Biomedical Engineering Reference
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results. Moreover, they have analyzed the effects of the tip-to-substrate approach-
ing speed on the unbinding process. Notably, they found that the unbinding force
is practically unchanged while the adhesion probability increases as long as lower
speed values are applied. Such a behavior has been attributed to the occurrence of
a physiological mechanism finalized to prevent the rebinding during the rolling pro-
cess. This experiment witnesses once more how DFS could be an extremely suitable
tool for studying some aspects of biorecognition processes that cannot be investi-
gated by standard techniques.
Hanley et al. have extended the work of Fritz et al. (1998) to include the inves-
tigation of the interaction of P-selectin with its ligand, which was embedded within
the leukocyte cells, in turn immobilized on the substrate (Hanley et al., 2003). The
authors found a dissociation rate value higher than that determined for isolated pro-
teins (see Table 6.5); such an effect having been attributed to a structural modulation
of the molecules when they are inserted in the cell surfaces. They have therefore
drawn the attention to the crucial role played by the environment to establish the
effective interaction properties of biomolecules with ligands. Furthermore, P-selectin
molecules have been shown to display a different ability to bind ligands located on
carcinoma cells with respect to normal ones. In this context, they have shown that an
important contribution to the final binding affinity between the partners could arise
from the protein density at the cell surface. Interestingly, these results have disclosed
the possibility to evaluate the efficiency of therapeutic agents by monitoring the inter-
action properties of biomolecules upon changing the environmental conditions and
under the action of mechanical stress.
Marshall et al. have also investigated the complex formed between the P-selectin
and the P-selectin glycoprotein ligand (Marshall et al., 2003). They have observed a
prolonged lifetime of the complex, followed by a shortening of it, upon the applica-
tion of a mechanical force The observation of this phenomenon constitutes the first
experimental evidence for the occurrence of “catch bonds” in a biological system. As
previously mentioned, the application of a mechanical force to a system, instead of
inducing a decrease of the energy barrier between the bound and the unbound states,
sometimes could give rise to an increase of it with a concomitant rise of the bond
lifetime (Prezhdo and Pereverzev, 2009). Catch bonds are expected to likely involve
allosteric changes within the biomolecules, coupled with high-order fluctuations of
the energy barrier and could have evolved in biological systems to fulfill specific
functions. Moreover, the occurrence of catch bonds in the P-selectin-involved com-
plex has been put into relationship to the capability of this system to support the
rolling of leucocytes on a wall at low shear stresses. More generally, it has been sug-
gested that transitions between catch and slip bonds might provide a general mech-
anism for the precise regulation of cell adhesion during mechanical stress. Notably,
the discovery of catch bonds has been possible thanks to the DFS capability to follow
a single biomolecular complex undergoing a controlled force. Successively, the same
authors have focused the attention on the discrepancies among the results obtained
by DFS for complexes involving P-selectin (Marshall et al., 2005). They deduced
that the effective
k
off
value extracted from DFS experiments depends not only on the
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