Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA.
Proc Natl Acad Sci U S A. 2013 Jan 29;110(5):1692-5. doi: 10.1073/pnas.1215400110. Epub 2013 Jan 14.
The ionization states of individual amino acid residues of membrane proteins are difficult to decipher or assign directly in the lipid-bilayer membrane environment. We address this issue for lysines and arginines in designed transmembrane helices. For lysines (but not arginines) at two locations within dioleoyl-phosphatidylcholine bilayer membranes, we measure pK(a) values below 7.0. We find that buried charged lysine, in fashion similar to arginine, will modulate helix orientation to maximize its own access to the aqueous interface or, if occluded by aromatic rings, may cause a transmembrane helix to exit the lipid bilayer. Interestingly, the influence of neutral lysine (vis-à-vis leucine) upon helix orientation also depends upon its aqueous access. Our results suggest that changes in the ionization states of particular residues will regulate membrane protein function and furthermore illustrate the subtle complexity of ionization behavior with respect to the detailed lipid and protein environment.
在脂质双层膜环境中，膜蛋白中单个氨基酸残基的电离状态很难直接破译或确定。我们针对设计的跨膜螺旋中的赖氨酸和精氨酸解决了这个问题。对于二油酰基磷脂酰胆碱双层膜内两个位置的赖氨酸（但不是精氨酸），我们测量到低于 7.0 的 pK(a) 值。我们发现，埋藏的带电荷的赖氨酸，与精氨酸类似，将调节螺旋方向以最大程度地使其自身进入水相界面，或者如果被芳环阻断，可能导致跨膜螺旋离开脂质双层。有趣的是，中性赖氨酸（相对于亮氨酸）对螺旋方向的影响也取决于其水相可及性。我们的结果表明，特定残基的电离状态变化将调节膜蛋白的功能，并且进一步说明了与详细的脂质和蛋白质环境有关的电离行为的微妙复杂性。
