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

蛋白质的溶解度通常用疏水性标度来表征，疏水性标度是指分子从水溶液到非极性溶液的转移自由能，本身并不是“溶解倾向标度”。在这里，我们测量了短多氨基酸标签(客体)对宿主蛋白的溶解性的影响，该宿主蛋白是一种简化的牛胰蛋白酶抑制物(BPTI)，它们在C末端与宿主蛋白融合。我们分析了10种氨基酸，代表了所有的生物物理性质(酸性、碱性、极性和疏水性)。正如预期的那样，在pH 4.7和8.7时，带正电荷的残基显著增加了模型蛋白的溶解度，而非常疏水的PolyIle显著降低了BPTI的溶解度。此外，为了确保测量的溶解度值与上下文无关，并能够提供真正的“溶解倾向标尺”，我们确认标签保持与宿主蛋白的结构、热稳定性和生化活性无关。这些观察表明，这种方法对于测量氨基酸的溶解倾向是有价值的，这最终可能允许根据其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.