First, what is a drug? What makes a drug?
A drug is a small molecule that has a binding affinity to its target (this usually is a protein).
What does that mean?
Drugs are specific to their target. The job of the drug is to disable and stop its target from functioning. Essentially, the better the drug and the protein fit with each other, the more effective the drug is!
If the target is a protein, it has an area where the drug joins/connects to it. This area can either be a receptor or a pocket.
This isn’t the only way drugs work. Sometimes, drugs can be ‘dummy’ molecules that are substitutes for real molecules. In such a case, the drug works by standing in place for the molecule, and thus inhibiting the molecule from completing its function. A popular example of this is remdesivir, an adenosine analog. Once the viral RNA polymerase attaches remdesivir to a copied strand of the virus, the virus’s replication cannot continue. Without copying its genes, the virus cannot assemble, grow, and exit the cell, which ultimately prohibits the virus from spreading to other cells.
The way a drug works is called its mechanism of action.
In drug design, we want to design small molecule drugs that are either agonists or antagonists.
Agonists are drugs that bind to a receptor to produce a similar response as the normal ligand.
Antagonists bind to the receptor to prevent the protein from doing its daily tasks. Essentially, antagonists block the protein from doing its job by binding to the protein before the agonist can.
TLDR?
Antagonists turn receptors “off” and agonists turn receptors “on”.
Types of Agonists and Antagonists
Agonists can be both natural and artificial. Natural agonist examples are ligands that exist in our body such as serotonin or dopamine. We can synthetically increase our feeling of “happiness” using artificial agonists which bind to the “happiness” receptors.
Antagonists counteract agonist functions. For example, when Demi Lovato overdosed and lost consciousness, she was medicated with Narcan, which brought her back to life. Narcan or naloxone attaches to all opioid receptors preventing morphine or heroin from binding to its own receptors and activating these receptors. This type of drug works by competitive inhibition.
Non competitive antagonists work by changing the shape of the binding site, so that the natural ligand can no longer bind to the receptor anymore. One example is Ketamine, an anesthesia drug that binds to the NDMA receptor.
(Full credit to the article linked here)