Lecture 8 Introduction to Receptor Superfamilies and Drug Targets

Overview of Drug Targets and Molecular Mechanism

• Fundamental Principle of Drug Action: A drug will not work unless it is bound. Most drugs produce their pharmacological effects by binding to specific protein targets.
• Primary Protein Targets:
  • Receptors: The most significant target for drug action.
  • Ion Channels: Proteins in the cell membrane that allow ions to cross the lipid bilayer.
  • Enzymes: Proteins that catalyze essential biochemical reactions within the body.
  • Transporters (Carrier Molecules): Proteins that carry small organic molecules and ions across cell membranes.

The Four Receptor Superfamilies

• Basis of Classification: Receptors are divided into four major superfamilies based on how they transduce the signal (the signal transduction pathway). This describes how drug binding leads to a change in cellular function.
• What Does Not Distinguish Superfamilies:
  • They are not distinguished by the chemical signals or agonists/drugs that stimulate them.
  • They are not distinguished by the specific change in cellular function that occurs.
• Commonality within Superfamilies: Receptors within a specific superfamily possess similar general structures and use similar signal transduction mechanisms.
• The Four Superfamilies:
  1. Ion channel receptors.
  2. G protein coupled receptors (GPCRs).
  3. Enzyme-linked receptors.
  4. Nuclear receptors.

Superfamily 1: Ion Channel Receptors (Ligand-Gated Ion Channels)

• Location: Situated within the cell membrane.
• Structure: These are proteins typically made of 4 to 5 subunits that surround a central pore (an ion channel).
• Mechanism of Action:
  • In the absence of an agonist or endogenous substance, the channel is closed, and no ions flow.
  • Agonist binding causes a conformational change that opens the channel.
  • Ions (which are charged and cannot diffuse through the lipid bilayer) travel through the open pore.
• Kinetics: Respond to "fast" neurotransmitters. The timescale of action is very fast, occurring within milliseconds ($ms$).
• Example: Nicotinic Acetylcholine Receptor ($nAChR$):
  • Agonist: Acetylcholine ($ACh$) is the endogenous neurotransmitter.
  • Effect: Binding of $ACh$ causes the channel to open, allowing sodium ions ($Na^+$) to enter the cell.
  • Antagonist Example: Pancuronium acts as an antagonist at the nicotinic acetylcholine receptor. It prevents the endogenous substance ($ACh$) from binding, thereby preventing the movement of $Na^+$ and resulting in muscle paralysis. Clinically, it is used during anesthesia.

Superfamily 2: G Protein Coupled Receptors (GPCRs)

• Location: Located in the cell membrane.
• Structure:
  • A very large class of receptors.
  • Characteristic feature: Seven transmembrane spanning domains (the protein folds seven times through the cell membrane).
  • Contains an extracellular $N$-terminus and an intracellular $C$-terminus.
  • Possesses a G protein coupling domain where the G protein binds.
• Mechanism of Action: Signal transduction is mediated via G proteins, which link the receptor to an effector protein (either an enzyme or an ion channel).
• Kinetics: Typically respond to hormones and slow neurotransmitters. The timescale between agonist binding and cellular effect is in seconds.
• Examples: Adrenoceptors ($\alpha$ and $\beta$ receptors) and Muscarinic Acetylcholine receptors.

G Proteins and Second Messenger Cascades

• The G Protein: Intracellular effector systems consisting of three subunits: $\alpha$, $\beta$, and $\gamma$. They are named because they bind guanosine nucleotides ($GTP$ and $GDP$).
• Classes of $G_{\alpha}$ Proteins:
  • $G_{s}$ (Stimulatory): Activation of receptors linked to $G_{s}$ activates the effector protein adenylate cyclase. This generates the second messenger cyclic AMP ($cAMP$).
  • $G_{i}$ (Inhibitory): Activation of receptors linked to $G_{i}$ inhibits the activity of adenylate cyclase.
  • $G_{q}$: Activation of receptors linked to $G_{q}$ activates the enzyme phospholipase C. This generates two second messengers: inositol trisphosphate ($IP_3$) and diacylglycerol ($DAG$).
• Clinical Example: Salbutamol:
  • Used in the treatment of asthma. It is an agonist at the $\beta_2$ adrenoceptor in airway smooth muscle.
  • The airway $\beta_2$ receptor is linked to $G_{s}$, which activates adenylate cyclase to produce $cAMP$.
  • Increased $cAMP$ causes the airway smooth muscle to relax, relieving asthma symptoms.

