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NMR challenges current protein hydration dogma

SBKB [doi:10.1038/sbkb.2011.08]
Technical Highlight - March 2011
Short description: For decades, scientists have struggled to understand protein interactions with water. NMR spectroscopy using reverse micelles suggests that the dynamics of water on the surface of proteins are quite variable and tend to cluster, overturning the current orthodoxy.

A view of ubiquitin surrounded by water molecules.

Remarkably little is known about protein interactions with water. Early attempts using NMR struggled because of the difficulties in distinguishing which signals arose from direct interactions between the protein and water molecules from those that were due to various confounding artefacts. As a result, only very long-lived interactions were detectable, and these were generally explicitly part of the structure.

This meant that for many years information captured through X-ray crystallographic experiments was relied on instead to provide evidence of so-called hydration water. Many people had reservations about this approach, but the lack of alternatives led to it being widely adopted.

A new method using NMR spectroscopy on proteins encapsulated in the water core of reverse micelles now allows the interaction between the protein surface and hydration water to be examined comprehensively, without fear of contamination by artifactual signals.

Reverse micelles are nanoscale-sized compartments in which water and protein are surrounded by an assembly of surfactant molecules. They were first developed by Joshua Wand and colleagues as a tool for optimizing the NMR performance of large proteins—dissolving the reverse micelle in a low-viscosity fluid increases the tumbling rate of the protein, allowing satisfactory spectra to be produced—and have since found numerous other applications in structural biology and biophysics.

Nucci et al. realized that the slowing of hydration dynamics and the hydrogen-exchange chemistry within the water core of a reverse micelle would enable the reliable characterization of protein–hydration waters interactions for the first time using solution NMR methods.

The initial views of protein hydration using ubiquitin have illuminated several unanticipated aspects of protein hydration. Particularly intriguing is the clustering of hydration water with similar residence times. This has significant implications for protein thermodynamics, molecular recognition and a host of other functionally important aspects of proteins. These findings challenge the view that the interaction of solvent water with the protein solute is uniform, and they expose an unanticipated richness that will undoubtedly be the focus of intense study over the coming years.

Maria Hodges

References

  1. N.V. Nucci et al. Site-resolved measurement of water-protein interactions by solution NMR.
    Nature Struct. Mol. Biol. 18, 245-249 (2011). doi:10.1038/nsmb.1955

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