Drug-Target Interaction
Master the mechanisms of Enzymes, Receptors, Agonists, and Antagonists.
When you take a pill, how does it know where to go and what to do? Drugs are chemical molecules that interact with the massive biological macromolecules inside our bodies. These macromolecules are the "targets." The most crucial targets for medicinal chemistry are proteins—specifically Enzymes and Receptors.
1. Enzymes as Drug Targets
Enzymes are proteins that act as biological catalysts. To understand how drugs target them, we must first understand the two main functions of an enzyme:
- Holding the Substrate: The enzyme's active site holds the substrate molecule in a suitable position so it can be attacked by a reagent. The substrate binds via ionic bonds, hydrogen bonds, van der Waals forces, or dipole-dipole interactions.
- Providing Functional Groups: The enzyme provides chemical functional groups that will attack the substrate to carry out the chemical reaction.
Enzyme Inhibitors
Drugs can block the binding site of the enzyme and prevent the binding of the natural substrate. Such drugs are called Enzyme Inhibitors. They do this in two vastly different ways:
These drugs have a chemical shape very similar to the natural substrate. They compete with the natural substrate for the Active Site of the enzyme. If the drug gets there first, the substrate is blocked.
These drugs do not bind to the active site. Instead, they bind to a different site on the enzyme called the Allosteric Site. Binding here physically distorts the shape of the enzyme, changing the shape of the active site so the natural substrate can no longer fit (like changing the shape of a lock so the key won't work).
Figure 1: Comparison of Competitive and Non-Competitive (Allosteric) enzyme inhibition.
Usually, drug-enzyme bonds are weak (H-bonds, van der Waals). However, if the bond formed between an enzyme and an inhibitor is a strong covalent bond and cannot be broken easily, the enzyme is permanently blocked. The body will then degrade the dead enzyme-inhibitor complex and synthesize a brand new enzyme.
2. Receptors as Drug Targets
Receptors are proteins that are crucial to the body's communication process. The majority of them are embedded in the cell membrane.
How do Receptors work?
Receptor proteins project their active site outwards from the cell membrane. In the body, message transmission happens via Chemical Messengers (like hormones or neurotransmitters).
- The chemical messenger binds to the active site of the receptor.
- This binding forces the receptor protein to change its shape.
- This shape change transfers the message to the inside of the cell.
- Crucial Point: Chemical messengers transfer the message into the cell without actually entering the cell.
3. Antagonists vs. Agonists
When designing drugs to target receptors, chemists create molecules that fall into one of two major categories based on what the patient's body needs:
A. Antagonists (The Blockers)
Drugs that bind to the receptor site and completely inhibit its natural function are called Antagonists.
When are they useful? They are highly useful when blocking a message is required. For example, if a chemical messenger is causing excessive acid production or allergic inflammation, an antagonist drug will block the receptor, stopping the "make acid" message.
B. Agonists (The Mimics)
Drugs that mimic the natural messenger by switching "on" the receptor are called Agonists.
When are they useful? They are useful when there is a lack or deficiency of the natural chemical messenger. If the body isn't producing enough of a hormone, an agonist drug acts as an artificial substitute to keep the communication channel running.
Receptors show selectivity for one chemical messenger over the other because their binding sites have different shapes, structures, and amino acid compositions. Drugs are synthesized to match only the specific "lock" of the target receptor.
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