Fig. 5. The C₃ domain catalytic site showing the position of PPant-Gly_stab.

a PPant-Gly_stab substrate extends fully into the active site, approaching the active site HHxxxDE motif (H2696 to E2702); electron density shown as a 2Fo-Fc map (PDB ID 7KW0). b The Gly_stab substrate is stabilized by a network of hydrophilic interactions. Note that residues M2917, S2919, Q2921, P2941, and E2950 are in a position that could potentially interact with the side chain of alternate acceptor substrates. c Mechanism of peptide bond formation via concerted N–C bond formation and N-deprotonation (upper pathway) or sequential N–C bond formation and N-deprotonation (lower pathway); donor PCP shown in green, acceptor PCP shown in cyan, peptide is shown in gray. d Zwitterionic intermediate in the sequential N–C bond formation/N-deprotonation pathway, in which the oxyanion is stabilized by two water molecules and the ammonium ion forms a hydrogen bond to histidine (see Source Data).