Temperature Dependence of
Internal Motions of Protein
Side-Chain NH<sub>3</sub><sup>+</sup> Groups: Insight into Energy
Barriers for Transient Breakage of Hydrogen Bonds
- Publication date
- Publisher
Abstract
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
NH<sub>3</sub><sup>+</sup> groups were established recently. Using
this methodology, we have studied the temperature dependence of the
internal motions of the lysine side-chain NH<sub>3</sub><sup>+</sup> groups that form ion pairs with DNA phosphate groups in the HoxD9
homeodomain–DNA complex. For these NH<sub>3</sub><sup>+</sup> groups, we determined order parameters and correlation times for
bond rotations and reorientations at 15, 22, 28, and 35 °C. The
order parameters were found to be virtually constant in this temperature
range. In contrast, the bond-rotation correlation times of the NH<sub>3</sub><sup>+</sup> groups were found to depend strongly on temperature.
On the basis of transition state theory, the energy barriers for NH<sub>3</sub><sup>+</sup> rotations were analyzed and compared to those
for CH<sub>3</sub> rotations. Enthalpies of activation for NH<sub>3</sub><sup>+</sup> rotations were found to be significantly higher
than those for CH<sub>3</sub> 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 NH<sub>3</sub><sup>+</sup> rotations to a level comparable to those for
CH<sub>3</sub> rotations. This entropic reduction in energy barriers
may accelerate molecular processes requiring hydrogen bond breakage
and play a kinetically important role in protein function