Department of Chemistry and Biochemistry, UCLA-DOE Institute, Molecular Biology Institute, University of California , Los Angeles, California 90095, United States.
Department of Biochemistry and Center for Structural Biology, Vanderbilt University , Nashville, Tennessee 37240, United States.
J Am Chem Soc. 2017 Aug 9;139(31):10742-10749. doi: 10.1021/jacs.7b04819. Epub 2017 Jul 25.
Although backbone hydrogen bonds in transmembrane (TM) helices have the potential to be very strong due to the low dielectric and low water environment of the membrane, their strength has never been assessed experimentally. Moreover, variations in hydrogen bond strength might be necessary to facilitate the TM helix breaking and bending that is often needed to satisfy functional imperatives. Here we employed equilibrium hydrogen/deuterium fractionation factors to measure backbone hydrogen bond strengths in the TM helix of the amyloid precursor protein (APP). We find an enormous range of hydrogen bond free energies, with some weaker than water-water hydrogen bonds and some over 6 kcal/mol stronger than water-water hydrogen bonds. We find that weak hydrogen bonds are at or near preferred γ-secretase cleavage sites, suggesting that the sequence of APP and possibly other cleaved TM helices may be designed, in part, to make their backbones accessible for cleavage. The finding that hydrogen bond strengths in a TM helix can vary widely has implications for membrane protein function, dynamics, evolution, and design.
尽管跨膜（TM）螺旋中的骨架氢键由于膜的低介电常数和低水环境而具有很强的潜力，但它们的强度从未在实验中进行过评估。此外，氢键强度的变化可能对于促进 TM 螺旋的断裂和弯曲是必要的，这通常需要满足功能要求。在这里，我们使用平衡氢/氘分馏因子来测量淀粉样前体蛋白（APP）TM 螺旋中的骨架氢键强度。我们发现氢键自由能的范围非常大，有些比水-水氢键弱，有些比水-水氢键强 6 千卡/摩尔以上。我们发现弱氢键位于或接近 γ-分泌酶的优先切割位点，这表明 APP 的序列和可能其他被切割的 TM 螺旋可能部分设计为使它们的骨架易于切割。TM 螺旋中氢键强度变化很大这一发现对膜蛋白的功能、动力学、进化和设计具有重要意义。
