Salt bridges
In proteins, salt bridges[1] occur between amino acid side-chains with opposite positive or negative full-electron charges, namely, (at neutral pH) Glu- or Asp- vs. Arg+ or Lys+. They may also occur between ionized organic ligands, such as acetylcholine+ (or example at right: 1cbr), or inorganic ions, such as K+ or SO4=, and amino acid side-chains. A salt bridge is generally considered to exist when the centers of charge are 4 Å or less apart[2]. The center of charge of the arginine sidechain is the zeta carbon[3]. The energetic significance of such complementary charge pairs is a complex function of the local environment. Proteins from thermophiles have more salt bridges than do proteins from mesophiles[4][5]. These additional salt bridges contribute to stability, resisting denaturation by high temperature[6][7]. ExamplesThermophile vs. mesophileGlutamate dehydrogenase structures have been determined at about 2 Å resolution for both a thermophile, Pyrococcus furiosus (1gtm), and a mesophile, Clostridium symbiosum (1hrd)[5]. The thermophile's protein has 1.7 fold more N and O atoms engaged in salt bridges than does the protein from the mesophile (301 vs. 175 respectively, as counted by FirstGlance). Ultraviolet-B receptorUVR8 is an ultraviolet-B receptor in plants such as Arabidopsis. It is a homodimer that, upon irradiation, dissociates into a monomer involved in transcriptional activation of UV protective proteins[8]. Unexpectedly, high ionic strength was found to dissociate the dimer. The homodimer 4dnw contains many salt bridges and cation-pi interactions at the interface. More.
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Visualization
Putative protein-protein salt bridges involving charged amino acid sidechains and/or charged chain termini can be displayed by 1cbr.
