Tara Fryatt

Binding Mode and Structure-Activity Relationships around Direct Inhibitors of the Nrf2-Keap1 Complex.

An X‐ray crystal structure of Kelch‐like ECH‐associated protein (Keap1) co‐crystallised with (1S,2R)‐2‐[(1S)‐1‐[(1,3‐dioxo‐2,3‐dihydro‐1H‐isoindol‐2‐yl)methyl]‐1,2,3,4‐tetrahydroisoquinolin‐2‐carbonyl]cyclohexane‐1‐carboxylic acid (compound (S,R,S)‐1 a) was obtained. This X‐ray crystal structure provides breakthrough experimental evidence for the true binding mode of the hit compound (S,R,S)‐1 a, as the ligand orientation was found to differ from that of the initial docking model, which was available at the start of the project. Crystallographic elucidation of this binding mode helped to focus and drive the drug design process more effectively and efficiently.

The multiple roles of computational chemistry in fragment-based drug design

Fragment-based drug discovery (FBDD) represents a change in strategy from the screening of molecules with higher molecular weights and physical properties more akin to fully drug-like compounds, to the screening of smaller, less complex molecules. This is because it has been recognised that fragment hit molecules can be efficiently grown and optimised into leads, particularly after the binding mode to the target protein has been first determined by 3D structural elucidation, e.g. by NMR or X-ray crystallography. Several studies have shown that medicinal chemistry optimisation of an already drug-like hit or lead compound can result in a final compound with too high molecular weight and lipophilicity. The evolution of a lower molecular weight fragment hit therefore represents an attractive alternative approach to optimisation as it allows better control of compound properties. Computational chemistry can play an important role both prior to a fragment screen, in producing a target focussed fragment library, and post-screening in the evolution of a drug-like molecule from a fragment hit, both with and without the available fragment-target co-complex structure. We will review many of the current developments in the area and illustrate with some recent examples from successful FBDD discovery projects that we have conducted.

Study of human Orexin-1 and -2 G-protein-coupled receptors with novel and published antagonists by modeling, molecular dynamics simulations, and site-directed mutagenesis.

The class A G-protein-coupled receptors (GPCRs) Orexin-1 (OX1) and Orexin-2 (OX2) are located predominantly in the brain and are linked to a range of different physiological functions, including the control of feeding, energy metabolism, modulation of neuro-endocrine function, and regulation of the sleep-wake cycle. Site-directed mutagenesis (SDM) and domain exchange (chimera) studies have provided important insight into key features of the OX1 and OX2 binding sites. However, the precise determinants of antagonist binding and selectivity are still not fully known. In this work, we used homology modeling of OX receptors to direct further SDM studies. These SDM studies were followed by molecular dynamics (MD) simulations to rationalize the full scope of the SDM data and to explain the role of each mutated residue in the binding and selectivity of a set of OX antagonists: Almorexant (dual OX1 and OX2 antagonist), SB-674042 (OX1 selective antagonist), EMPA (OX2 selective antagonist), and others. Our primary interest was focused on transmembrane helix 3 (TM3), which is identified as being of great importance for the selectivity of OX antagonists. These studies revealed conformational differences between the TM3 helices of OX1 and OX2, resulting from differences in amino acid sequences of the OX receptors that affect key interhelical interactions formed between TM3 and neighboring TM domains. The MD simulation protocol used here, which was followed by flexible docking studies, went beyond the use of static models and allowed for a more detailed exploration of the OX structures. In this work, we have demonstrated how even small differences in the amino acid sequences of GPCRs can lead to significant differences in structure, antagonist binding affinity, and selectivity of these receptors. The MD simulations allowed refinement of the OX receptor models to a degree that was not possible with static homology modeling alone and provided a deeper rationalization of the SDM data obtained. To validate these findings and to demonstrate that they can be usefully applied to the design of novel, very selective OX antagonists, we show here two examples of antagonists designed in house: EP-109-0092 (OX1 selective) and EP-009-0513 (OX2 selective).

Cross-Linking of Protein Crystals as an Aid in the Generation of Binary Protein-Ligand Crystal Complexes, Exemplified by the Human Pde10A-Papaverine Structure.

Protein crystallography has proven to be an effective method of obtaining high-resolution structures of protein-ligand complexes. However, in certain cases only apoprotein structures are readily available and the generation of crystal complexes is more problematic. Some crystallographic systems are not amenable to soaking of ligands owing to crystal-packing effects and many protein-ligand complexes do not crystallize under the same conditions as used for the apoprotein. Using crystals of human phosphodiesterase 10a (hPDE10a) as an example of such a challenging crystallographic system, the structure of the complex with papaverine was obtained to 2.8 A resolution using protein crystals cross-linked by glutaraldehyde prior to soaking of the ligand. Inspection of the electron-density maps suggested that the correct mode of binding was obtained in one of the two monomers in the asymmetric unit and inspection of crystal-packing contacts explained why cocrystallization experiments and soaking of crystals that were not cross-linked were unsuccessful.