Figure 2

Salt bridge interactions between Arg or Lys and Asp or Glu. (a) Energy profile (in kcal/mol) as a function of the interresidue distance d (in Å) in globular proteins (dashed black line), extramembrane regions (blue line) and transmembrane regions (red line). (b) Difference between the energy profiles of salt bridges involving Arg (small dashed red) and Lys residues (large dashed red). (c,d) Salt bridges occurring in the transmembrane region of the protein structures 5AYN (iron transporter ferroportin) and 2WJR (NanC porin), respectively. The residues Arg and Lys are drawn in red, and Glu and Asp in blue.