MS09-P07 Crystal structures of Bowman-Birk Inhibitor in complex with α-chymotrypsin Eva Johansson (Department of Protein and Peptide Structure, Novo Nordisk A/S, Måløv, Denmark) Christian Wenzel Tornøe (Department of Protein and Peptide Chemistry 1, Novo Nordisk A/S, Måløv, Denmark)email: evjh@novonordisk.comBowman-Birk Inhibitor (BBI) protein from soy bean is a serine protease inhibitor of 71 amino acid residues containing seven disulphide bonds. It has two distinct 9 amino acid loops which inhibit α‑chymotrypsin and trypsin, respectively. Chemical protein synthesis, using a divergent strategy, was used to prepare analogues of BBI to improve α-chymotrypsin inhibition. Four BBI analogues were prepared, and a four-fold improvement in chymotrypsin inhibition was obtained. Crystal structures of co-crystallised α-chymotrypsin:BBI complexes were determined for both wtBBI from soy bean and synthetic 27L,42T,43F,45I,47P-BBI variant. The crystal structures confirmed the correct protein fold of the synthetic BBI and showed a similar overall structure to the wtBBI.
The improved inhibition of α-chymotrypsin by the modified BBI may be explained comparing the two complex structures. The entire Phe43 amino acid residue is clearly pulled further into the chymotrypsin P1 pocket. This also results in withdrawal of the Thr42 backbone carbonyl group preventing a hydrogen bond formation across the inhibitory loop present in the α-chymotrypsin:wtBBI structure. However, the A42T modification provides the possibility of an alternative hydrogen bond formation utilising the threonine residue side chain hydroxyl group instead of the backbone carbonyl. Furthermore, the introduction of the Pro47 residue makes the structure more rigid and facilitates the hydrogen bond formation mentioned above.

Tornøe, C., Johansson, E. & Wahlund, P.-O. (2017) Synlett, 28, 1901-1906
Keywords: Bowman-Birk inhibitor, chymotrypsin, chemical protein synthesis