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].

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.)