## 1 Introduction
GST monomers are, in general, made of 200 to 250 residues with a molecular weight generally comprising between 25 and 30 kDa [18; 19] and are organized into two subdomains (Figure 1A): the typically 80-residue-long N-terminal subdomain (I) has the typical fold of thioredoxin. It contains a first active site where GSH is hosted in catalytic conformation, named G site (Figure 1B). The thioredoxin fold is composed of a characteristic sequence of \(\alpha\)-helices and \(\beta\)-strands encountered in the thioredoxin protein family, i.e., \(\beta_{1}-\alpha_{1}-\beta_{2}-\alpha_{2}-\beta_{3}-\beta_{4}-\alpha_{3}\), which is characteristic of enzymes dealing with glutathione, such as glutaredoxin or glutathione peroxidase [20]. In addition, it has been shown that the region \(\beta_{3}-\beta_{4}-\alpha_{3}\) is well conserved among GSTs and enables GSH recognition by the enzyme [7]. Residues forming the G site are generally conserved among GSTs [19]. A noticeable exception is the residue of \(\alpha_{1}\)-helix which interacts with the sulfur atom of GSH and which can be a cysteine, a serine, or a tyrosin (as is the case for human GSTA1). Depending on the nature of this residue, GSTs develop slightly different catalytic functions and target a different range of substrates [21]. The second subdomain (II) is all-helical and contains the H site which binds the substrate. The number of helices varies between four and seven among GSTs. Subdomain (II) is hydrophobic, and therefore attractive for hydrophobic molecules. Together with the G site, the combined architecture of GST monomers is adapted to bind GSH to hydrophobic substrates [22]. Contrary to the G site,the residues of the H site strongly vary from one GST to the others, resulting in H sites of different natures and the ability to recognize different substrates [23]. Hereafter, we focus our interest on the structure of human GSTA1 (hGSTA1), which is a homodimer with each monomer made of 222 residues [24]. Among the 29 experimental  
Figure 1: (**A**) Cartoon representation of human GSTA1 monomer structures. Left panel: subdomains (I) and (II) are indicated in magenta and red, respectively. Right panel: the color code is the following: N-term (blue) to C-term (red). Secondary structures labels are also indicated. (**B**) Ligand binding G (top panel) and H (bottom panel) sites of human GSTA1. Ligands are shown in green spheres, and residues belonging to the binding sites are shown in yellow and purple sticks, respectively. The color code is the following: hGSTA1 monomer A in red, hGSTA1 monomer B in blue. (**C**) Catalytic cycle of GSH conjugation to electrophilic substrate. Three forms of hGSTA1 during the conjugation reaction cycle are highlighted: APO (no-compound-bound), GSH (glutathione-bound), and GS-R (glutathione-S-conjugated substrate). The R form (substrate-bound) is not considered in the present work. The color code is the same as in panel (**B**). structures of hGSTA1 available in the Protein Data Bank, 21 of them contain at least one ligand, which is GSH, substrates (R), or GS-conjugates (GS-R). For each structure, a sequence analysis is provided, featuring the residues of hGSTA1 which are in contact with the ligand. Collecting these data (Figure S1), we determined the list of residues bound to GSH and therefore involved in the G site of hGSTA1: Tyr9, Arg15, Arg45, Gln54, Val55, Pro56, Gln67, Thr68, Asp101, Arg131, and Phe220 (Asp101 and Arg131 belong to the opposite monomer; see Figure 1B). We performed the same analysis to identify residues bound to GS-conjugates and therefore involved in the H site of hGSTA1: Phe10, Gly14, Ser18, Arg69, Leu72, Ile96, Glu97, Ala100, Ile106, Leu107, Leu108, Vall111, His159, Met208, Leu213, and Phe222.