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The charged amino acids (His, Lys, Arg, Asp, Glu) are generally distributed in surface-accessible positions in protein structures, although at these positions the degree of conservation is not as great as in the core[Fiser et al., 1996]. Extensive surface networks of charged ion pairs appear to stabilise a number of enzymes from thermophilic organisms[Hennig et al., 1995,Korndorfer et al., 1995,Yip et al., 1995], whilst in mesophiles charge pairs are fewer in number and are not generally thought to be a significant stabilising force[Dill, 1990]. Indeed, pairwise interactions between distant homologues are not very well conserved[Russell & Barton, 1994].
The distribution of cysteine residues in proteins stabilised by disulphide bonds will obviously encode important structural information since these bonds are often well conserved[Carrington & Boothroyd, 1996]. In intracellular proteins the distribution of cysteine residues is not of structural importance. Proline is often found in turns at the surface of all types of proteins[Hutchinson & Thornton, 1994].