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Agonist: Definition & Effect

Instructor: Catherine Konopka

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.

Have you ever wondered how hormones, pharmaceuticals and illicit drugs actually cause changes in your body? In this lesson you will learn about one way that chemicals can alter the activity of receptor proteins and their cells.

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).

A signaling cascade is a series of protein-protein interactions that starts with the activation of a receptor and ends with various changes in cellular activity.
Agonist activate receptors and initiate signaling cascades

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.

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