Müller A, Saenger W
Institut für Kristallographie, Freie Universität Berlin, Federal Republic of Germany.
J Biol Chem. 1993 Dec 15;268(35):26150-4.
In proteinase K, Cys73 is located "below" the imidazole of the active site His69. In a 2.4-A resolution x-ray crystal structure of the complex formed between the enzyme and HgAc2, two Hg(II) positions are found: a fully occupied site, covalently bound to Cys73 (S gamma), which disrupts the catalytic triad (Asp39-His69-Ser224), and a 2-fold disordered (25 and 35% occupancy), noncovalent complexation to His72, Cys73, and Thr76 of lower affinity. The enzyme is inhibited noncompetitively at low concentrations and competitively above stoichiometric concentrations of Hg(II), but it retains 7% residual activity. This can be rationalized if the molecule is flexible enough to permit transient formation of the catalytic triad. Except for the active site, only minor structural changes are observed upon binding of Hg(II), but the thermal stability is reduced by 4 degrees C.
在蛋白酶K中，半胱氨酸73位于活性位点组氨酸69的咪唑“下方”。在该酶与HgAc2形成的复合物的2.4埃分辨率X射线晶体结构中，发现了两个汞（II）位置：一个完全占据的位点，与半胱氨酸73（Sγ）共价结合，这破坏了催化三联体（天冬氨酸39-组氨酸69-丝氨酸224）；另一个是与组氨酸72、半胱氨酸73和苏氨酸76形成的2倍无序（占有率为25%和35%）、亲和力较低的非共价络合物。该酶在低浓度下受到非竞争性抑制，在汞（II）化学计量浓度以上受到竞争性抑制，但仍保留7%的残余活性。如果分子足够灵活以允许催化三联体的瞬时形成，这是可以解释的。除活性位点外，汞（II）结合后仅观察到微小的结构变化，但热稳定性降低了4摄氏度。
