Protein surface hydration mapped by site-specific mutations
Water motion at protein surfaces is fundamental to protein structure, stability, dynamics, and function. By using intrinsic tryptophans as local optical probes, and with femtosecond resolution, it is possible to probe surface-water motions in the hydration layer. Here, we report our studies of local hydration dynamics at the surface of the enzyme Staphylococcus nuclease using site-specific mutations. From these studies of the WT and four related mutants, which change local charge distribution and structure, we are able to ascertain the contribution to solvation by protein side chains as relatively insignificant. We determined the time scales of hydration to be 3–5 ps and 100–150 ps. The former is the result of local librational/rotational motions of water near the surface; the latter is a direct measure of surface hydration assisted by fluctuations of the protein. Experimentally, these hydration dynamics of the WT and the four mutants are also consistent with results of the total dynamic Stokes shifts and fluorescence emission maxima and are correlated with their local charge distribution and structure. We discuss the role of protein fluctuation on the time scale of labile hydration and suggest reexamination of recent molecular dynamics simulations.
Additional Information© 2006 by the National Academy of Sciences. Contributed by Ahmed H. Zewail, July 25, 2006. Published online before print September 12, 2006, 10.1073/pnas.0606235103. We thank Prof. Patrik R. Callis for catalysis of the initial collaboration for the work with W.E.S. and for constructive comments on the manuscript, Bradford Bullock for help with preparing protein samples, Prof. Sherwin Singer for helpful discussion, Tanping Li for efforts in preparing Fig. 5, and Dr. Wenyun Lu for initial help with the experiments. This work was supported by Petroleum Research Fund Grant PRF-42734-G4, the Packard Foundation Fellowship (to D.Z.), National Science Foundation grants (to D.Z. and A.H.Z.), and National Institutes of Health Grant NCRR COBRE P20 RR15569 (to W.E.S.). Author contributions: D.Z. and A.H.Z. designed research; W.Q., Y.-T.K., L.Z., Y.Y., and L.W. performed research; W.E.S. contributed proteins; W.Q., Y.-T.K., and L.Z. analyzed data; and D.Z. and A.H.Z. wrote the paper. The authors declare no conflict of interest.
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