Hormones and signal transduction

In short

Hormones are chemical messengers secreted by endocrine glands into blood, acting at very low concentrations (10⁻⁹–10⁻¹² M) on target cells that carry specific receptors. Signal transduction is the chain by which receptor binding is converted into an intracellular response — second messengers, protein kinase cascades, altered enzyme activity and altered gene expression — so that a few hormone molecules can reorganise the metabolism of a whole cell. The topic is the regulatory "wiring diagram" that links the individual metabolic pathways into an integrated whole-body response to feeding, fasting, stress and growth. It carries heavy weight in B.Sc/M.Sc Biochemistry papers, CSIR-UGC NET Life Sciences, GATE Biotechnology/BT and XL, ICMR-JRF, DBT-BET and in technical interviews at pharma, diagnostics and CRO employers (Biocon, Dr Reddy's, Sun Pharma, Serum Institute, Syngene, Thyrocare, Metropolis, SRL, Agappe). Practically it underlies hormone assays (TSH, insulin, cortisol), receptor-targeted drugs and the entire diagnostics market for thyroid and diabetes testing in India.

Explained

Hormones: definitions, classes and transport

• Hormone: a substance secreted by one cell type that travels (usually in blood) to act on a distant target cell bearing a specific receptor. Endocrine = via blood; paracrine = on neighbouring cells; autocrine = on the secreting cell itself; neurocrine/neuroendocrine = released from neurons (e.g. hypothalamic releasing hormones).
• Chemical classes: (i) peptide/protein — insulin, glucagon, GH, ACTH, TSH, PTH, leptin; (ii) amine (catecholamine, from tyrosine) — epinephrine, norepinephrine, dopamine; (iii) steroid (from cholesterol) — cortisol, aldosterone, testosterone, estradiol, progesterone, calcitriol (vitamin D hormone); (iv) thyroid hormones (iodinated tyrosine) — T₄, T₃; (v) eicosanoids (from arachidonic acid) — prostaglandins, thromboxanes, leukotrienes; (vi) gases — nitric oxide.
• Water-soluble vs lipid-soluble: peptides and catecholamines are hydrophilic, circulate free, have short half-lives (minutes), and bind cell-surface receptors. Steroids and thyroid hormones are hydrophobic, travel bound to carrier proteins (CBG, SHBG, TBG, albumin), have long half-lives (hours to days for T₄), and bind intracellular/nuclear receptors.
• Only free hormone is biologically active — hence clinical assays of free T₄ and free T₃ in addition to total.
• Synthesis and storage: peptide hormones are made as preprohormones → prohormones → active hormone (preproinsulin → proinsulin → insulin + C-peptide) and stored in secretory granules; steroid hormones are not stored, they are synthesised on demand from cholesterol by cytochrome P450 enzymes (StAR protein moves cholesterol into mitochondria; CYP11A1/desmolase is the committed, rate-limiting step).

Receptors, specificity and the logic of signalling

• Receptor = protein with a ligand-binding site (high affinity, Kd in the hormone's physiological range) plus an effector/transducing function. Specificity resides in the receptor, not the hormone — the same cAMP signal makes liver release glucose and adipose release fatty acids.
• Four receptor superfamilies: (1) G-protein-coupled receptors (GPCR) — 7-transmembrane serpentine receptors (glucagon, epinephrine β, ACTH, TSH, PTH, LH/FSH); (2) receptor enzymes — receptor tyrosine kinases (insulin, IGF-1, EGF) and receptor guanylyl cyclase (ANP); (3) cytokine receptors acting through JAK–STAT (GH, prolactin, leptin, erythropoietin); (4) nuclear receptors acting as ligand-activated transcription factors (steroids, T₃, retinoic acid). Ion-channel receptors (nicotinic acetylcholine) are the fifth type, mainly neural.
• Universal features of signal transduction: specificity (shape complementarity), amplification (enzyme cascades multiply the signal), modularity (interchangeable domains: SH2, SH3, PH, PTB), integration (opposing inputs summed), desensitisation/adaptation (receptor phosphorylation, arrestin binding, internalisation, down-regulation of receptor number).
• Signal termination is as regulated as initiation: GTP hydrolysis by the Gα subunit, phosphodiesterases degrading cAMP/cGMP, protein phosphatases (PP1, PP2A, PTP1B), Ca²⁺ pumps (SERCA, PMCA), receptor internalisation and lysosomal degradation.
• Agonist vs antagonist: an agonist activates the receptor; an antagonist occupies it without activating (propranolol on β-adrenergic receptors). Partial agonist/inverse agonist give intermediate or below-basal activity.

