The aryl hydrocarbon receptor (AhR) is a transcription factor involved in detoxification of polyaromatic hydrocarbons by regulating the expression of metabolic enzymes such as cytochrome CYP450. Elevated AhR activity is linked to cancer and neurodegeneration, leading to premature ageing. AhR consists of bHLH-PASA-PASB region, where PASB is considered the ligand binding domain, and PASA domain being undruggable by small molecules. However, the molecular mechanisms of activation triggered by the ligand binding are still unclear. Due to challenges in expression of the protein, there is no available crystal structure of PASB domain yet, resulting in the PASB domain being modelled.<br/><br/>This work focuses on assessing the energetics of two distinct conformations of the human PASB domain, specifically the role of its loop1 region (residues 310-330), and how it links to the transitional energy between the conformations. This region can adopt either a partially disordered loop, or an α-helix conformation, which are both ‘druggable’, i.e. able to bind small molecule ligands. The interchangeability between the two conformations is assessed in terms of entropic and enthalpic contributions, as well as in terms of ability to bind agonists and antagonists. We have observed that the N-terminal disordered region of PASB domain regulates the molecular dynamics of loop1, and is crucial for maintaining either the α-helix or the loop conformation. Truncated N-terminal region (starting at residue 287) stabilise α-helical conformation of loop1, while the presence of a long, intrinsically unstructured N-terminal segment induces unwinding of the loop1 α-helix, shifting it into the partially disordered loop conformation. The C-terminal segment of PASB domain stabilises the domain via favourable enthalpic factors. The influence of known agonists and antagonists of AhR on the intrinsic thermodynamics has also been assessed, and their enthalpic and entropic contributions were quantified.<br/><br/>Finding the triggers for transitioning between the two distinct conformations of loop1, can be crucial for addressing the challenges in expression and crystallisation of human PASB, as well as differentiation between the binding modes of agonists and antagonists to AhR, thus enabling the rational design of drug molecules targeting AhR.

Conformational analysis and investigation of energetics of the human AhR PAS-B domain: Impact on druggability

2019 Fall National Meeting

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