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Many Proteins have ionic charges that can attract each other (e.g. Formation of salt bridges) or repel each other. On the other Hand, Proteins are mostly immersed in water that screens most of the Electric charges of the proteins. The reason is that water is a dielectric, the H2O-molecules can be polarized easily. So the Electrostatic Forces are reduced in aqueous solution.

Now I am asking which Proteins can be easily affected when applying an Electric field? That means which Proteins will Change their conformation substantially even when weak Electric fields are applied?

Can some proteins immersed in water also be dependent on external Electric fields?

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There are various proteins sensitive to changes in electric field. More precisely, they are activated by changes in voltage (surrounding the protein), and immersed in water, as all cells are mainly composed of an aqueous solution. You are correct that most charges in the side chains of the proteins' amino acids are 'masked' by the dipole character of water molecules, but the electrochemical potential of the cell is primarily due to ions, and not to the interaction between side chains in a protein. However, it is important to note that within these proteins sensitive to changes in the electrochemical field, there are domains that contain charged aminoacids (i.e. not covered by water), very important to be able to 'sense' the changes in voltages.

You can check out some of the voltage-sensitive ion channels, which participate in different cellular pathways, including the generation and transmission of the action potential in neurons:

enter image description here

Wikipedia has a good entry about them.

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As you mentioned water is a dielectric and because of that and the presence of ions in solution, electrostatic forces have a very short range inside a cell. If you apply a potential difference (voltage) in solution, you will generate a constant electric field between the two electrodes. Proteins will respond to this electric field by moving towards the anode or cathode depending on the charge of the protein. This is what we know as electrophoresis (note that for the electrophoresis of proteins you usually need tens to hundreds of Volts)

Now, when you have the same voltage difference but there is an electrical insulator in the middle, most of the electric field will be concentrated within the insulating material.

This is what happens in the cell membrane which insulates the small voltage difference between the inside and outside of the cell (-40 to -90 mV depending on the cell). This small voltage difference generates a huge electric field with the membrane which now can be sensed by charged residues of proteins within the membrane such as the arginines in the voltage sensor of a potassium channel. The electric fields sensed by these channels are in the order of 10^8 to 10^9 V/m.

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