Free-Energy Landscape Analysis of Protein-Ligand Binding: The Case of Human Glutathione Transferase A1

Abstract

Glutathione transferases (GSTs) are a superfamily of enzymes which have in common the ability to catalyze the nucleophilic addition of the thiol group of reduced glutathione (GSH) onto electrophilic and hydrophobic substrates. This conjugation reaction, which occurs spontaneously but is dramatically accelerated by the enzyme, protects cells against damages caused by harmful molecules. With some exceptions, GSTs are catalytically active as homodimers, with monomers generally constituted of 200 to 250 residues organized into two subdomains. The first is the N-terminal subdomain, which contains an active site named G site, where GSH is hosted in catalytic conformation and which is generally highly conserved among GSTs. The second subdomain, hydrophobic, which binds the substrate counterpart (H site), can vary from one GST to another, resulting in structures able to recognize different substrates. In the present work, we performed all-atom molecular dynamics simulations in explicit solvent of human GSTA1 in its APO form, bound to GSH ligand and bound to GS-conjugated ligand. From MD, two probes were analyzed to (i) decipher the local conformational changes induced by the presence of the ligand and (ii) map the communication pathways involved in the ligand-binding process. These two local probes are, first, coarse-grained angles (θ,γ), representing the local conformation of the protein main chain and, second, dihedral angles χ representing the local conformation of the amino-acid side chains. From the local probes time series, effective free-energy landscapes along the amino-acid sequence were analyzed and compared between the three different forms of GSTA1. This methodology allowed us to extract a network of 33 key residues, some of them being located in the experimentally well-known binding sites G and H of GSTA1 and others being located as far as 30Å from the original binding sites. Finally, the collective motions associated with the network of key residues were established, showing a strong dynamical coupling between residues Gly14-Arg15 and Gln54-Val55, both in the same binding site (intrasite) but also between binding sites of each monomer (intersites).