Second Contact Shell Mutation Diminishes Streptavidin-Biotin Binding Affinity Through Transmitted Effects on Equilibrium Dynamics

摘要

We report a point mutation in the second contact shell of the high-affinity streptavidin-biotin complex that appears to reduce binding affinity through transmitted effects on equilibrium dynamics. The Y54F streptavidin mutation causes a 75-fold loss of binding affinity with 73-fold faster dissociation, a large loss of binding enthalpy (ΔΔH, 3.4 kcal/mol at 37 °C) and a small gain in binding entropy (TΔΔS, 0.7 kcal/mol). The removed Y54 hydroxyl is replaced by a water molecule in the bound structure, but there are no observable changes in structure in the first contact shell and no additional changes surrounding the mutation. Molecular dynamics simulations reveal a large increase in atomic fluctuations for W79, a key biotin contact residue, compared to the wild type complex. The increased W79 fluctuations are caused by loss of water-mediated hydrogen bonds between the Y54 hydroxyl group and peptide backbone atoms in and near W79. We propose that the increased fluctuations diminish the integrity of the W79-biotin interaction and represent a loosening of the “tryptophan collar” which is critical to the slow dissociation and high affinity of streptavidin-biotin binding. These results illustrate how changes in protein dynamics distal to the ligand binding pocket can have a profound impact on ligand binding, even when equilibrium structure is unperturbed.