Lin G, Bode W, Huber R, Chi C, Engh R A
Shanghai Institute of Biochemistry, People's Republic of China.
Eur J Biochem. 1993 Mar 1;212(2):549-55. doi: 10.1111/j.1432-1033.1993.tb17692.x.
The structure of the Bowman-Birk-type inhibitor from mung bean Phaseolus aureus has been determined in ternary complex with porcine trypsin. The complex formed crystals of the trigonal space group P3(1)21 which diffracted to a resolution of 250 pm. Each of the two mung bean protease reactive sites is bound to trypsin according to the standard mechanism for serine proteinase inhibition. The binding loops thereby adopt the canonical conformation for the standard mechanism; however, the sub-van der Waals contact between the active-site serine O gamma (195) and the P1 carbonyl carbon of both loops is significantly smaller (210 pm) than hitherto observed, with continuous electron density connecting the two atoms. The inhibitor is formed by two double-stranded antiparallel beta-sheets, which are connected into a moderately twisted beta-sheet by a network of hydrogen bonds involving main-chain atoms and two water molecules. All contacts with neighbors in the crystal lattice occur between trypsin molecules. This apparently gives rise to an unusual form of disorder where the complexes pack in two orientations Ta:MaMb:Tb and Tb:MbMa:Ta (Ta, Tb = trypsin, Ma = mung bean loop I, Mb = mung bean loop II), such that the asymmetric unit consists of the ternary complex in two orientations, each with half occupancy. This is nearly equivalent to an asymmetric unit which has one trypsin molecule with full occupancy and one mung bean inhibitor with half occupancy and a crystallographic twofold symmetry axis through its center. Because of the approximate twofold symmetry of the inhibitor itself, however, the electron density was interpretable for most of the inhibitor (17 residues at the termini were not resolved) and shows evidence of its double orientation.
已确定来自绿豆（Phaseolus aureus）的鲍曼-伯克型抑制剂与猪胰蛋白酶形成的三元复合物的结构。该复合物形成了三方晶系空间群P3(1)21的晶体，其衍射分辨率为250皮米。绿豆蛋白酶的两个反应位点均按照丝氨酸蛋白酶抑制的标准机制与胰蛋白酶结合。因此，结合环采用了标准机制的典型构象；然而，活性位点丝氨酸Oγ（195）与两个环的P1羰基碳之间的亚范德华接触明显更小（210皮米），小于迄今观察到的情况，且两个原子之间有连续的电子密度。该抑制剂由两个双链反平行β-折叠组成，它们通过涉及主链原子和两个水分子的氢键网络连接成一个适度扭曲的β-折叠。晶体晶格中与相邻分子的所有接触都发生在胰蛋白酶分子之间。这显然导致了一种不寻常的无序形式，其中复合物以两种取向堆积：Ta:MaMb:Tb和Tb:MbMa:Ta（Ta、Tb = 胰蛋白酶，Ma = 绿豆环I，Mb = 绿豆环II），使得不对称单元由处于两种取向的三元复合物组成，每种取向占有率为一半。这几乎等同于一个不对称单元，其中有一个占有率为100%的胰蛋白酶分子和一个占有率为50%的绿豆抑制剂，且有一条通过其中心的晶体学二重对称轴。然而，由于抑制剂本身近似二重对称，大部分抑制剂的电子密度是可解释的（末端17个残基未解析），并显示出其双重取向的证据。