Pathway 1 — cAMP / protein kinase A (the classical second-messenger cascade)

1. Hormone (glucagon, epinephrine via β-receptor) binds the GPCR → conformational change.
2. Receptor acts as a GEF: the heterotrimeric G protein Gs (αβγ) exchanges GDP for GTP on Gα.
3. Gsα–GTP dissociates from βγ and activates adenylate cyclase (membrane enzyme): ATP → cyclic AMP + PPi.
4. cAMP binds the two regulatory (R) subunits of protein kinase A (R₂C₂), releasing two active catalytic (C) subunits.
5. PKA phosphorylates serine/threonine residues: phosphorylase kinase (activated), glycogen synthase (inactivated), hormone-sensitive lipase and perilipin (activated), pyruvate kinase and acetyl-CoA carboxylase (inactivated), PFK-2/FBPase-2 (switched to the bisphosphatase mode in liver).
6. PKA also phosphorylates CREB, which with CBP/p300 turns on genes bearing a CRE (e.g. PEPCK — gluconeogenesis).
7. Termination: intrinsic GTPase of Gα (accelerated by RGS proteins) → Gα–GDP reassociates with βγ; phosphodiesterase (PDE) hydrolyses cAMP to 5′-AMP (inhibited by caffeine/theophylline, and by sildenafil for cGMP-PDE5); phosphoprotein phosphatase-1 reverses the phosphorylations.
8. Gi (α₂-adrenergic, somatostatin, opioid, adenosine A1) inhibits adenylate cyclase — the same cell integrates Gs and Gi inputs.
9. Toxin pharmacology (examination favourite): cholera toxin ADP-ribosylates Gsα and blocks its GTPase → permanently "on" → massive cAMP in intestinal epithelium → Cl⁻ and water secretion → watery diarrhoea. Pertussis toxin ADP-ribosylates Giα and blocks GDP release → Gi cannot be activated → cAMP rises.

Pathway 2 — Phosphoinositide, IP₃/DAG and Ca²⁺–calmodulin

• Hormone (epinephrine via α₁-receptor, vasopressin V1, angiotensin II, TRH, GnRH, oxytocin) → Gq → activates phospholipase C-β.
• PLC cleaves membrane PIP₂ (phosphatidylinositol 4,5-bisphosphate) into IP₃ (water-soluble) and DAG (stays in membrane).
• IP₃ opens IP₃-gated Ca²⁺ channels on the endoplasmic reticulum → cytosolic Ca²⁺ rises from ~10⁻⁷ M to ~10⁻⁶ M (10⁻⁵ M locally).
• DAG + Ca²⁺ activate protein kinase C (conventional isoforms need both); PKC phosphorylates many targets and is the receptor for the tumour promoter phorbol ester (TPA).
• Ca²⁺ acts largely through calmodulin (4 Ca²⁺-binding EF-hand sites) → Ca²⁺–calmodulin activates CaM kinases, phosphorylase kinase (the δ subunit of phosphorylase kinase *is* calmodulin — this is how muscle contraction and glycogenolysis are coupled), myosin light-chain kinase, NO synthase, calcineurin (a phosphatase).
• Signal ends with Ca²⁺ pumped out by SERCA/PMCA, IP₃ dephosphorylated (lithium inhibits inositol monophosphatase — the basis of its mood-stabilising action), DAG phosphorylated or hydrolysed.
• PI 3-kinase branch: PIP₂ → PIP₃, which recruits PDK1 and Akt (PKB) through PH domains — the main growth/metabolic limb of insulin signalling; reversed by the tumour suppressor phosphatase PTEN.

