Paper I — 2018 July (Supplementary) (2010 Scheme) — Q1
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Paper I
2018 July (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Define a receptor. Discuss the various transducer mechanisms by which the receptors
act. (1+5)
Q16 marksEssays
Answer
Definition
A receptor is a specific macromolecule (usually a protein) present on the cell surface or within the cell, which recognizes and binds a specific ligand (drug/hormone/neurotransmitter) with high affinity and specificity, and translates this binding into a biological/pharmacological response.
Transducer mechanisms (signal transduction) by which receptors act
Ligand-gated ion channels (ionotropic receptors): The receptor itself forms an ion channel; ligand binding causes conformational change opening the channel, allowing ion flux (Na⁺, K⁺, Ca²⁺, Cl⁻) — very rapid response (milliseconds). Example: Nicotinic acetylcholine receptor, GABA-A receptor
G-protein coupled receptors (GPCRs, metabotropic receptors): Seven-transmembrane-domain receptors coupled to G-proteins; ligand binding activates the G-protein, which in turn activates/inhibits effector enzymes (adenylyl cyclase, phospholipase C) generating second messengers (cAMP, IP3/DAG) — response in seconds. Example: β-adrenergic receptors, muscarinic receptors
Enzyme-linked (catalytic) receptors: Ligand binding activates intrinsic or associated enzymatic activity (usually tyrosine kinase), triggering phosphorylation cascades and gene expression changes — response in minutes to hours. Example: Insulin receptor, growth factor receptors
Intracellular (nuclear) receptors: Located in cytoplasm/nucleus; lipophilic ligands (steroids, thyroid hormone) diffuse across the cell membrane and bind these receptors, which then act as transcription factors, modulating gene transcription — slow response (hours to days). Example: Glucocorticoid receptor, oestrogen receptor
Receptors that directly regulate transcription without a classical ligand-binding intracellular receptor (e.g., cytokine receptors signalling via JAK-STAT pathway) — also enzyme-linked-associated in a broader classification
