Abstract
The 15-kDa trypsin inhibitors from rice bran (RBTI) are members of the Bowman-Birk protease inhibitor (BBI) family. The crystal structure of a 1:1 complex between RBTI and bovine pancreatic trypsin (BPT) was determined by combination of molecular replacement and electron density modification methods. This complex model has been refined to a crystallographic R-factor of 26.2% and free R-factor of 31.6% at 3.0 Å resolution. The RBTI structure consists of seven beta-strands and loops without alpha-helices structure and folds into two compact domains (N- and C-domain) which are similar to each domain from barley BBI. However, orientation between two domains is quite different to barley BBI, this makes the distance between two P1 residues (17Lys and 83Lys) in RBTI is only 23 Å apart rather than distance of 40 Å in barley BBI. The closer distance provides evidence to support result from activity assay that two domains show different abilities to inhibit trypsin. RBTI C-domain with protruding 84Met at P1’ position, moreover, mainly leads into breakdown of the classically canonical conformation of reactive site loop. Major interaction with BPT is achieved by RBTI N-domain but C-domain plays an auxiliary role to block trypsin molecule.