Structure-based Stability Engineering of the Mouse IgG1 Fab Fragment by Modifying Constant Domains

Abstract

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 C L 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.