BMS 123 · Common Core (Tronc commun)Section 5.3 · Pharmacodynamics: mechanisms and receptors
Section 20 of 27
The four main drug targets
Name the four main drug targets, give a named drug for each, and distinguish selectivity from specificity.
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Why this matters
Almost every drug you will meet in this course works by binding one of four kinds of protein. Knowing which kind tells you what the drug can do, what it cannot do, and which unwanted effects are likely. It also explains why drugs are grouped into clinical classes, and why two drugs with the same target may still behave very differently in a patient.
What you need to know first
You need the idea from 5.2 that most clinically useful drugs work by binding biomolecules, and some everyday knowledge of enzymes and cell membranes from biology. Every technical term used here is defined in the section, so no previous pharmacology is required.
Words to know (6)
Drug target
A biomolecule, almost always a protein macromolecule, to which a drug binds to produce its effect. Xanthine oxidase is the target of allopurinol.
Enzyme
A protein that catalyses a biological reaction, speeding up the building or breaking of a substance. Cholinesterase breaks down acetylcholine at the synapse.
Ion channel
A pore in the cell membrane through which ions such as sodium, potassium or calcium pass. Local anaesthetics block sodium channels in nerves.
Carrier molecule
A membrane protein that carries ions and small organic molecules from one side of the membrane to the other. Digoxin binds the sodium pump, a carrier.
Physiological receptor
A protein or glycoprotein that receives the body’s own chemical signals and can be activated or inactivated by a ligand. Salbutamol binds β₂ adrenoceptors in the bronchi.
Pharmacophore
The small part of a ligand that actually makes contact with the binding site of the receptor. Two drugs of the same class often share a pharmacophore.
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In one sentence The majority of clinically useful drugs act by binding biomolecules called drug targets, which are mostly large proteins.
Most drugs bind protein macromolecules
The majority of clinically useful drugs produce their action by binding with biomolecules at the molecular level. These biomolecules are often called drug targets or receptors. Drug targets are predominantly proteins of large molecular weight, and are therefore referred to as macromolecules. Four groups account for almost all of them: enzymes, carrier molecules, ion channels and physiological receptors. A smaller fifth group, structural proteins such as tubulin, is also targeted. Naming the group is the first step in predicting what a drug will do.
For example
Allopurinol acts on an enzyme, digoxin on a carrier, local anaesthetics on an ion channel and salbutamol on a receptor. Four drugs, four different target types, four different clinical pictures.
Think of it like
Think of the body as a workshop full of protein machines. A drug is a stranger that can jam a machine (enzyme), block a doorway (ion channel), overload a conveyor belt (carrier) or press a button (receptor). Unlike a worker, the drug is not choosing: it binds wherever its shape fits, which is why one molecule can jam more than one machine.
Pause and check yourself
Name the four main drug targets.
Show the answer
Enzymes, carrier molecules, ion channels and physiological receptors. Structural proteins such as tubulin form a smaller additional group.
Read detailed explanations
Why the same four targets keep appearing
The four target families are not an arbitrary list: they are the four ways a cell can receive, transmit or act on a chemical message. Enzymes are the machinery that makes and breaks molecules; ion channels are the fast electrical switches; carriers control the traffic of ions and small molecules across membranes; receptors are the antennas that recognise hormones and neurotransmitters. A drug must change one of these four processes to change cell behaviour, and there is no fifth way of producing a specific, graded, reversible effect. This is why drug discovery begins by choosing a target before a molecule is designed. It also explains why the same four families appear in every tissue: bone marrow, heart, gut and brain use the same components, arranged differently. The arrangement is what gives a drug its selectivity. A tissue that expresses many β₂ receptors and few β₁ receptors will respond mostly through β₂, and the same drug will look selective in that tissue and unselective in another. Comparing the pattern of targets across tissues is therefore a powerful way to predict both the therapeutic action and the adverse effect spectrum of a new molecule.
