Curmi P M, Barden J A, dos Remedios C G
Eur J Biochem. 1982 Feb;122(2):239-44. doi: 10.1111/j.1432-1033.1982.tb05872.x.
The presence of bound Ln ions gives rise to changes in the ultraviolet absorbance of G-actin. Previously, Ln ions have been shown to mobilize the adenosine moiety of ATP bound to the actin monomer. The changes in ultraviolet absorbance appear to be fully attributable to this mobilization. Any structural changes induced in G-actin by the substitution of Ln ions for Ca2+ must be small enough so as not to perturb the environment of any aromatic chromophores other than those exposed by the mobility of the nucleotide. This conclusion is supported by measurements of the rates of proteolytic digestion of actin with and without bound Ln ions. The effect of Ln ions on the actin-bound nucleotide is not simply due to the increase in positive charge on the actin monomer. This is demonstrated by the absence of any ultraviolet spectral changes on the addition of a fivefold excess of Ca2+. Analysis of the ultraviolet difference spectrum obtained when Ln ions bind to G-actin indicates that at least one tryptophan residue is partially exposed to the solvent coincident with mobilization of the nucleotide. Tb3+ luminescence enhancement studies indicate that there are no Trp, Tyr or Phe residues in the vicinity of the high-affinity cation site. These results suggest that the exposed Trp may be close to the nucleotide.
结合的镧系离子的存在会导致G-肌动蛋白的紫外吸光度发生变化。此前，已表明镧系离子会使与肌动蛋白单体结合的ATP的腺苷部分发生移动。紫外吸光度的变化似乎完全归因于这种移动。用镧系离子取代Ca2+在G-肌动蛋白中引起的任何结构变化必须足够小，以免干扰除核苷酸移动所暴露的那些芳香发色团之外的任何芳香发色团的环境。这一结论得到了有或没有结合镧系离子时肌动蛋白蛋白水解消化速率测量结果的支持。镧系离子对肌动蛋白结合核苷酸的影响不仅仅是由于肌动蛋白单体上正电荷的增加。这通过加入五倍过量的Ca2+时没有任何紫外光谱变化得到证明。对镧系离子与G-肌动蛋白结合时获得的紫外差光谱的分析表明，至少一个色氨酸残基在核苷酸移动的同时部分暴露于溶剂中。Tb3+发光增强研究表明，高亲和力阳离子位点附近没有色氨酸、酪氨酸或苯丙氨酸残基。这些结果表明，暴露的色氨酸可能靠近核苷酸。
