What are the differences between a ligand-gated channel, a G-protein, and a second messenger system.  

Expert Answers
txmedteach eNotes educator| Certified Educator

Well, the differences start when we consider what each sort of messenger really does in the cell. Also note that there are many, many more receptors and channels than what you mentioned here! Let's just do a quick summary of each of those things, and then we'll compare/contrast them.

Ligand-gated channels are channels that are opened on the binding of a substrate (ligand) to the channel. It's almost like a keycard that needs to be swiped to open a door. One of the best examples of this is the nicotinic receptor for acetylcholine (ACh). When nerves send signals, they use ion channels to let the signal travel. For one nerve to tell another nerve to start communicating, the first nerve needs to open the second nerve's ion channels. This effect is accomplished by using ACh to open the ligand-gated channels at the synapse between the two nerves.

G-Protein Coupled Receptors (GPCR's) work by binding a ligand and then releasing a molecule (that started attached to the receptor) to carry out one of a few side reactions. These side reactions then inhibit or promote certain activities in the cell. For example, in the heart, norepinepherine binds its GPCR, which then activates adenylate cyclase to make cAMP, which then activates a protein kinase, which then does a few things, culminating in fight-or-flight effects like increased contraction strength. A way to think of this is like setting off dynamite in a cartoon. Your ligand pushes on the handle, and the G-protein (in the box) sends the signal to the dynamite (other things in the cell) to explode!

The molecules involved in those side reactions are actually what is termed "second messengers." For example, in the particular GCPR we discussed, the second messenger would be cAMP, which activates the other molecules in the cascade. In a way, it acts like the Postal Service. You (the ligand) give a letter to the courier (activate the GPCR and send out your second messenger). The courier then brings the letter and signals whomever you sent it to.

Now, hopefully that it is clear that most of the similarities and differences here have to do with things being inside and outside the cell. With both ligand-gated ion channels and GPCR's, the ligand remains outside the cell. However, ligand-gated channels allow transport of something (ions) across the membrane, while GPCR's simply change shape to throw off a molcule that does the signaling. Nothing is moved across the membrane with G-proteins!

Now to relate these to second messengers. Ligand-gated ion channels have no second messengers. GPCR's always work with second messengers, either stimulating or inhibiting them. However, not all second messengers involve G-proteins! Tyrosine kinases are a great example of non GPCR second messenger systems.

A common misconception, by the way, is that G Proteins are channels of some sort. They might activate one through a second messenger. However, they by themselves are not membrane channels.

I hope that helps!

aribic01 | Student

The main difference between a ligand-gated channel, G-protein and the second messenger system lies in the sequence of their recruitment. Ligand-gated channels and G-proteins are primary responders-they are the first ones to respond to the stimulus that comes to the surface of the cell they are present on. Secondary messengers are linked to the primary responders and transmit their message further into the cell.

In case of nerve cells and muscle cells that have ligand-gated channels on their surface for example, neurotransmitters such as glutamate will bind to a site on the ligand-gated channel, change its conformation and open it to allow the activation of the cell. Ligand-gated channels are also major drug targets because of this. Benzodiazepines bind to a specific ligand-gated channel and their binding mimics the normal activation of these channels.

G-proteins bind larger molecules, from odors to small peptides and hormones. For example, when we inhale air, small odor molecules hit the G-proteins that are expressed on the cells lining the interior of our noses. Specific odors will activate their specific receptors, which is the basis of our sense of smell.

After ligand-gated channel and G-proteins are activated, their changes in conformation activate secondary messenger systems. Secondary messengers act in specific groups, where each molecule has a specific function. For example, one molecule will detect a change in conformation of a ligand-gated channel, change its own conformation and thereby activate another molecule that may perform another function, such as adding phosphate groups to yet another molecule, etc. Secondary messenger systems relay the information from the cell surface to the cell nucleus, where longer-term changes happen. Such as system provides feedback communication between cell nucleus and cell surface, which is the basis for balanced cell activity under all conditions. For example, if a neurons is receiving too much glutamate, the messenger systems will transmit that information to the nucleus, which will in turn tone down the production of glutamate receptors and, in turn, their export to the cell surface.

ac1950 | Student

G-proteins are part of the signal transduction system which causes cells to react to external stimuli.  The process typically begins with the binding of a ligand to a receptor (such as a G protein) on the cell's external membrane.  This causes a change in the shape of the receptor which often releases another substance (a second messenger) which causes a response within the cell.

A ligand-gated channel is a pore on the cell membrane which opens or closes in response to the binding of an appropriate ligand.  The ligand molecule has a shape and charge distribution which 'fits' the pore's receptor.  These gated channels are used extensively in the nervous system to control the flow of ions which can polarize or depolarize the membrane of an axon to generate nerve impulses.

There are a wide variety of messengers and receptors which regulate important bodily systems include the immune system, the nervous system, reproductive organs, etc.

rawand70 | Student