Crystal Structure of Bovine Trypsin and Wheat Germ Trypsin Inhibitor (I-2b) Complex (2:1) at 2.3 A Resolution1

The Bowman-Birk trypsin inhibitor (BBI) from wheat germ (I-2b) consists of 123 ami no acid residues with two inhibitory loops. The crystal structure of a bovine trypsin-wheat germ trypsin inhibitor (I-2b) complex (2:1) has been determined at 2.3 A resolution to a final iJ-factor of 0.177. A distance of 37.2 A between the contiguous contact loops allows them to bind and inhibit two trypsin molecules simultaneously and independently. Each domain shares the same overall fold with 8 kDa BBIs. The five disulfide bridges in each domain are a subset of seven disulfide bridges in the 8 kDa BBIs. I-2b consists of ten P-strands and the loops connecting these strands but it hicks a-helices. The conformations of the contiguous contact loops of I-2b are in a heart-like structure. The reactive sites in both domains, Arg 17 and Lys 76, are located on the loop connecting anti-parallel P-strands, [i 1/(32 and pfi/p7. Strands pi and P6 are in direct contact with trypsin molecules and form stable triple stranded P-sheet structures via hydrogen bonds. Key words: Bowman-Birk type inhibitor, contiguous contact loop, double-headed inhibi-tor, trypsin inhibitor, wheat germ. Plant seeds contain a large number of different types o