Selectivity, specificity and the arithmetic of side effects
In everyday speech “specific” means one and only one, and beginners carry that meaning into pharmacology. In pharmacology, specificity is an ideal that almost no drug reaches, and selectivity is the practical quality that matters: the drug binds its preferred target more strongly than the others, but it still binds the others. The arithmetic is simple and sobering. If a drug is a hundred times more selective for the wanted receptor, then at the concentration needed for the therapeutic effect the unloved receptors are still being touched, and the proportion of them occupied rises with the dose. That is why simply raising the dose to get more benefit also raises the burden of side effects, and why drugs are used at the lowest dose that produces the needed effect. Selectivity can be improved chemically, by changing the shape of the molecule so that it fits the wanted binding site and no longer fits the others, and clinically, by choosing the route that puts the drug where it is needed. Inhaled salbutamol is the classic example: most of the dose reaches the bronchi, so the heart sees relatively little of it, and the off-target effect stays small. Understanding that balance between wanted and unwanted receptors is one of the most useful practical skills in this unit.
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The four main drug targets
Most clinically useful drugs bind protein macromolecules. The four main targets are enzymes, ion channels, carrier molecules and physiological receptors; structural proteins such as tubulin form a fifth, smaller group.
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01 / 5Enzymes
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Enzymes
Protein catalysts that build or break down substances. Because almost every biological reaction needs an enzyme, this target is central: a drug that inhibits one enzyme can slow a whole pathway. Most drug actions on enzymes are inhibitory.
Schematic, not to scale. The four groups are separated for teaching; one drug can bind more than one type of target at the same time, which is often the reason it has side effects as well as benefits.
Key points to remember
✓Most clinically useful drugs bind protein macromolecules called drug targets.
✓The four main targets are enzymes, ion channels, carrier molecules and physiological receptors.
✓Structural proteins such as tubulin form a smaller fifth group, targeted by vincristine and colchicine.
✓Ligands bind the binding site of the receptor with a small region called the pharmacophore.
✓Selectivity is relative: most drugs act on several receptor subtypes, which broadens use and creates off-target effects.
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Worked example
Classifying four unfamiliar drugs by their target. You are given four molecules with no names. Experiment shows that the first stops a chemical reaction, the second changes the movement of sodium across a membrane, the third alters the reuptake of a small molecule back into a nerve ending, and the fourth changes the shape of the cell’s internal scaffold. You must name the target type for each and predict the kind of effect to expect.
The first molecule stops a chemical reaction, so it is acting on an enzyme. Expect a change in the amount of the substance that the enzyme normally makes or breaks down.
The second changes the movement of sodium across a membrane, so it is acting on an ion channel. Expect a change in the excitability of the tissue that carries that channel — nerve, muscle or heart.
The third alters the reuptake of a small molecule into a nerve ending, so it is acting on a carrier molecule. Expect the natural cargo to accumulate outside the cell and to prolong signalling.
The fourth changes the cell’s internal scaffold, so it is acting on a structural protein such as tubulin. Expect effects on cell division and on cell movement rather than on a chemical signal.
For each molecule, ask next whether the natural ligand for that target is also present. A drug that competes with a natural substrate behaves differently from one that acts where nothing else is competing.
Identifying the target type constrains the possible effects before any clinical trial is done. It tells you which tissue will respond, whether the effect will be fast or slow, and what kind of toxicity to look for. This is why pharmacodynamics begins with the question “what is the site of action?”.
Common misconception
A widespread beginner error is to treat “receptor” and “drug target” as exact synonyms. In the lecture notes the two words are used loosely, but a receptor is only one of the four main targets: enzymes, carriers and ion channels are targets that are not receptors. Calling a calcium channel a receptor then makes it impossible to explain why a calcium channel blocker does not behave like a receptor antagonist.
Check your reasoning
A drug is described as highly selective for β₂ receptors, yet patients report a racing heart. How can both statements be true?
Reveal an answer
Selectivity is relative, not absolute. At the concentration reached in the body the drug still binds a fraction of β₁ receptors in the heart, and that fraction is enough to speed the rate. Dose, route and the number of receptors in each tissue decide how large the off-target effect becomes in a given patient.
Where this comes from
BMS 123 General Pharmacology II lecture notes · Part Two, §2.1.2 Receptor-mediated mechanisms of drug action; §3.1 Drug–receptor interactions
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About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.