Firefly luciferase (FLuc), an ATP-dependent bioluminescent reporter enzyme, is broadly used in chemical biology and drug discovery assays. PTC124 (Ataluren; (3-[5-(2-fluorophenyl)-1,2,4-oxadiazol-3-yl]benzoic acid) discovered in an FLuc-based assay targeting nonsense codon suppression, is an unusually potent FLuc-inhibitor. Paradoxically, PTC124 and related analogs increase cellular FLuc activity levels by posttranslational stabilization. In this study, we show that FLuc inhibition and stabilization is the result of an inhibitory product formed during the FLuc-catalyzed reaction between its natural substrate, ATP, and PTC124. A 2.0 Å cocrystal structure revealed the inhibitor to be the acyl-AMP mixed-anhydride adduct PTC124-AMP, which was subsequently synthesized and shown to be a high-affinity multisubstrate adduct inhibitor (MAI;
K
D
= 120 pM) of FLuc. Biochemical assays, liquid chromatography/mass spectrometry, and near-attack conformer modeling demonstrate that formation of this novel MAI is absolutely dependent upon the precise positioning and reactivity of a key
meta
-carboxylate of PTC124 within the FLuc active site. We also demonstrate that the inhibitory activity of PTC124-AMP is relieved by free coenzyme A, a component present at high concentrations in luciferase detection reagents used for cell-based assays. This explains why PTC124 can appear to increase, instead of inhibit, FLuc activity in cell-based reporter gene assays. To our knowledge, this is an unusual example in which the “off-target” effect of a small molecule is mediated by an MAI mechanism.
萤火虫荧光酶(FLuc)是一种依赖ATP的生物发光报告酶,在化学生物学和药物发现实验中被广泛使用。PTC124(Ataluren;(3-[5-(2-氟苯基)-1,2,4-噁二唑-3-基]苯甲酸)是在基于FLuc的抑制无义密码子的荧光酶实验中发现的一种异常强效的FLuc抑制剂。矛盾的是,PTC124和相关类似物通过后转录稳定化增加了细胞内FLuc活性水平。在这项研究中,我们展示了FLuc的抑制和稳定是由FLuc催化的反应产生的一种抑制性产物,该反应发生在其天然底物ATP和PTC124之间。2.0 Å的共晶结构显示抑制剂是酰-AMP混合酸酐加合物PTC124-AMP,随后合成并表明它是FLuc的高亲和力多底物加合物抑制剂(MAI;KD = 120 pM)。生化测定、液相色谱/质谱和近攻击构象建模表明,这种新型MAI的形成绝对依赖于PTC124的关键meta-羧酸在FLuc活性位点内的精确定位和反应性。我们还证明了PTC124-AMP的抑制活性可以通过游离辅酶A来解除,后者是细胞基础实验中用于荧光酶检测试剂的高浓度成分。这解释了为什么PTC124在细胞基础报告基因实验中似乎会增加而不是抑制FLuc活性。据我们所知,这是一个罕见的例子,其中小分子的“非靶效应”是通过MAI机制介导的。