What is the Difference Between Ligand-Gated Ion Channels and G-Protein Coupled Receptors

The main difference between ligand-gated ion channels and G-protein coupled receptors is that ligand-gated ion channels are pores in the cellular membrane that allow ions to pass in and out of the cell upon binding with neurotransmitters, whereas G-protein coupled receptors recognize a wide range of ligands including photons, small molecules, and proteins to open or close ion channels through the activation of the intracellular messenger, G-protein.

Simply, ligand-gated ion channels and G-protein coupled receptors are two types of transmembrane proteins that make up postsynaptic ion channels. But, ligand-gated ion channels are ionotropic receptors while G-protein coupled receptors are metabotropic receptors.

Key Areas Covered

1. What are Ligand-Gated Ion Channels
   • Definition, Characteristics, Importance
2. What are G-Protein Coupled Receptors
   • Definition, Characteristics, Importance
3. Similarities Between Ligand-Gated Ion Channels and G-Protein Coupled Receptors
   • Outline of Common Features
4. Difference Between Ligand-Gated Ion Channels and G-Protein Coupled Receptors
   • Comparison with Key Differences

Key Terms

G-Protein Coupled Receptors, Ligand-Gated Ion Channels, Nicotinic Acetylcholine Receptor

What are Ligand-Gated Ion Channels

Ligand-gated ion channels are a group of transmembrane ion-channel proteins that open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger such as a neurotransmitter. Their receptors are ionotropic receptors that directly link to the ion channel. Generally, there are two functional domains in the receptor. They are the extracellular site, which binds to the neurotransmitters, and a membrane-spanning domain, which forms the ion channel. Therefore, the two functional domains are in the same molecule entity. Moreover, ligand-gated ion channels contain four or five protein subunits, each contributing to the pore of ion channels.

Figure 1: Ligand-Gated Ion Channels

Furthermore, the prototypic ligand-gated ion channel is the nicotinic acetylcholine receptor. It is a pentamer containing five protein subunits and contains two binding sites for acetylcholine. Normally, the binding of acetylcholine to the ionotropic receptor changes the conformation of the ligand-gated ion channel. However, this results in the constriction of the pore, allowing sodium ions to pass through the ion channel. Here, the sodium ions flow down into the cell, resulting in the depolarization of the plasma membrane of the postsynaptic neuron.

What are G-Protein Coupled Receptors

G-protein coupled receptors are the proteins located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule called a G protein. These receptors are metabotropic receptors. Therefore, one or more metabolic steps engage in the movement of ions through the receptor. Metabotropic receptors do not have ion channels in their structure and therefore activate intermediate molecules such as G-proteins. There, the intracellular domain of the receptor binds to G-protein while the extracellular domain binds to the neurotransmitter.

Figure 2: G-Protein Coupled Receptors

Moreover, the binding of the neurotransmitter activates the G-protein, which is coupled to the metabotropic receptor in the inactive form. This binding also results in the dissociation of the G-protein from the receptor. Usually, the activated G-protein binds with ion channels or other effector proteins such as enzymes. Therefore, G-proteins serve as intracellular messengers that open and close ion channels.

Similarities Between Ligand-Gated Ion Channels and G-Protein Coupled Receptors

• Ligand-gated ion channels and G-protein coupled receptors are two types of postsynaptic ion channels responsible for the depolarization and hyperpolarization of the postsynaptic neurons.
• Neurotransmitters bind ligand-gated ion channels and G-protein coupled receptors at the synaptic cleft.
• Both types of postsynaptic ion channels occur in the postsynaptic neuron.
• Both are transmembrane proteins responsible for the exchange of ions and molecules through the plasma membrane of the postsynaptic neuron.

Difference Between Ligand-Gated Ion Channels and G-Protein Coupled Receptors

Definition

Ligand-gated ion channels refer to a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger such as a neurotransmitter, while G-protein coupled receptors refer to the proteins located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule called a G protein.

Type of Receptor

Ligand-gated ion channels are ionotropic receptors while G-protein coupled receptors are metabotropic receptors.

Structure

Usually, ligand-gated ion channels are multimers having at least four or five individual proteins, subunits while G-protein coupled receptors are seven-pass transmembrane domain receptors.

Ion Channels

Ligand-gated ion channels couple with ion channels while G-protein coupled receptors do not couple with ion channels.

Importance

Ligand-gated ion channels involve the direct opening of ion channels with the binding of a neurotransmitter, while G-protein coupled receptors are involved in the indirect binding of ion channels with metabolic activation of G-protein.  

Conclusion

In brief, ligand-gated ion channels and G-protein coupled receptors are postsynaptic receptors that occur on the postsynaptic neurons. Their function is to mediate ion channels to depolarize or hyperpolarize the plasma membrane of the postsynaptic neuron. Significantly, ligand-gated ion channels activate upon the binding of the neurotransmitter while G-protein coupled receptors allow the activation of G-protein upon binding the neurotransmitter, which activates ion channels. Therefore, the main difference between ligand-gated ion channels and G-protein coupled receptors is their activation of the ion channels.

References:

1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Two Families of Postsynaptic Receptors.

Image Courtesy:

1. “LGIC” By Bensaccount at en.wikipedia – Own work (Public Domain) via Commons Wikimedia
2. “GPCR in membrane” By Repapetilto – Own Work (CC BY-SA 3.0) via Commons Wikimedia