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Temperature Dependence of Fast Dynamics in Proteins

Xiang-jin Song ^*, Peter F. Flynn , Kim A. Sharp ^* and A. Joshua Wand ^*

^* Johnson Research Foundation and Department of Biochemistry & Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania; and Department of Chemistry, University of Utah, Salt Lake City, Utah

Correspondence: Address reprint requests and inquiries to A. Joshua Wand, Tel.: 215-573-7288; E-mail: wand{at}mail.med.upenn.edu.

The temperature dependence of the internal dynamics of recombinant human ubiquitin has been measured using solution NMR relaxation techniques. Nitrogen-15 relaxation has been employed to obtain a measure of the amplitude of subnanosecond motion at amide N-H sites in the protein. Deuterium relaxation has been used to obtain a measure of the amplitude of motion of methyl-groups in amino-acid side chains. Data was obtained between 5 and 55°C. The majority of amide N-H and methyl groups show a roughly linear (R² > 0.75) temperature dependence of the associated Lipari-Szabo model-free squared generalized-order parameter (O²) describing the amplitude of motion. Interestingly, for those sites showing a linear response, the temperature dependence of the backbone is distinct from that of the methyl-bearing side chains with the former being characterized by a significantly larger -value, where is defined as d ln(1 – O)/d lnT. These results are comparable to the sole previous such study of the temperature dependence of protein motion obtained for a calmodulin-peptide complex. This suggests that the distinction between the main chain and methyl-bearing side chains may be general. Insight into the temperature dependence is gathered from a simple two-state step potential model.