Development of a novel amino acid solubility propensity scale for the calculation of polypeptide solubility

蛋白质的溶解度通常以疏水性尺度来表征，疏水性尺度指的是分子从水到非极性溶液的自由转移能，而不是“溶解度倾向尺度”本身。在这里，我们测量了短聚氨基酸标签（guest）对宿主蛋白（一种简化的牛胰蛋白酶抑制剂（BPTI））溶解度的影响，它们在c端融合在一起。我们分析了10种氨基酸类型，代表了生物物理性质的全部范围（酸性、碱性、极性和疏水性）。正如预期的那样，带正电的残基显著增加了模型蛋白在pH值为4时的溶解度。7和8。而极疏水性聚ile则显著降低了BPTI的溶解度。此外，为了确保测量的溶解度值与环境无关，并能提供真正的“溶解度倾向量表”，我们证实了这些标签与宿主蛋白的结构、热稳定性和生化活性无关。这些观察结果表明，这种方法对于测量氨基酸的溶解度倾向是有价值的，它最终可以通过氨基酸序列计算多肽的相对溶解度。

Protein solubility is usually characterized in terms of a hydrophobicity scale, which refers to the free energy of transfer of a molecule from an aqueous to a nonpolar solution and is not a "solubility propensity scale" per se. Here, we measured the effects of short poly-amino-acid tags(guests) on the solubility of a host protein, a simplified bovine pancreatic trypsin inhibitor(BPTI), to which they were fused at the C-terminus. We analyzed 10 amino acid types, representing the full range of biophysical properties(acidic, basic, polar, and hydrophobic). As anticipated, positively charged residues significantly increased the solubility of the model protein, at both pH 4. 7 and 8. 7, whereas very hydrophobic poly-Ile markedly reduced the solubility of BPTI. Furthermore, to ensure that the measured solubility values were context independent and could provide a genuine "solubility propensity scale", we confirmed that the tags remained independent of the structure, thermal stability, and biochemical activity of the host protein. These observations suggest that this approach is valuable for measuring the solubility propensities of amino acids, which could eventually allow the calculation of a polypeptide's relative solubility from its amino acid sequence.