Sommer H, Lu P
J Biol Chem. 1976 Jun 25;251(12):3774-9.
By comparing the fluorescence emission properties of the wild type lac repressor with two lac repressors altered at tryptophan 190 and 209, respectively, we show that tryptophan residue 209 has its environment changed, either by its own motion, or that of the surrounding amino acids when an inducer molecule is bound. Substitution of this tryptophan with other amino acids results in lac respressor molecules with reduced affinity for inducer molecules, indicating that the geometry at residue 209 affects the geometry of the inducer-binding site. From the results of potassium iodide quenching of fluorescence from the tryptophans, and from attempts to react the native lac repressor with dimethyl(2-hydroxy-5-nitrobenzyl) sulfonium bromide and 2-hydroxy-5-nitrobenzyl bromide, we propose that tryptophan residue 209 is involved in a conformational change of the protein upon binding of inducer, but does not come in direct contact with inducer.
通过比较野生型乳糖阻遏物与分别在色氨酸190和209处发生改变的两种乳糖阻遏物的荧光发射特性，我们发现当结合诱导剂分子时，色氨酸残基209的环境会因其自身运动或周围氨基酸的运动而发生变化。用其他氨基酸取代该色氨酸会导致乳糖阻遏物分子对诱导剂分子的亲和力降低，这表明残基209处的几何结构影响诱导剂结合位点的几何结构。从色氨酸荧光的碘化钾猝灭结果，以及使天然乳糖阻遏物与二甲基(2-羟基-5-硝基苄基)溴化锍和2-羟基-5-硝基苄基溴反应的尝试结果来看，我们提出色氨酸残基209在诱导剂结合时参与了蛋白质的构象变化，但不与诱导剂直接接触。
