The Sodium Channel
Patch clamp recordings of the sodium channel reconstituted in vesicles shows it to be a voltage-gated channel. Two potent neurotoxins, tetrodotoxin (from Puffer fish) and saxitoxin bind to the channel and act as antagonists (inhibit the activity of the receptor by blocking sodium influx).
Figure: Puffer fish
Figure: Tetrodoxin and Saxitoxin - Structures
The guanidino group of the tetrodotoxin appears to bind with high affinity to the entrance of the channel that interacts with the hydrated sodium ion. Affinity chromatography using tetrodotoxin beads has been used to purify the protein. The protein sequence consists of 4 repeating units, each containing 6 transmembrane helices. Ion selectivity of the sodium channel is determined by the size of the ion when it is hydrated by one water molecule.
Figure: Ion selectivity of the sodium channel
Hence Li and Na ions can pass through, but not the large K ion. Hydroxylamine and hydrazine both have the same width as a water-Na+ complex, and are both positively charged. These can also pass through the channel. At the narrowest part of the channel a water (and presumably the hydroxylamine and hydrazine) form H-bonds to side chains in the pore while the positive charge on the ion forms transient salt-bridges to either a Glu or Asp in the pore. The open channel can likewise spontaneously inactivate by undergoing another conformational change. Comparison of the sequence of this channel with calcium and potassium channels shows extensive sequence and structural similarities. One of the 6 transmembrane helices, S4, appears to be the voltage sensor. Depolarization of the membrane potential may result in an outward movement and rotation of the positively charged helixes containing Lys and Arg side chains which had presumably formed salt-bridges with negatively charged side chains within the protein. Depolarization of the membrane results in breaking a few salt bridges, and effectively leads to the movement of 1 or 2 charges on the helix through the membrane. Work occurs when charges are moved through an electric field. Work is also related to the ΔG for the system, which is also dependent on the ratio of the open and closed form of the channel. Other voltage-gated ion channels (for potassium and chloride) have a similar membrane topology and an S4 voltage-sensor helix.