Fig. 30.25 Two long helices orient their positively polarized, N-terminal ends toward the narrowest position in the channel in the crystal structure of the voltage-gated ClC channel (a). Glu148 is found at this position, which acts as a gatekeeper and opens and closes the channel. A conformational rearrangement of the negatively charged residue opens passage for chloride ions. Upon passage, the chloride ion sheds its water shell. This is replaced by coordination to the hydroxyl groups of Ser107 and Tyr445 and two contacts to NH groups from the main chain (b).