FIGURE 2.

Catalytic mechanism. A, GlpG catalyzes the hydrolysis of DCI to form an α-hydroxy acid. The complex between 7-amino-4-chloro-3-methoxyisocoumarin and GlpG is stabilized by two covalent bonds. B, the covalent adduct between DFP and GlpG mimics the tetrahedral transition state. C and D, the crystal structures of GlpG in complex with isocoumarin and DFP, respectively (Protein Data Bank codes 2XOW and 3TXT) (25, 33). E, hypothetical model of substrate (green) bound to rhomboid protease (side view; 90° from that in Fig. 1B). The protease TM helices are shown as cylinders, and the loops are omitted for clarity. The extended cleavage site and helical TM segment of the substrate are connected by a sharp turn (green dots). According to this model, Ala-253 is adjacent to the side chain of the substrate P1 residue (inset). The red arrows indicate the scissile bond.