Superfamily 3: Enzyme-Linked Receptors

• Location: Cell membrane.
• Structure:
  • Extracellular portion: Agonist binding domain ($N$-terminus).
  • Intracellular portion: Catalytic domain (enzyme) ($C$-terminus).
• Mechanism of Action: The drug binds to the extracellular portion, which alters the activity of the intracellular enzyme (e.g., a kinase that phosphorylates proteins).
• Biological Role: Respond to growth-type factors involved in metabolism, growth, and differentiation.
• Kinetics: Timescale of action is minutes (relatively slow).
• Example: The Insulin Receptor:
  • The enzyme linked to this receptor is Tyrosine Kinase.
  • Binding of insulin activates tyrosine kinase, leading to the phosphorylation of intracellular proteins and metabolic pathway activation.
  • Specifically, it activates the glucose transport pathway, causing the movement of the transporter protein GLUT4 from the cytoplasm to the cell membrane.
  • Once at the membrane, GLUT4 allows glucose to enter the cell for energy production. Without insulin receptor activation, glucose cannot effectively cross the membrane.

Superfamily 4: Nuclear Receptors (DNA-Linked Receptors)

• Also Known As: Intracellular receptors or DNA-linked receptors.
• Location: Found intracellularly (within the cytoplasm or nucleus).
• Mechanism of Action:
  • The drug (e.g., steroid hormone) must be lipophilic to diffuse across the cell membrane.
  • The drug binds the receptor in the cytoplasm; the complex then migrates to the nucleus.
  • The complex typically dimerizes (forms a homodimer with another identical complex).
  • The complex binds to DNA and changes gene transcription, leading to the synthesis of new proteins.
• Kinetics: The timescale of action is hours. It is very slow because protein synthesis takes significant time.
• Examples: Glucocorticoid and other steroid hormone receptors.
• Clinical Example: Prednisolone:
  • An anti-inflammatory drug that acts as an agonist at nuclear receptors to reduce inflammation via changes in gene transcription.

Non-Receptor Drug Targets

Ion Channels

• Blockers: Physically block the channel to prevent ion flow.
  • Example: Amlodipine is a calcium channel blocker. By blocking calcium entry into vascular smooth muscle (required for contraction), it reduces muscle tone and lowers blood pressure. Used to treat hypertension.
• Modulators: Change or increase/decrease the activity of the channel.
  • Example: Benzodiazepines used for anxiety; they increase the activity of GABA channels, which increases the movement of chloride ions ($Cl^-$) into the cell.

Enzymes

• Inhibitors (Substrate Analogues): The drug has a similar structure to the endogenous substance and replaces it at the active site to inhibit activity.
  • Example: Paracetamol (Panadol) inhibits cyclooxygenase ($COX$), reducing the production of prostaglandins that cause pain (analgesic effect).
• False Substrates: The enzyme metabolizes the drug instead of the endogenous substance, producing an abnormal/non-functional metabolite.
  • Example: Fluorouracil replaces uracil in purine biosynthesis for cancer treatment, blocking DNA and protein synthesis.
• Prodrugs: The drug is inactive until converted by an enzyme within the body.
  • Example: Ciclesonide (asthma treatment). It is converted by an enzyme specifically in the lung to its active form, minimizing adverse effects in other tissues where the enzyme is absent.

Transporters

• Inhibitors: Block the movement of molecules.
  • Example: Fluoxetine (Prozac), an antidepressant, blocks the transport of serotonin ($5-HT$) back into the nerve terminal, keeping more serotonin in the synapse.
• False Substrates: The drug takes the place of the endogenous substance in the transport process and is moved across the membrane.
  • Example: Amphetamine (CNS stimulant) takes the place of noradrenaline in the noradrenaline transporter. It enters the terminal and releases stored noradrenaline and serotonin.

Questions & Discussion

Q: What is the effect of a drug that targets an ion channel?

• A: It will either allow more ions to move through the channel or block the channel so fewer ions move through.

Q: What is the effect of a drug targeting an enzyme?

• A: It can inhibit enzyme activity (producing less product) or activate the enzyme (producing more product).

Q: Practice Question: Comparing Drug A, B, and C on a concentration-response curve.

• Definitions Recap:
  • $EC_{50}$: The concentration of drug required to produce $50\%$ of its own maximum response. Low $EC_{50}$ equals high potency.
  • Potency: Indicated by the position of the curve on the X-axis (left-shifted is more potent).
  • Efficacy: The maximum response ($E_{max}$) an agonist can produce.
• Analysis of Options:
  • Option A (Drug A is a partial agonist): Cannot be determined without a comparison to a full agonist producing a larger response.
  • Option B (Drug A is more potent than B & C): TRUE. If Drug A has the lowest $EC_{50}$, it is the most potent because a lower concentration is needed for the same relative effect.
  • Option C (Drug C has higher efficacy than A & B): Cannot be determined solely from $EC_{50}$ data; efficacy refers to the height of the maximum response.