Both the enzymes and receptors are targets for drug action and many biomolecules in the body.
Further, they both are proteinaceous in nature and help to regulate body physiology.
But, they have stark differences in terms of their structure, mechanism, and functions in the body.
Receptors are the signal receivers and form the front end of the messenger communication system. They evoke and bring changes in body physiology when a ligand binds to them.
In pharmacology, few enzymes act as receptors, and even a few receptors have enzymes within them.
See the types of receptors for an idea.
The similarities between enzymes and receptors
Enzymes and receptors are similar in the following ways like
Enzymes and receptors are quite specific regarding
- the chemicals stimulating them,
- reactions or effects involved,
- their location in the body, and
- the environment around them.
Let me take the examples of enzyme nucleases and nicotinic receptors to describe them in detail for you.
♦ Chemicals stimulating them
Both the receptors and enzymes act only upon stimulation by a specific chemical.
The enzyme nuclease is active only in the presence of nucleic acids like DNA and RNA as substrate.
Similarly, the nicotinic receptor is active only in the presence of the ligands acetylcholine, nicotine, or other related chemicals.
They are not stimulated by any other substrate or ligands other than those mentioned above.
♦ Location in the body
- Enzyme nucleases are formed in the pancreas and released into the small intestine. They are not found in any other regions of the body.
- Likewise, the nicotinic receptor is found only in the ganglia and neuromuscular junction regions of the body.
Both enzymes and receptors require a suitable environment to function.
Enzymes require optimal temperature and pH to function.
At a very high temperature, they get denatured (destroyed), and at low temperatures, they are less active.
At optimal pH, the enzyme helps convert substrate to product. While at unfavorable pH, they become inactive.
Receptors also require optimal temperature and pH to bind with ligands efficiently. The ligand-receptor binding varies with temperature.
This is because the drug (ligand) receptor binding occurs via electrostatic, covalent, or hydrophobic bonds. These bonds vary in strength with temperature.
Both are inert and not active unless required or stimulated.
They can produce an effect only in the presence of a substrate or ligand, respectively.
For example, the enzyme amylase shows its effect only in the presence of carbohydrates like starch.
It does not induce a biochemical reaction unless there is a carbohydrate.
Similarly, a nicotinic receptor is active only in the presence of a ligand like acetylcholine, which can stimulate it. In the absence, it is inert.
3. Drug Targets
Both are involved in drug actions.
For example, the medicine lisinopril inhibits the enzyme angiotensin-converting enzyme and helps lower high blood pressure.
On the other hand, the drug metoprolol acts on the β-receptors to lower blood pressure.
Thus, enzymes and receptors are explored as drug targets in therapeutic care.
4. Reaction kinetics
Kinetic equations like the rate of association and dissociation constants can describe both functions.
As long as the enzyme is bound to the substrate, there is a product formation, and when the substrate dissociates from the enzyme, no product is formed.
Similarly, as long as the ligand is bound to the receptor, there is a response. When ligands dissociate, there is no further response from receptors.
5. Degree of response
The concentration of enzymes and receptors matters for the extent of activity or response.
An increased concentration produces a maximal response, while a decrease in concentration shows sub-maximal responses when stimulated by substrate or ligand, respectively.
Competitors can control their actions for binding. I.e., a substrate that binds to the enzyme and a ligand that binds to the receptor can be displaced by a competitive binder.
Thus, the action of enzymes and receptors can be manipulated.
Inhibitors are those substances that can decrease the rate of activity or completely abolish the activity.
Both the enzymes and receptors have certain inhibitors that block their function.
These inhibitors bind to enzymes and receptors, preventing the original ligands from binding and evoking a response.
Examples: The compound sarin acetylcholinesterase enzyme inhibitor. This compound irreversibly binds with cholinesterase enzyme, leading to toxicity.
Similarly, atenolol is a β receptor blocker that helps in minimizing blood pressure.
8. Allosteric binding sites
An allosteric binding site is a region on the enzyme or receptors that is adjacent to the actual binding site.
These sites can help control the overall response of the enzyme or receptors positively or negatively.
This is a phenomenon where the enzyme or receptor has other sites of binding. In general, a substrate binds to an enzyme at the main site, and a ligand binds to the receptor at the key site.
When a substance binds to these allosteric sites, both the enzyme and receptors show abnormal activity.
Thus, allosteric sites are common for both enzymes and receptors.