Electrostatics in Protein Binding and Function

Protein electrostatic properties stem from the proportion and distribution of polar and charged residues. Polar and charged residues regulate the electrostatic properties by forming short-range interactions, like salt-bridges and hydrogen-bonds, and by defining the over-all electrostatic environment in the protein. Electrostatics play a major role in defining the mechanisms of protein-protein complex formation, molecular recognitions, thermal stabilities, conformational adaptabilities and protein movements. For example:- Functional hinges, or flexible regions of the protein, lack short-range electrostatic interactions; Thermophilic proteins have higher electrostatic interactions than their mesophilic counter parts; Increase in binding specificity and affinity involve optimization of electrostatics; High affinity antibodies have higher, and stronger, electrostatic interactions with their antigens; Rigid parts of proteins have higher and stronger electrostatic interactions. In this review we address the significance of electrostatics in protein folding, binding and function. We discuss that the electrostatic properties are evolutionally selected by a protein to perform an specific function. We also provide bona fide examples to illustrate this. Additionally, using continuum electrostatic and molecular dynamics approaches we show that the "hot-spot" inter-molecular interactions in a very specific antibody-antigen binding are mainly established through charged residues. These "hot-spot" molecular interactions stay intact even during high temperature molecular dynamics simulations, while the other inter-molecular interactions, of lesser functional significance, disappear. This further corroborates the significance of charge-charge interactions in defining binding mechanisms. High affinity binding frequently involves "electrostatic steering". The forces emerge from over-all electrostatic complementarities and by the formation of charged and polar interactions. We demonstrate that although the high affinity binding of barnase-barstar and anti-hen egg white lysozyme (HEL) antibody-HEL complexes involve different molecular mechanisms, it is electrostatically regulated in both the cases. These observations, and several other studies, suggest that a fine tuning of local and global electrostatic properties are essential for protein binding and function.