Refined crystal structure (2.3 A) of a double-headed winged bean alpha-chymotrypsin inhibitor and location of its second reactive site

The crystal structure of a double-headed alpha-chymotrypsin inhibitor, WCI, from winged bean seeds has now been refined at 2.3 A resolution to an R-factor of 18.7% for 9,897 reflections. The crystals belong to the hexagonal space group P6(1)22 with cell parameters a = b = 61.8 A and c = 212.8 A. The final model has a good stereochemistry and a root mean square deviation of 0.011 A and 1.14 degrees from ideality for bond length and bond angles, respectively. A total of 109 ordered solvent molecules were localized in the structure. This improved structure at 2.3 A led to an understanding of the mechanism of inhibition of the protein against alpha-chymotrypsin. An analysis of this higher resolution structure also helped us to predict the location of the second reactive site of the protein, about which no previous biochemical information was available. The inhibitor structure is spherical and has twelve anti-parallel beta-strands with connecting loops arranged in a characteristic beta-trefoil fold common to other homologous serine protease inhibitors in the Kunitz (STI) family as well as to some non homologous functionally unrelated proteins. A wide variation in the surface loop regions is seen in the latter ones.

Structure of a Kunitz-Type Chymotrypsin from Winged Bean Seeds at 2.95 Å Resolution

Thc crystal structure of an alpha-chymotrypsin inhibitor (P6(1)22; a = 61.4, c = 210.9 A) isolated from winged bean (Psophocarpus. tetragonolobus) seeds has been determined at 2.95 A resolution by the molecular-replacement method using the 2.6 A coordinates of Erythrina trypsin inhibitor (ETI) as the starting model (57% sequence homology). This protease inhibitor, WCI, belongs to the Kunitz (STI) family and is a single polypeptide chain with 183 amino-acid residues having a molecular weight of 20 244 Da. Structure refinement with RESTRAIN and X-PLOR has led to a crystallographic R factor of 19.1% for 3469 observed reflections (I > 2sigma) in the resolution range 8-2.95 A. A total of 56 water molecules have been incorporated in the refined model containing 181 amino-acid residues. In the refined structure the deviations of bond lengths and bond angles from ideal values are 0.015 A and 2.2 degrees, respectively. The inhibitor molecule is spherical and consists of 12 antiparallel beta-strands with connecting loops arranged in a characteristic folding (a six-stranded beta-barrel and a six-stranded lid on one hollow end of the barrel) common to other homologous serine protease inhibitors in the Kunitz (STI) family as well as to some non-homologous proteins like interleukin-lalpha and interleukin-lbeta. In the structure the conformation of the protruding reactive-site loop is stabilized through hydrogen bonds mainly formed by the side chain of Asnl4, which intrudes inside the cavity of the reactive-site loop, with the side-chain and main-chain atoms of some residues in the loop region. A pseudo threefold axis exists parallel to the barrel axis of the structure. Each of the three subdomains comprises of four beta-strands with connecting loops.

Crystallization and preliminary X-ray studies of psophocarpin B1, a chymotrypsin inhibitor from winged bean seeds

Psophocarpin B1 is a 20,000 Mr protein of winged bean (Psophocarpus tetragonolobus) seeds having chymotrypsin inhibitory activity. Single crystals of this protein suitable for X-ray crystallographic studies have been obtained by the vapour diffusion method using ammonium sulphate. The crystals are hexagonal, space group P6(4)22 or P6(2)22, cell dimensions a = b = 61 A, c = 210 A. They are stable to irradiation with X-rays and diffract to at least 2.6 A resolution.