Insulin Dissociates by Diverse Mechanisms of Coupled Unfolding and Unbinding

The
protein hormone insulin exists in various oligomeric forms,
and a key step in binding its cellular receptor is dissociation of
the dimer. This dissociation process and its corresponding association
process have come to serve as paradigms of coupled (un)­folding and
(un)­binding more generally. Despite its fundamental and practical
importance, the mechanism of insulin dimer dissociation remains poorly
understood. Here, we use molecular dynamics simulations, leveraging
recent developments in umbrella sampling, to characterize the energetic
and structural features of dissociation in unprecedented detail. We
find that the dissociation is inherently multipathway with limiting
behaviors corresponding to conformational selection and induced fit,
the two prototypical mechanisms of coupled folding and binding. Along
one limiting path, the dissociation leads to detachment of the C-terminal
segment of the insulin B chain from the protein core, a feature believed
to be essential for receptor binding. We simulate IR spectroscopy
experiments to aid in interpreting current experiments and identify
sites where isotopic labeling can be most effective for distinguishing
the contributions of the limiting mechanisms.