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].
Examples
Thermophile vs. mesophile
Glutamate 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 receptor
UVR8 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.
Chains and clumps of salt bridges
6nie contains a chain of salt bridges: D236-K170-D140-R237-E120-K301. The chain branches at R237 which is salt bridged to D119. A branched chain could be described as a "clump". (K301 is an unusual monomeric amino acid ligand.)
