Fig. 6. Active site of the TGEV Mpro. (A) Difference electron density (|Fo| – |Fc| at 3.0σ above the mean; red) for the oxidized active site Cys144, indicating three oxygen atoms bound to the sulfur. (B) The catalytic Cys144 and His41 residues are shown. The region forming the oxyanion hole (main chain amides of Gly142, Thr143 and Cys144) is highlighted in pink. The water molecule, which occupies a position equivalent to that of the catalytic aspartate of serine proteinases, is shown together with its hydrogen-bonding partners, His41, His163 and Asp186. (C) Superposition of the active site residues of chymotrypsin (shown in red) with the spatially equivalent residues of TGEV Mpro (blue) and HAV 3Cpro (green). The equivalent to the third catalytic residue (Asp102) of chymotrypsin is Asp84 in HAV 3Cpro (side chain oriented differently) and Val84 in TGEV Mpro.