Figure 2.

Sulfate binding in the sTrpRS complexes. (A) Structural comparison of the bound ligands among the sTrpRS complexes and the hTrpRS–TrpNH₂O–ATP complex. There is a sulfate ion bound at each active site of the three sTrpRS complexes. Analyses of the sTrpRS complexes and the hTrpRS–TrpNH₂O–ATP complex show that during the amino activation reaction the Trp moieties remain at similar positions, while the α-phosphate appears to approach Trp from a distant position observed in the hTrpRS–TrpNH₂O–ATP complex (orange) to an intermediate position similar to that of the sulfate ion in monomer A of the sTrpRS–TrpNH₂O complex (magenta), and to a final position in the sTrpRS–TrpAMP complex (cyan) when the reaction is completed. The sulfate ions in the sTrpRS–Trp (green) and sTrpRS–TrpAMP (cyan) complexes occupy a position equivalent to that of the β-phosphate of ATP in the hTrpRS–TrpNH₂O–ATP complex. For clarity, all the ligands are colored in gray except the α-phosphate of ATP and the sulfate ions. (B–E) Stereoviews showing the interactions of the sulfate ions with the surrounding residues at the active site in (B) the sTrpRS-Trp complex, (C) the sTrpRS–TrpAMP complex, and (D) monomer A and (E) monomer B of the sTrpRS–TrpNH₂O complex, respectively.