Backbone dynamics of ribonuclease T1 and its complex with 2?GMP studied by two-dimensional heteronuclear NMR spectroscopy

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Abstract
The backbone dynamics of free ribonuclease T1 and its complex with the competitive inhibitor 2′GMP have been studied by 15N longitudinal and transverse relaxation experiments, combined with {1H, 15H} NOE measurements. The intensity decay of individual amide cross peaks in a series of (1H, 15N)-HSQC spectra with appropriate relaxation periods (Kay, L.E. et al. (1989) Biochemistry, 28, 8972–8979; Kay, L.E. et al. (1992) J. Magn. Reson., 97, 359–375) was fitted to a single exponential by using a simplex algorithm in order to obtain 15N T1 and T2 relaxation times. These experimentally obtained values were analysed in terms of the ‘model-free’ approach introduced by Lipari and Szabo (Lipari, G. and Szabo, A. (1982) J. Am. Chem. Soc., 104, 4546–4559; 4559–4570). The microdyramical parameters accessible by this approach clearly indicate a correlation between the structural flexibility and the tertiary structure of ribonuclease T1, as well as restricted mobility of certain regions of the protein backbone upon binding of the inhibitor. The results obtained by NMR are compared to X-ray crystallographic data and to observations made in molecular dynamics simulations.