Rational design of isoform specific ligands
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Cyclophilins (Cyp) are proteins that catalyze the interconversion of trans/cis isomers of proline belonging to the peptidyl-prolyl isomerases family (PPIase). In addition to their PPIase activity, Cyps have diverse biological roles and have been implicated in a number of different diseases such as HIV-1 and HCV. Although several Cyp inhibitors have been reported in the literature, none are able to inhibit with high specificity various Cyp isoforms. To facilitate the development of isoform-specific Cyp ligands, we have pursued detailed studies of Cyp dynamics and ligand binding thermodynamics using molecular simulations, biophysical assays and protein X-ray crystallography. Research efforts were focussed on the identification of novel Cyp inhibitors using X-ray crystallographic studies and Surface Plasmon Resonance (SPR) experiments on fragments from an in-house bespoke library of small compounds. These biophysical studies revealed a number of fragments that are able to bind to diverse Cyp isoforms with high micromolar – low millimolar activity. To further examine the binding of these fragments to cyclophilins, identify interactions with the proteins and explain specificity trends from SPR and X-ray results, molecular dynamics (MD) simulations and free energy calculations were pursued. Models of apo and holo Cyps in complex with fragments that we had experimentally tested were set up using the Amber, AmberTools and FESetup software. Free energy calculations were performed using the thermodynamic integration (TI) technique with the Sire/OpenMM software. The results were analysed with custom scripts. Correlations between computed and measured binding energies, and calculated and observed binding modes were analysed to help develop guidelines for the development of isoform specific cyclophilin ligands. A detailed comparison of the merits and drawbacks of the experimental and computational techniques used in this work has also been made, and strategies for effective combination of the methodologies in structure-based projects are outlined.