Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.
A Review of Receptor Proteins and Signaling Cascades
Most of the activities and functions of our body are brought about by the action of proteins. If we think of the cell as a little community, proteins are the people - they can make things happen, they can transport items, they communicate and interact with each other, and they can build large structures. Proteins can even be large structures - think of a human pyramid or kids holding hands in a game of red rover.
Proteins are often classified by what they do. Receptors are proteins that bind to small signaling molecules like hormones, neurotransmitters, and ions. A molecule that binds a receptor is a ligand. In many cases, once a receptor binds a ligand, the receptor changes shape. This starts a cascade of protein-protein interactions and brings about a change in cellular activities - in either small or big ways (see figure).
For example, the insulin receptor (IR) is a protein in the plasma membrane of many cells. When the hormone insulin reaches the cells, it binds to the IR. The IR changes shape, which allows it to interact with other proteins and change their activity. These newly activated proteins then go on to activate other proteins and so forth.
Ultimately, these protein interactions cause several changes in the cell, including the uptake of glucose from the blood and cell growth. It's very much like a series of dominoes. A receptor protein is the very first domino and your finger is the ligand which starts the domino interaction cascade.
Agonists 'Activate' Receptors
Now that we have a basic understanding of signaling pathways, we are going to look at chemicals that can control those pathways. An agonist is any chemical that activates or 'turns on' the activity of a receptor protein. They are often referred to as stimulating agents.
In the example above, insulin is an agonist. In the domino analogy, your finger is an agonist. Agonists can be endogenous, which means they are made by our bodies, and include familiar hormones like insulin, estrogen and epinephrine (aka adrenaline). Agonists can also be exogenous, which means they originate from outside the body. Prescription and illicit drugs are examples of exogenous agonists.
As you might have guessed, if receptors can have agonists, they also have antagonists - chemicals that block or inhibit the function or activity of a receptor. They are often referred to as blocking agents. Think of these agents as similar to the antagonists of a book, or those characters who provide obstacles for the protagonist.
An antagonist can either block an agonist from binding or prevent the receptor from changing shape even if an agonist is bound. In the domino analogy, an antagonist would be someone else's fingers holding up the first domino, not letting it fall over even if you try to knock it down. When describing a chemical as an agonist or antagonist, you need to reference the receptor. For instance, epinephrine is a beta-adrenergic receptor agonist, while propranolol is an beta-adrenergic receptor antagonist, or a so called beta-blocker given to prevent heart attacks.
Simply put, agonists activate or stimulate their receptors and antagonists inhibit or block the activity of their receptors. The downstream effect on the cell depends on the action of the receptor and subsequent protein-protein interactions. The human body is very complex, so of course it's not that simple. There are various types of agonists and antagonists depending on exactly how the chemicals act. But for our lesson, that definition is sufficient. The endogenous agonists in our body and the exogenous drugs we put into our body all have the same effect on their receptors. They ACTIVATE!
Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.
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