Inversion of receptor binding preferences by mutagenesis: free energy thermodynamic integration studies on sugar...

The Escherichia coli L-arabinose-binding protein (ABP) participates as a specific receptor in the transport of L-arabinose, D-fucose, and D-galactose through the periplasmic space. The wild-type protein binds L-arabinose about 40 times more strongly than D-fucose. A mutation of the protein at position 108 (Met-->Leu) causes a specificity change. The Met108Leu ABP slightly prefers binding of D-fucose over L-arabinose. Molecular dynamics (MD) and thermodynamic integration (TI) computer simulations were performed to study the mechanism of sugar discrimination and specificity change based on the known high-resolution X-ray structures. The specificity change was evaluated by calculating the difference in free energy of L-arabinose versus D-fucose bound to wild-type and Met108Leu ABP. The calculated free energy differences are consistent with the experimentally observed specificity of wild-type and Met108Leu ABP. The simulations indicate that the specificity change of Met108Leu ABP is accomplished mainly by reduced Lennard-Jones interactions of residue 108 with L-arabinose and improved interactions with D-fucose. In addition to MD/TI calculations on sugar binding, finite difference Poisson-Boltzmann calculations were performed to identify the most stable ionization state of buried ionizable residues in ABP.