MODULATION OF A SALT LINK DOES NOT AFFECT BINDING OF PHOSPHATE TO ITSSPECIFIC ACTIVE-TRANSPORT RECEPTOR

Electrostatic interactions are among the key forces determining the st ructure and function of proteins. These are exemplified in the ligande d form of the receptor, a phosphate binding protein from Escherichia c oli. The phosphate, completely dehydrated and buried in the receptor, is bound by 12 hydrogen bonds as well as a salt link with Arg 135. We have modulated the ionic attraction while preserving the hydrogen bond s by mutating Asp 137, also salt linked to Arg 135, to Asn, Gly, or Th r. High-resolution crystallographic analysis revealed that Gly and Thr (but not Asn) mutant proteins have incorporated a more electronegativ e Cl- in place of the Asp carboxylate. That no dramatic effect on phos phate affinity was produced by these ionic perturbations indicates a m ajor role for hydrogen bonds and other local dipoles in the binding an d charge stabilization of ionic ligands.