Pathway 3 — Insulin, receptor tyrosine kinases and the MAPK cascade

1. Insulin receptor is a disulphide-linked α₂β₂ tyrosine kinase; insulin binds the α subunits → the β subunits autophosphorylate on tyrosines (activation loop first).
2. Phosphotyrosines are docking sites; the receptor phosphorylates IRS-1/IRS-2 on multiple tyrosines.
3. Metabolic limb: IRS-1-P binds PI3K (SH2 domain) → PIP₃ → PDK1 → Akt/PKB. Akt outputs: GLUT4 vesicles translocate to the membrane (muscle, adipose) → glucose uptake; glycogen synthase kinase-3 is inhibited → glycogen synthase activated; FOXO1 is excluded from the nucleus → gluconeogenic genes (PEPCK, G6Pase) switched off; mTORC1 activated → protein synthesis; phosphodiesterase-3B activated → cAMP falls → lipolysis suppressed.
4. Growth limb (MAPK cascade): Grb2 (SH2–SH3 adaptor) + Sos (GEF) load GTP onto Ras → Raf (MAPKKK) → MEK (MAPKK) → ERK (MAPK) → phosphorylates transcription factors (Elk-1, c-Myc) → growth and division. Three-tier cascade gives huge amplification and sharp (sigmoidal) switching.
5. Insulin also activates the phosphoprotein phosphatase PP1, which dephosphorylates glycogen phosphorylase and glycogen synthase — hence insulin's effects are broadly the mirror image of glucagon's.
6. Termination/pathology: PTP1B and SHIP dephosphorylate; serine phosphorylation of IRS-1 (by TNF-α, free fatty acids, mTOR/S6K) blocks tyrosine phosphorylation — a molecular basis of insulin resistance and type 2 diabetes. Oncogenic *Ras* mutations lock Ras in the GTP form.
7. Other receptor enzymes: ANP receptor is a receptor guanylyl cyclase → cGMP → PKG → natriuresis; TGF-β receptor is a serine/threonine kinase acting through SMADs.

Pathway 4 — JAK–STAT, nitric oxide–cGMP, and nuclear receptors

• JAK–STAT: GH, prolactin, leptin, erythropoietin, interferons → receptor dimerises → associated JAK (Janus kinase) phosphorylates the receptor and STAT proteins → STAT dimerises through SH2–phosphotyrosine interaction → moves to nucleus → transcription (GH → IGF-1). Switched off by SOCS proteins and SHP phosphatases. Erythropoietin biosimilars made in India act through this route.
• NO–cGMP: acetylcholine/shear stress → Ca²⁺–calmodulin activates endothelial NO synthase (arginine + O₂ + NADPH → NO + citrulline) → NO diffuses into smooth muscle → binds the haem of soluble guanylyl cyclase → cGMP → PKG → myosin light-chain phosphatase activated, Ca²⁺ lowered → vasodilation. Nitroglycerine works by releasing NO; sildenafil inhibits PDE5 so cGMP persists.
• Nuclear (intracellular) receptors: steroid/thyroid hormone diffuses through the membrane → binds receptor with a DNA-binding (zinc-finger) domain, ligand-binding domain and transactivation domain. Type I (steroid) receptors sit in cytosol with HSP90, bind ligand, dimerise, enter the nucleus and bind hormone response elements (GRE, ERE); Type II (T₃, RA, vitamin D, PPAR) are already on DNA as heterodimers with RXR, with corepressors replaced by coactivators on ligand binding.
• Timescale: second-messenger responses appear in seconds to minutes (phosphorylation of existing enzymes); nuclear-receptor responses take hours to days (new protein synthesis) and are blocked by actinomycin D or cycloheximide.
• Thyroid hormone also has genomic action via TR on TREs; T₄ is a prohormone converted to the more active T₃ by 5′-deiodinase (a selenoenzyme) in peripheral tissues.

Integration, axes and feedback

• Hierarchy: hypothalamus (releasing hormone, e.g. CRH, TRH, GnRH, GHRH) → anterior pituitary (tropic hormone: ACTH, TSH, LH/FSH, GH) → target gland (cortisol, T₃/T₄, sex steroids) → negative feedback on both pituitary and hypothalamus. Long-loop, short-loop and ultra-short-loop feedback.
• Positive feedback is rare: the LH surge before ovulation, oxytocin in labour.
• Diagnostic consequence: in primary hypothyroidism TSH is high with low T₄; in secondary (pituitary) hypothyroidism both are low — which is why TSH is the first-line test and why an abnormal TSH is interpreted only with free T₄.
• Fed state: high insulin/low glucagon → glycogen, fat and protein synthesis. Fasting: glucagon + epinephrine + cortisol + GH → glycogenolysis, gluconeogenesis, lipolysis, ketogenesis. Stress: catecholamines (seconds) then cortisol (minutes to hours).
• Receptor-level disease: type 2 diabetes (post-receptor resistance), pseudohypoparathyroidism (Gs defect), testicular feminisation (androgen receptor defect), laron dwarfism (GH receptor defect), McCune–Albright (activating Gsα mutation), Graves' disease (stimulating autoantibody against the TSH receptor), nephrogenic diabetes insipidus (V2 receptor defect).
• Indian practice and regulation: hormone assays are done by CLIA/ECLIA/ELISA in NABL-accredited laboratories (ISO 15189) under the Clinical Establishments (Registration and Regulation) Act, 2010 where adopted by the state; diagnostic kits and hormone drugs (insulin, HCG, thyroxine) fall under the Drugs and Cosmetics Act, 1940 with the New Drugs and Clinical Trials Rules, 2019, regulated by CDSCO, with recombinant products also cleared through DBT/RCGM–GEAC biosafety review; radioimmunoassay work needs AERB licensing. The Drugs and Magic Remedies (Objectionable Advertisements) Act, 1954 restricts claims on hormone products. ICMR runs the national diabetes registry; thyroid and diabetes assays are among the highest-volume tests in Indian labs.

