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Catch bond
A catch bond is a type of noncovalent bond whose dissociation lifetime increases with tensile force applied to the bond. Normally, bond lifetimes are expected to diminish with force. In the case of catch bonds, the lifetime of the bond actually increases up to a maximum before it decreases like in a normal bond. Catch bonds work in a way that is conceptually similar to that of a Chinese finger trap. While catch bonds are strengthened by an increase in force, the force increase is not necessary for the bond to work. Catch bonds were suspected for many years to play a role in the rolling of leukocytes, being strong enough to roll in presence of high forces caused by high shear stresses, while avoiding getting stuck in capillaries where the fluid flow, and therefore shear stress, is low. The existence of catch bonds was debated for many years until strong evidence of their existence was found in bacteria. Definite proof of their existence came shortly thereafter in leukocytes.
==Discovery==
Catch bonds were first proposed in 1988 in the Proceedings of the Royal Society by M. Dembo et al. while at Harvard University. The concept of catch bonds were used to explain unexpected observations in which the strength of certain bonds increased as force was applied.
No decisive evidence of catch bonds was found until 2003. This is due to experimental conditions that were unfavorable for detecting catch bonds, as well as the counterintuitive nature of the bonds themselves. For example, most early experiments were conducted in 96 well plates, an environment that does not provide any flow. Some experiments failed to produce shear stress that is now known to be critical to lengthen the lifetimes of catch bonds, while other experiments conducted under flow conditions too weak or too strong for optimal shear-induced strengthening of these bonds. Finally, Marshall and coworkers found that L-selectin:PSGL-1 bonds exhibited increasing bond lifetime as step loads were applied between 0 and ~25 pN, and fell exponentially at higher loads.〔 These data were collected using an atomic force microscope, and have subsequently been duplicated using a biomembrane force probe and in shear flow assays. L-selectin:PSGL-1 bonds display catch bond behavior at low loads, and slip bond behavior (meaning that bond lifetime decreases with increasing load) at high loads ("catch-slip" bonds). Catch-slip behavior has also been reported for dissociation of L-selectin from endoglycan, P-selectin from PSGL-1, FimH from mannose,〔 and myosin from actin. Emphasizing their importance and general acceptance, in the three years following their discovery there were at least 24 articles published on catch bonds.
Recently, Sivasankar and his research team have found that the mechanism behind the puzzling phenomenon is due to long-lived, force-induced hydrogen bonds. Using data from previous experiments, the team used molecular dynamics to discover that two rod-shaped cadherins in an X-dimer formed catch bonds when pulled and in the presence of calcium ions. The calcium ions keep the cadherins rigid, while pulling brings the proteins closer together, allowing for hydrogen bonds to form. The mechanism behind catch bonds helps to explain the biophysics behind cell-cell adhesion. According to the researchers, "Robust cadherin adhesion is essential for maintaining the integrity of tissue such as the skin, blood vessels, cartilage and muscle that are exposed to continuous mechanical assault."
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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