Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online





Learning Goals/Objectives for Chapter 9B:
After class and this reading, students will be able to

  • describe how a transmembrane ion gradient and nongated/gated membrane ion channels specific for given ions can give rise to a transmembrane electric potential across membranes
  • given ion concentrations and the electrical potential across a membrane, predict likely changes in the membrane potential and ion concentrations on the opening of specific channnels;
  • use the Goldman equation to predict transmembrane electrical potentials;
  • state difference between the communication across the neuromuscular junction and a synapse between two neurons;
  • state the difference between nongated and gated ion channels;
  • describe different ways to open/close gated ion channels
  • describe the immediate changes in the muscle cells when acetylcholine is released into the neuromuscular junction
  • describe the roles of stimulatory neurotransmitter receptors, voltage-gated Na+and K+ channels and the Na/K-ATPase  in the activation of a neuron;
  • explain the mechanism for selectivity of K+ over the smaller Na+ ion in the K+ channel;
  • briefly explain how membrane protein channels can be gated open by changes in transmembrane potential;

B7.  Voltage-Gated 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 antagonist (inhibits the activity of the receptor by blocking sodium influx). 

Figure:   Puffer fish

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 tetrodoxin beads have been used to purify the protein. The protein sequence consists of 4 repeating units, each containing 6 transmembrane helices. Ion selectively of the sodium channel is determined by the size of the the ion when it is hydrated by one water molecule.

Tetrodotoxin:  Ancient Alkaloid from the Sea

Saxitonin:  From Food Poisoning to Chemical Warfare

Figure:  Tetrodoxin and Saxitoxin - Structures

Figure:  Ion selectively 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 as 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 show extensive sequence and structural similarities. One of the 6 transmembrane helices, S4, appears to be the voltage sensor. Depolarization of the membrane potential rmay 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.

Animation:  Na channel


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Archived version of full Chapter 9B:  Neural Signaling


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Biochemistry Online by Henry Jakubowski is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.