The electrostatic potential of the alpha helix (electrostatic potential/alpha-helix/secondary structure/helix dipole)

The active sites of many enzymes are very close to the N-terminus of an alpha-helix. The helix dipole has been postulated to enhance the binding of anions and speed charge relays in catalysis. We present electrostatic potential maps of alpha-helices of various lengths using a point charge model. We show that the potential field of the helix can be mimicked by two equal and opposite charges, one at each terminus. The magnitude of these equivalent charges reaches its limiting value of +/- 0.2 to 0.3 electron at a helix length of approximately 7-10 residues. We also comment on the relative importance of the helix dipole to that of ionized residues in determining the electrostatics of a protein and discuss what consequences this has for enzymology.