Using chemical shift perturbation to characterise ligand binding

Chemical shift perturbation (CSP, chemical shift mapping or complexation-induced changes in chemical shift, CIS) follows changes in the chemical shifts of a protein when a ligand is added, and uses these to determine the location of the binding site, the affinity of the ligand, and/or possibly the structure of the complex. A key factor in determining the appearance of spectra during a titration is the exchange rate between free and bound, or more specifically the off-rate k(off). When k(off) is greater than the chemical shift difference between free and bound, which typically equates to an affinity K-d weaker than about 3 mu M, then exchange is fast on the chemical shift timescale. Under these circumstances, the observed shift is the population-weighted average of free and bound, which allows K-d to be determined from measurement of peak positions, provided the measurements are made appropriately. H-1 shifts are influenced to a large extent by through-space interactions, whereas C-13 alpha and C-13 beta shifts are influenced more by through-bond effects. N-15 and C-13' shifts are influenced both by through-bond and by through-space (hydrogen bonding) interactions. For determining the location of a bound ligand on the basis of shift change, the most appropriate method is therefore usually to measure N-15 HSQC spectra, calculate the geometrical distance moved by the peak, weighting N-15 shifts by a factor of about 0.14 compared to H-1 shifts, and select those residues for which the weighted shift change is larger than the standard deviation of the shift for all residues. Other methods are discussed, in particular the measurement of (CH3)-C-13 signals. Slow to intermediate exchange rates lead to line broadening, and make K-d values very difficult to obtain. There is no good way to distinguish changes in chemical shift due to direct binding of the ligand from changes in chemical shift due to allosteric change. Ligand binding at multiple sites can often be characterised, by simultaneous fitting of many measured shift changes, or more simply by adding substo-ichiometric amounts of ligand. The chemical shift changes can be used as restraints for docking ligand onto protein. By use of quantitative calculations of ligand-induced chemical shift changes, it is becoming possible to determine not just the position but also the orientation of ligands. (C) 2013 Elsevier B.V. All rights reserved.