Temperature Dependence of Fast Dynamics in Proteins

Abstract

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 ( R 2 > 0.75) temperature dependence of the associated Lipari-Szabo model-free squared generalized-order parameter ( O 2) 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 Λ-value, where Λ is defined as d ln(1 − O)/ d ln T. 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.