  Lifetimes of bound states of protein complexes or biomolecule folded states
typically decrease when subject to mechanical force. However, a plethora of
biological systems exhibit the counter-intuitive phenomenon of catch bonding,
where non-covalent bonds become stronger under externally applied forces. The
quest to understand the origin of catch-bond behavior has lead to the
development of phenomenological and microscopic theories that can
quantitatively recapitulate experimental data. Here, we assess the successes
and limitations of such theories in explaining experimental data. The most
widely applied approach is a phenomenological two-state model, which fits all
of the available data on a variety of complexes: actomyosin,
kinetochore-microtubule, selectin-ligand, and cadherin-catenin binding to
filamentous actin. With a primary focus on the selectin family of cell-adhesion
complexes, we discuss the positives and negatives of phenomenological models
and the importance of evaluating the physical relevance of fitting parameters.
We describe a microscopic theory for selectins, which provides a structural
basis for catch bonds and predicts a crucial allosteric role for residues
Asn82--Glu88. We emphasize the need for new theories and simulations that can
mimic experimental conditions, given the complex response of cell adhesion
complexes to force and their potential role in a variety of biological
contexts.
