Determination of the Backbone Mobility of Ribonuclease T1 and its 2′GMP Complex Using Molecular Dynamics Simulations and NMR Relaxation Data

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Abstract
The results of 1-nanosecond molecular dynamics simulations of the enzyme ribonuclease T1 and its 2′GMP complex in water are presented. A classification of the angular reorientations of the backbone amide groups is achieved via a transformation of NH-vector trajectories into several coordinate frames, thus unravelling contributions of NH-bond librations and backbone dihedral angle fluctuations. The former turned out to be similar for all amides, as characterized by correlation times of librational motions in a subpicosecond scale, angular amplitudes of about 10–12° for out-of-peptide-plane displacements of the NH-bond and 3–5° for the in-plane displacements, whereas the contributions of much slower backbone dihedral angle fluctuations strongly depend on the secondary structure. Correlation functions relevant for NMR were obtained and analyzed utilizing the ‘model-free’ approach (Lipari, G. and Szabo, A. (1982) J. Am. Chem. Soc. 104, 4546–4559,4559-4570; Clore et al., (1990) J. Am. Chem. Soc. 112, 4989–4991). The dependence of the amplitude of local motion on the residue location in the backbone is in good agreement with the results of NMR relaxation measurements and X-ray data. The protein dynamics is characterized by a highly restricted local motion of those parts of the backbone with defined secondary structure as well as by a high flexibility in loop regions. The comparison of results derived from different periods of the trajectory (of 50 ps and 1 ns duration, 1000 points sampled) reveals a dependence of the observed dynamic picture on the characteristic time scale of the experimental method used. Comparison of the MD data for the free and liganded enzyme clearly indicates a restriction of the mobility within certain regions of the backbone upon inhibitor binding.