Figure 2.

(A) Structure of peptide substrate IT1-SNAC. (B) The intermediate in the reaction of IT1-SNAC with the active-site serine of TycC TE modeled in the proposed cyclizing conformation of the natural substrate tyrocidine A. The substrate meets the criteria of the minimal substrate model and introduces numerous amino acid side-chain and stereochemical alterations relative to the natural substrate. (C) The proposed structure in solution of cyclo-IT1, the product of enzymatic cyclization of IT1-SNAC. The β type II′ turn-inducing d-Ala-N^α-Me-Arg sequence could position Arg at the i+2 position of the turn.