Fig. 6. Substrate binding. (A) 2F_o – F_c electron density contoured at 1σ for the substrate pentapeptide is shown in green, that of the substrate recognition clamp in blue. (B) The substrate peptide (P-Ser19 to P-Cys22) forms a regular parallel β-sheet with β-strand S7 of the substrate clamp (Phe149 to Ile151) and backbone H-bonds to the protein surface on the opposite side to Asn14 (carbonyl and amide of P-Tyr20) and Asn117 (amide and carboxylate of P-Cys22). (C) Surface representation of the binding pocket for P-Tyr20 coloured according to the H-bonding properties of the contributing protein residues (grey, hydrophobic; red, H-bond donors; blue, H-bond acceptors).