Binding Affinities of Factor Xa Inhibitors Estimated by Thermodynamic Integration and MM/GBSA.

We present free energy estimates of nine 3-amidinobenzyl-1H-indole-2-carboxamide inhibitors of factor Xa. Using alchemical thermodynamic integration (TI) calculations, we estimate the difference in binding free energies with high accuracy and precision, except for mutations involving one of the amidinobenzyl rings. Crystal studies show that the inhibitors may bind in two distinct conformations, and using TI, we show that the two conformations give a similar binding affinity. Furthermore, we show that we can reduce the computational demand, while still retaining a high accuracy and precision, by using fewer integration points and shorter protein-ligand simulations. Finally, we have compared the TI results to those obtained with the simpler MM/GBSA method (molecular-mechanics with generalized Born surface-area solvation). MM/GBSA gives better results for the mutations that involve a change of net charge, but if a precision similar to that of the TI method is required, the MM/GBSA method is actually slightly more expensive. Thus, we have shown that TI could be a valuable tool in drug design

The Normal-Mode Entropy in the MM/GBSA Method: Effect of System Truncation, Buffer Region, and Dielectric Constant

We have performed a systematic study of the entropy term in the MM/GBSA (molecular Mechanics combined with generalized Born and surface area solvation) approach to calculate ligand-binding affinities The entropies are calculated by a normal mode analysis of harmonic frequencies from minimized snapshots of molecular dynamics simulations. For computational reasons, these calculations have normally been performed on truncated systems. We have studied the binding of eight inhibitors of blood clotting factor Xa, nine ligands of ferritin, and two ligands of HIV-1 protease and show that removing protein residues with. distances. larger than 8-16 angstrom to the ligand, including a 4 angstrom shell of fixed protein residues and water molecules, change the absolute entropies by 1-5 kJ/mol on average. However, the change is systematic, so relative entropies for different ligands change by only 0.7-1.6 kJ/mol on average. Consequently, entropies from truncated systems give relative binding affinities that are identical to those obtained for the Whole protein within statistical uncertainty (172 kJ/mol). We have also tested to use a distance dependent dielectric constant in the minimization and. frequency calculation (epsilon = 4r), but it typically gives slightly different entropies and poorer binding, affinities. Therefore, we recommend entropies calculated with the smallest truncation radius (8 angstrom) and epsilon =1 Such an approach also gives an improved precision for the calculated binding free energies

Tautomeric equilibria of nucleobases in the hachimoji expanded genetic alphabet

Evolution has yielded biopolymers that are constructed from exactly four building blocks and are able to support Darwinian evolution. Synthetic biology aims to extend this alphabet, and we recently showed that 8-letter (hachimoji) DNA can support rule-based information encoding. One source of replicative error in non-natural DNA-like systems, however, is the occurrence of alternative tautomeric forms, which pair differently. Unfortunately, little is known about how structural modifications impact free-energy differences between tautomers of the non-natural nucleo¬bases used in the hachimoji expanded genetic alphabet. Determining experimental tautomer ratios is technically difficult and so strategies for improving hachimoji DNA replication efficiency will benefit from accurate computational predictions of equilibrium tautomeric ratios. We now report that high-level quantum-chemical calculations in aqueous solution by the embedded cluster reference interaction site model (EC-RISM), benchmarked against free energy molecular simulations for solvation thermodynamics, provide useful quantitative information on the tautomer ratios of both Watson-Crick and hachimoji nucleobases. In agreement with previous computational studies, all four Watson-Crick nucleobases adopt essentially only one tautomer in water. This is not the case, however, for non-natural nucleobases and their analogs. For example, although the enols of isoguanine and a series of related purines are not populated in water, these heterocycles possess N1-H and N3-H keto tautomers that are similar in energy thereby adversely impacting accurate nucleobase pairing. These robust computational strategies offer a firm basis for improving experimental measurements of tautomeric ratios, which are currently limited to studying molecules that exist only as two tautomers in solution

Computational and Experimental Characterization of Proteins With Respect to Protein-Solvent Interactions

The complexity of biopharmaceuticals requires intensive protein characterization and a thorough study of production processes to ensure process efficiency, product quality, and most notably patient safety. The present work focuses on the investigation of the interactions between proteins and aqueous solvents. Therefore appropriate methods for assessing protein characteristics were developed and protein phase behavior and protein partitioning in aqueous two phase systems were investigated

Rapid, Accurate, Precise and Reproducible Binding Affinity Calculations using Ensembles of Molecular Dynamics Simulations

The accurate prediction of the binding affinities of ligands to proteins is a major goal in drug discovery and personalised medicine. The use of in silico methods to predict binding affinities has been largely confined to academic research until recently, primarily due to the lack of their reproducibility, as well as unaffordably longer time to solution. In this thesis, I mainly describe the ensemble based molecular dynamics approaches, ESMACS and TIES, that provide a route to reliable predictions of free energies meeting the requirements of speed, accuracy, precision and reliability. The performance of both these methods when applied to a diverse set of protein targets and ligands is reported. The results are in very good agreement with experimental data while the methods are repeatable by construction. Statistical uncertainties of the order of 0.5 kcal/mol or less are achieved. These methods have been further extended to incorporate enhanced sampling techniques based on replica exchange (also known as parallel tempering) to handle situations where conformational sampling is difficult using standard molecular dynamics. A critical assessment of free energy estimators like MBAR has been made for their application in binding affinity prediction. The methodologies described are shown to have a positive impact in the drug design process in the pharmaceutical domain as well as in personalised medicine, with concomitant potential major industrial and societal impact. Finally, our automated workflow, comprising the Binding Affinity Calculator (BAC) together with the FabSim are described. These tools and services help us complete the entire execution in 8 hours or less, depending on the high performance architecture and hardware available