ELECTROSTATIC STEERING AND IONIC TETHERING IN ENZYME-LIGAND BINDING -INSIGHTS FROM SIMULATIONS

To bind at an enzyme's active site, a ligand must diffuse or be transp orted to the enzyme's surface, and, if the binding site is buried, the ligand must diffuse through the protein to reach it. Although the dri ving force for ligand binding is often ascribed to the hydrophobic eff ect, electrostatic interactions also influence the binding process of both charged and nonpolar ligands. First, electrostatic steering of ch arged substrates into enzyme active sites is discussed. This is of par ticular relevance for diffusion-influenced enzymes. By comparing the r esults of Brownian dynamics simulations and electrostatic potential si milarity analysis for triose-phosphate isomerases, superoxide dismutas es, and beta-lactamases from different species, we identify the conser ved features responsible for the electrostatic substrate-steering fiel ds. The conserved potentials are localized at the active sites and are the primary determinants of the bimolecular association rates. Then w e focus on a more subtle effect, which we will refer to as ''ionic tet hering.'' We explore, by means of molecular and Brownian dynamics simu lations and electrostatic continuum calculations, how salt links can a ct as tethers between structural elements of an enzyme that undergo co nformational change upon substrate binding, and thereby regulate or mo dulate substrate binding. This is illustrated for the lipase and cytoc hrome P450 enzymes. Ionic tethering can provide a control mechanism fo r substrate binding that is sensitive to the electrostatic properties of the enzyme's surroundings even when the substrate is nonpolar.