Zbyryt T, Otlewski J
Institute of Biochemistry, University of Wroclaw, Poland.
Biol Chem Hoppe Seyler. 1991 Apr;372(4):255-62. doi: 10.1515/bchm3.1991.372.1.255.
Squash seeds proteinase inhibitors form stoichiometric complexes with bovine trypsinogen. In terms of association constants (Ka), the interaction is weak. The inhibitors bind to the zymogen with Ka values of approx. 10(4)M-1 i.e. 2 X 10(7) times weaker than to bovine beta-trypsin. Squash inhibitor with Lys at the P1 position binds to trypsinogen with a Ka value 2.1-fold higher than the inhibitor with Arg at P1. The Ile-Val binding cleft and the Ca2+ binding site of trypsinogen are cooperatively linked to the inhibitor binding site. Although these three sites are spatially separated, either binding of calcium ion or Ile-Val dipeptide to trypsinogen increase the Ka values 3-fold and more than 100-fold, respectively. In the presence of Ile-Val trypsinogen resynthetizes extremely slowly (about 10(4) times slower than beta-trypsin) the reactive site peptide bond in squash inhibitors.
南瓜籽蛋白酶抑制剂与牛胰蛋白酶原形成化学计量复合物。就缔合常数（Ka）而言，这种相互作用较弱。抑制剂与酶原结合的Ka值约为10⁴M⁻¹，即比与牛β-胰蛋白酶结合弱2×10⁷倍。P1位为赖氨酸的南瓜抑制剂与胰蛋白酶原结合的Ka值比P1位为精氨酸的抑制剂高2.1倍。胰蛋白酶原的异亮氨酸-缬氨酸结合裂隙和钙离子结合位点与抑制剂结合位点协同相连。尽管这三个位点在空间上是分开的，但钙离子或异亮氨酸-缬氨酸二肽与胰蛋白酶原的结合分别使Ka值增加3倍和100倍以上。在异亮氨酸-缬氨酸存在的情况下，胰蛋白酶原重新合成南瓜抑制剂中反应位点肽键的速度极慢（比β-胰蛋白酶慢约10⁴倍）。
