Temperature Dependence of Internal Motions of Protein Side-Chain NH3+ Groups: Insight into Energy Barriers for Transient Breakage of Hydrogen Bonds
Temperature Dependence of Internal Motions of Protein Side-Chain NH3+ Groups: Insight into Energy Barriers for Transient Breakage of Hydrogen Bonds
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DOI：
10.1021/bi5012749
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发表时间：
2015-01-20
期刊：
影响因子：
2.9
通讯作者：
Iwahara, Junji
中科院分区：
文献类型：
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作者：
Zandarashvili, Levani;Iwahara, Junji
尽管带电荷侧链在蛋白质功能中起着重要作用，但其动力学性质尚不清楚。近年来建立了研究赖氨酸侧链NH3+基团动力学的核磁共振方法。利用这种方法，我们研究了在HoxD9同源结构域-DNA复合体中赖氨酸侧链NH3+基团与DNA磷酸基团形成离子对的内部运动的温度依赖性。对于这些NH3+基团，我们确定了在15、22、28和35℃时键旋转和重定向的顺序参数和相关时间，发现顺序参数在这个温度范围内几乎是恒定的。相反，发现NH3+基团的键旋转相关次数强烈依赖于温度。基于过渡态理论，分析了NH3+旋转的能垒，并与CH3旋转的能垒进行了比较。NH3+旋转的激活焓明显高于CH3旋转的激活焓，这可以归因于氢键断裂的需要。然而，激活熵大大降低了NH3+旋转的总激活自由能，使其与CH3旋转的自由能相当。这种能量垒的熵降低可能会加速需要氢键断裂的分子过程，并在蛋白质功能中发挥重要的动力学作用。
Although charged side chains play important roles in protein function, their dynamic properties are not well understood. Nuclear magnetic resonance methods for investigating the dynamics of lysine side-chain NH3+ groups were established recently. Using this methodology, we have studied the temperature dependence of the internal motions of the lysine side-chain NH3+ groups that form ion pairs with DNA phosphate groups in the HoxD9 homeodomain-DNA complex. For these NH3+ groups, we determined order parameters and correlation times for bond rotations and reorientations at 15, 22, 28, and 35 degrees C. The order parameters were found to be virtually constant in this temperature range. In contrast, the bond-rotation correlation times of the NH3+ groups were found to depend strongly on temperature. On the basis of transition state theory, the energy barriers for NH3+ rotations were analyzed and compared to those for CH3 rotations. Enthalpies of activation for NH3+ rotations were found to be significantly higher than those for CH3 rotations, which can be attributed to the requirement of hydrogen bond breakage. However, entropies of activation substantially reduce the overall free energies of activation for NH3+ rotations to a level comparable to those for CH3 rotations. This entropic reduction in energy barriers may accelerate molecular processes requiring hydrogen bond breakage and play a kinetically important role in protein function.