilizing the native structure of a protein. Whereas the basis for their stabilizing effect is well understood, their role in a protein folding reaction still seems to require further attention. We used the constant domain of the antibody light chain (C_L), a representative of the ubiquitous immunoglobulin (Ig)-superfamily, to delineate the kinetic role of its single buried disulfide bridge. Independent of its redox state, the monomeric C_L domain adopts a typical Ig-fold under native conditions and does not retain significant structural elements when unfolded. Interestingly, its folding pathway is strongly influenced by the disulfide bridge. The more stable oxidized protein folds via a highly structured on-pathway intermediate, whereas the destabilized reduced protein populates a misfolded off-pathway species on its way to the native state. In both cases, the formation of the intermediate species is shown to be independent of the isomerization state of the Tyr₁₄₁-Pro₁₄₂ bond. Our results demonstrate that the internal disulfide bridge in an antibody domain restricts the folding pathway by bringing residues of the folding nucleus into proximity thus facilitating the way to the native state.