Field of Science

So salt bridges are not stable in water? Shocking

Three salt bridges seen in this protein in the xtal structure were not observed by detailed NMR experiments in water. Here's the abstract:

NMR investigations have been carried out on the B1 domain of protein G. This protein has six lysine residues, of which three are consistently found to form surface-exposed salt bridges in crystal structures, while the other three are not. The Nζ and Hζ chemical shifts of all six lysines are similar and are not affected significantly by pH titration of the carboxylate groups in the protein, except for a relatively small titration of K39 Nζ. Deuterium isotope effects on nitrogen and proton are of the size expected for a simple hydrated amine (a result supported by density functional theory calculations), and also do not titrate with the carboxylates. The line shapes of the J-coupled 15N signals suggest rapid internal reorientation of all NH3+ groups. pKa values have been measured for all charged side chains except Glu50 and do not show the perturbations expected for salt bridge formation, except that E35 has a Hill coefficient of 0.84. The main differential effect seen is that the lysines that are involved in salt bridges in the crystal display faster exchange of the amine protons with the solvent, an effect attributed to general base catalysis by the carboxylates. This explanation is supported by varying buffer composition, which demonstrates reduced electrostatic shielding at low concentration. In conclusion, the study demonstrates that the six surface-exposed lysines in protein G are not involved in significant salt bridge interactions, even though such interactions are found consistently in crystal structures. However, the intrahelical E35−K39 (i,i+4) interaction is partially present.
The title was meant in half-jest of course and I don't mean to disparage such studies. But I think it just goes to show the kind of difficult, tedious and careful work that has to be often carried out in science even to reach "obvious" conclusions.

An an aside though, this conclusion was not at all obvious for a fair amount of time. There was a vigorous debate in the 90s kicked off by Bruce Tidor's paper arguing that salt bridges are not really that energetically important in protein stabilization, especially on surfaces. People who believed in the intense power of the holy electrostatic attraction did not really believe this. While the debate still continues, to my knowledge the general consensus is now on the side of the original Tidor proposition; salt bridges mostly provide only a marginal energetic gain (1-2 kcal/mol) to protein stability. This has been shown to be so primarily because of the loss in solvation and especially long-range solvation that formation of a salt-bridge incurs. Well, let the "obvious" research continue.

1. Tomlinson, J., Ullah, S., Hansen, P., & Williamson, M. (2009). Characterization of Salt Bridges to Lysines in the Protein G B1 Domain Journal of the American Chemical Society DOI: 10.1021/ja808223p

2. Z.S. Hendsch and B. Tidor. Do salt bridges stabilize proteins? A continuum electrostatic analysis. Protein Sci. 3: 211-226 (1994)

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