Biomedical Engineering Reference
In-Depth Information
Receptor: ICAM-1
AFM cantilever
Ligand: LFA-1
Jurkat
cell
Ligand
Receptor
Substrate
FIGURE 6.6
Sketch of the experimental setup used to investigate by DFS the interaction
between the receptor, ICAM-1, and the ligand, LFA-1, located on the surface of Jurkat cells.
(Adapted from Wojcikiewicz, E. P. et al. 2006.
Biomacromolecules
, 7:3188-3195.).
function of the logarithm loading rate, indicating that the unbinding process involves
a double energy barrier. In particular, they have hypothesized the presence of an
inner barrier associated with a fast process and an outer barrier responsible for the
slower process (see Table 6.2). These DFS data, analyzed also in connection with
the SPR data available in literature on the same systems, pointed out that the two
complexes are characterized by similar energy heights but different energy barrier
widths. Since ICAM-1 and ICAM-2 have a very similar structure, these differences
have been traced back to small changes in the structural organization of the bind-
ing sites. The authors have also shown that the addition of Mg
2
+
to the solution can
induce a strengthening of the interaction between the biomolecular partners, likely
due to some changes in the binding site environment, arising from a modulation
of the energy landscape. This work constitutes a remarkable example of how DFS
experiments, carried out in single-molecule regime and in near-native environment,
could be able to put into evidence even small differences in the binding properties of
biological complexes; these differences being usually hidden in bulk measurements.
Morfill et al. have explored the possibility to use DFS to discriminate among vari-
ants of recombinant antibodies through the analysis of their binding properties with
suitable ligands (Morfill et al., 2007). Indeed, selection of antibodies with high affin-
ity for specific ligands is required in many applications. The interaction of the GCN4
peptide with three fragments, obtained from recombinant antibodies at different steps
of the affinity maturation process, has been studied to determine their affinity. For all
the three complexes, the unbinding force plotted as a function of the logarithm of the
loading rate showed a linear trend with the same slope and slightly different inter-
cept (see Table 6.2). Accordingly, they have hypothesized the existence of a single
energy barrier whose height varies with the maturation degree of the antibody; such
an effect being related to a modification in the antibody binding pocket. Interestingly,
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