Section 23.2.5.1. Dominant role of local dipoles in stabilization of isolated charges

A novel finding of further paramount importance and wide implication is how the isolated charges of the protein-bound phosphate and sulfate are stabilized. No counter-charged residues or cations are associated with the sulfate completely buried in SBP. Although a salt link involving Arg135 is formed with the phosphate bound to PBP, it is shared with an Asp residue (Fig. 23.2.5.1b). Moreover, site-directed mutagenesis studies indicate that phosphate binding is quite insensitive to modulation of the salt link (Yao et al., 1996). These findings are a powerful demonstration of how a protein is able to stabilize the charges by means other than salt links. Experimental and computational studies indicate that local dipoles, including the hydrogen-bonding groups and the backbone NH groups from the first turn of helices, immediately surrounding the sulfate and phosphate are responsible for charge stabilization (Pflugrath & Quiocho, 1985; Quiocho et al., 1987; Åqvist et al., 1991; He & Quiocho, 1993; Yao et al., 1996; Ledvina et al., 1996). Helix macrodipoles play little or no role in charge stabilization of the anions. The same principle of charge stabilization by local dipoles also applies for the following buried uncompensated ionic groups: Arg151 of the arabinose-binding protein (Quiocho et al., 1987), the zwitterionic leucine ligand bound to the leucine/isoleucine/valine-binding protein (Quiocho et al., 1987), the potassium in the pore of the potassium channel (Doyle et al., 1998) and Arg56 of synaptobrevin-II in a SNARE complex (Sutton et al., 1998).