Arunkumar Radhika, Drummond Calum J, Greaves Tamar L
School of Science, College of Science, Engineering and Health, RMIT University, Melbourne, VIC, Australia.
Front Chem. 2019 Feb 13;7:74. doi: 10.3389/fchem.2019.00074. eCollection 2019.
Protein misfolding is a detrimental effect which can lead to the inactivation of enzymes, aggregation, and the formation of insoluble protein fibrils called Amyloids. Consequently, it is important to understand the mechanism of protein folding, and under which conditions it can be avoided or mitigated. Ionic liquids (ILs) have previously been shown as capable of increasing or decreasing protein stability, depending on the specific IL, IL concentration and which protein. However, a greater range of IL-proteins need to be systematically explored to enable the development of structure-property relationships. In this work, the secondary structure of four proteins, lysozyme, trypsin, β-lactoglobulin and α-amylase, were studied in aqueous solutions of 10 protic ionic liquids (PILs) with 0-50 mol% PIL present. The PILs consisted of ethyl-, ethanol-, diethanol- and triethanolammonium cations paired with nitrate, formate, acetate or glycolate anions. The secondary structure was obtained using ATR-FTIR spectroscopy. It was found that lysozyme and trypsin retained its secondary structure, consistent with a native folded state, for many of the aqueous IL solutions which contained a formate or nitrate anion at the most dilute concentrations. In contrast, α-amylase and β-lactoglobulin generally had poor stability and solubility in the IL solutions. This may be due to the isoelectric point of α-amylase and β-lactoglobulin being closer to the pH of the solvents. All four proteins were insoluble in ethyl-, ethanol- and diethanolammonium acetate, though α-amylase and trypsin retained their secondary structure in up to 20 and 30 mol% of triethanolammonium acetate, respectively. It was evident that the protein stability varied substantially depending on the protein-IL combination, and the IL concentration in water. Overall, the findings indicated that some ions and some ILs were in general better for protein solubility and stability than others, such as acetate leading to poor solubility, and EAN and EAF generally leading to better protein stability than the other PILs. This study of four proteins in 10 aqueous PILs clearly showed that there are many complexities in their interactions and no clear general trend, despite the similarities between the PIL structures. This highlights the need for more and larger studies to enable the selection and optimization of PIL solvents used with biomolecules.
蛋白质错误折叠是一种有害效应，可导致酶失活、聚集以及形成称为淀粉样蛋白的不溶性蛋白质原纤维。因此，了解蛋白质折叠机制以及在何种条件下可以避免或减轻这种情况非常重要。离子液体（ILs）此前已被证明能够根据特定的离子液体、离子液体浓度和蛋白质的不同，增加或降低蛋白质稳定性。然而，需要系统地探索更多种类的离子液体 - 蛋白质组合，以建立结构 - 性质关系。在这项工作中，研究了溶菌酶、胰蛋白酶、β - 乳球蛋白和α - 淀粉酶这四种蛋白质在10种质子型离子液体（PILs）的水溶液中的二级结构，其中PILs的含量为0 - 50 mol%。这些质子型离子液体由乙基、乙醇、二乙醇和三乙醇铵阳离子与硝酸根、甲酸根、乙酸根或乙醇酸根阴离子配对组成。使用衰减全反射傅里叶变换红外光谱（ATR - FTIR）获得二级结构。结果发现，对于许多最稀浓度下含有甲酸根或硝酸根阴离子的水性离子液体溶液，溶菌酶和胰蛋白酶保持其二级结构，与天然折叠状态一致。相比之下，α - 淀粉酶和β - 乳球蛋白在离子液体溶液中的稳定性和溶解性通常较差。这可能是由于α - 淀粉酶和β - 乳球蛋白的等电点更接近溶剂的pH值。所有四种蛋白质在乙基、乙醇和二乙醇铵乙酸盐中均不溶，不过α - 淀粉酶和胰蛋白酶分别在高达20 mol%和30 mol%的三乙醇铵乙酸盐中保留了它们的二级结构。很明显，蛋白质稳定性根据蛋白质 - 离子液体组合以及水中离子液体浓度的不同而有很大差异。总体而言，研究结果表明，一些离子和一些离子液体通常比其他离子和离子液体更有利于蛋白质的溶解性和稳定性，例如乙酸根会导致溶解性差，而EAN和EAF通常比其他质子型离子液体更能使蛋白质保持稳定。对10种水性质子型离子液体中四种蛋白质的这项研究清楚地表明，尽管质子型离子液体结构相似，但它们之间的相互作用存在许多复杂性，且没有明显的一般趋势。这突出表明需要进行更多且规模更大的研究，以便能够选择和优化与生物分子一起使用的质子型离子液体溶剂。
