Folding and oxidation of the antibody domain C_H3

The non-covalent homodimer formed by the C-terminal domains of the IgG1 heavy chains (C_H3) is the simplest naturally occurring model system for studying immunoglobulin folding and assembly. In the native state the intrachain disulfide bridge which connects a three-stranded and a four-stranded β-sheet is buried in the hydrophobic core of the protein. Here we show that the disulfide bridge is not required for folding and association since the reduced C_H3 domain folds to a dimer with defined secondary and tertiary structure. However the thermodynamic stability of the reduced C_H3 dimer is much lower than that of the oxidized state. This allows the formation of disulfide bonds either concomitant with folding (starting from the reduced denatured state) or after folding (starting from the reduced dimer). The analysis of the two processes revealed that under all conditions investigated one of the cysteine residues Cys 86 reacts preferentially with oxidized glutathione to a mixed disulfide that subsequently interacts with the less-reactive second thiol group of the intra-molecular disulfide bond. For folded C_H3 the second step in the oxidation process is slow. In contrast starting from the unfolded and reduced protein the oxidation reaction is faster. However the overall folding reaction of C_H3 during oxidative folding is a slow process. Especially dimerization is slow compared to the association starting from the denatured oxidized state. This deceleration may be due to mis-folded conformations trapped by the disulfide bridge.