Polyvalent Glycan Quantum Dots as a Multifunctional Tool for Revealing Thermodynamic, Kinetic, and Structural Details...

Multivalent lectin-glycan interactions (MLGIs) are widespread and vital for biology. Their binding biophysical and structural details are thus highly valuable, not only for the understanding of binding affinity and specificity mechanisms but also for guiding the design of multivalent therapeutics against specific MLGIs. However, effective techniques that can reveal all such details remain unavailable. We have recently developed polyvalent glycan quantum dots (glycan-QDs) as a new probe for MLGIs. Using a pair of closely related tetrameric viral-binding lectins, DC-SIGN and DC-SIGNR, as model examples, we have revealed and quantified their large affinity differences in glycan-QD binding are due to distinct binding modes: with simultaneous binding for DC-SIGN and cross-linking for DC-SIGNR. Herein, we further extend the capacity of the glycan-QD probes by investigating the correlation between binding mode and binding thermodynamics and kinetics and further probing a structural basis of their binding nature. We reveal that while both lectins' binding with glycan-QDs is enthalpy driven with similar binding enthalpy changes, DC-SIGN pays a lower binding entropy penalty, resulting in a higher affinity than DC-SIGNR. We then show that DC-SIGN binding gives a single second-order k_on rate, whereas DC-SIGNR gives a rapid initial binding followed by a much slower secondary interaction. We further identify a structural element in DC-SIGN, absent in DC-SIGNR, that plays an important role in maintaining DC-SIGN's MLGI character. Its removal switches the binding from being enthalpically to entropically driven and gives mixed binding modes containing both simultaneous and cross-linking binding behavior, without markedly affecting the overall binding affinity and kinetics.