  Receptor-ligand interactions are essential for biological function and their
binding strength is commonly explained in terms of static lock-and-key models
based on molecular complementarity. However, detailed information of the full
unbinding pathway is often lacking due, in part, to the static nature of atomic
structures and ensemble averaging inherent to bulk biophysics approaches. Here
we combine molecular dynamics and high-speed force spectroscopy on the
streptavidin-biotin complex to determine the binding strength and unbinding
pathways over the widest dynamic range. Experiment and simulation show
excellent agreement at overlapping velocities and provided evidence of the
unbinding mechanisms. During unbinding, biotin crosses multiple energy barriers
and visits various intermediate states far from the binding pocket while
streptavidin undergoes transient induced fits, all varying with loading rate.
This multistate process slows down the transition to the unbound state and
favors rebinding, thus explaining the long lifetime of the complex. We provide
an atomistic, dynamic picture of the unbinding process, replacing a simple
two-state picture with one that involves many routes to the lock and
rate-dependent induced-fit motions for intermediates, which might be relevant
for other receptor-ligand bonds.
