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 bringing changes in body physiology when a ligand binds to them.
In pharmacology, few enzymes act as receptors and even a few receptors have enzymes withing 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 in terms of the chemicals stimulating them, the 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 on stimulation by a specific chemical.
The enzyme nuclease is active only in presence of nucleic acids like the 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 do not get 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. It is not found in other regions of the body.
Likewise, the nicotinic receptor is found specifically in the ganglia and neuromuscular junction
Both enzymes and the receptors require a conducive environment to function.
Enzymes require optimal temperature and pH to function. At the very high temperature they get denatured destroyed) and at temperature, they are less active.
At optimal pH they enzyme help in the conversion of substrate to product. While at unfavorable pH, they become inactive.
Receptors too 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 by either electrostatic, covalent bonds, or hydrophobic bonds. These bonds vary in strength with temperature.
Both are by themselves inert and not active unless stimulated.
They can produce an effect only in 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.
Both can act as targets for drug molecules.
For example, lisinopril is given to control high blood pressure by inhibiting the enzyme angiotensin-converting enzyme.
While metoprolol is used to control blood pressure by acting on β-receptors.
Both functions can be described by kinetic equations like the rate of association and dissociation constants.
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.
Degree of response
The concentration of enzymes and receptors matters. A decrease in concentration than required can give sub-maximal responses when stimulated by substrate or ligand respectively.
Their actions can be controlled by competitors for binding. I.e. a substrate that binds to enzyme and ligand which binds to the receptor can be displaced by a competitive binder.
Thus the action of enzymes and receptors can be manipulated.
The function of enzymes and also receptors can be blocked by respective inhibitors. These inhibitors bind to enzyme and receptors and prevent the original ligands from binding and evoking a response.
Allosteric binding sites
An allosteric binding site is a site on the enzyme or receptors which is adjacent to the actual binding site.
These sites can help in controlling the overall response of the enzyme or receptors in a positive or negative way. Though the actual substrate or ligand binds, its response will be influenced by the molecule bound to the allosteric site.