Structure-based stability engineering of the mouse IgG1 Fab fragment by modifying constant domains

A semi-rational approach based on structural data was exploited in a search for C_H1 and C_L domains with improved intrinsic thermodynamic stabilities. Structural and amino acid level comparisons were carried out against known biophysically well-behaving and thermodynamically beneficial scFv and Fab fragments. A number of mutant Fab fragments were constructed by site-directed mutagenesis of regions in the C_H1 and CL domains expected to be most sensitive under physical stress conditions. These mutations were located on three sites in the Fab constant domains; a mobile loop in the C_H1 domain, residues surrounding the two largest solvated hydrophobic cavities located in the interface of the C_H1 and C_L domains and the hydrophobic core regions of both C_H1 and C_L. Expression levels of functional Fab fragments, denaturant-induced unfolding equilibria and circular dichroism spectroscopy were used to evaluate the relative stabilities of the wild-type and the mutant Fab fragments. The highest thermodynamic stability was reached through the mutation strategy, where the hydrophobicity and the packing density of the solvated hydrophobic cavity in the C_H1/C_L interface was increased by the replacement of the hydrophilic Thr178 in the C_L domain by a more hydrophobic residue, valine or isoleucine. The midpoint of the transition curve from native to unfolded states of the protein, measured by fluorescence emission, occurred at concentrations of guanidine hydrochloride of 2.4 M and 2.6 M for the wild-type Fab and the most stable mutants, respectively. Our results illustrate that point mutations targeted to the C_H1/C_L interface were advantageous for the overall thermodynamic stability of the Fab fragment.