Key points

• Hormone – chemical messenger active at 10⁻⁹–10⁻¹² M; specificity lies in the target cell's receptor, not the hormone.
• Hydrophilic hormones – peptides and catecholamines; surface receptors, second messengers, response in seconds to minutes.
• Lipophilic hormones – steroids and thyroid hormones; carrier-bound in blood, nuclear receptors, response in hours via transcription.
• Second messengers – cAMP, cGMP, IP₃, DAG, Ca²⁺, PIP₃; relay and amplify the extracellular signal inside the cell.
• GPCR + G protein – 7-TM receptor acts as GEF for Gα; Gs activates and Gi inhibits adenylate cyclase; Gq activates PLC.
• PKA – inactive R₂C₂; four cAMP molecules release two active catalytic subunits that phosphorylate Ser/Thr targets.
• Amplification cascade – one hormone → adenylate cyclase → PKA → phosphorylase kinase → phosphorylase, giving 10⁶–10⁸-fold amplification.
• PLC pathway – PIP₂ → IP₃ (releases ER Ca²⁺) + DAG (with Ca²⁺ activates PKC).
• Calmodulin – Ca²⁺ sensor with four EF hands; forms the δ subunit of phosphorylase kinase, linking contraction to glycogenolysis.
• Insulin receptor – α₂β₂ receptor tyrosine kinase; autophosphorylation → IRS-1 → PI3K/Akt (metabolic) and Ras/MAPK (growth).
• GLUT4 translocation – Akt-dependent; the reason muscle and adipose glucose uptake is insulin-sensitive while liver (GLUT2) and brain (GLUT1/3) are not.
• Insulin resistance – serine phosphorylation of IRS-1 by TNF-α/FFA/S6K blocks tyrosine signalling.
• MAPK cascade – Ras → Raf → MEK → ERK; three tiers give amplification plus switch-like, sigmoidal output.
• JAK–STAT – single-step route from cytokine receptor to transcription; used by GH, prolactin, leptin, EPO; braked by SOCS.
• NO–cGMP – eNOS makes NO from arginine; NO activates soluble guanylyl cyclase → cGMP → PKG → vasodilation.
• Desensitisation – receptor phosphorylation by GRK, β-arrestin binding, internalisation and down-regulation of receptor number.
• Termination – Gα GTPase, phosphodiesterases, protein phosphatases (PP1, PP2A, PTP1B), Ca²⁺ pumps.
• Feedback – hypothalamus–pituitary–target-gland axes with negative feedback; high TSH + low free T₄ = primary hypothyroidism.

Formulas and examples

Example 1 — Receptor occupancy at physiological hormone level. A hepatocyte glucagon receptor has Kd = 1 × 10⁻⁹ M. Plasma glucagon in fasting is 1 × 10⁻¹⁰ M and rises to 5 × 10⁻¹⁰ M.

• Fasting: θ = 10⁻¹⁰/(10⁻¹⁰ + 10⁻⁹) = 10⁻¹⁰/1.1 × 10⁻⁹ = 0.0909 → 9.1% occupancy.
• Stimulated: θ = 5 × 10⁻¹⁰/(5 × 10⁻¹⁰ + 10⁻⁹) = 5/15 = 0.333 → 33.3% occupancy.
• A 5-fold rise in hormone gives a 3.67-fold rise in occupancy — and because the cascade amplifies, a far larger rise in glucose output. Note: occupancy of only a few per cent is enough for a full response in many systems (spare receptors).

Example 2 — Amplification in the cAMP cascade. Assume one activated receptor activates 100 Gs proteins; each Gsα activates one adenylate cyclase which makes 1,000 cAMP; 4 cAMP activate 1 PKA holoenzyme (so 1,000 cAMP → 250 PKA); each PKA phosphorylates 10 phosphorylase kinase molecules; each phosphorylase kinase activates 100 phosphorylase molecules; each phosphorylase releases 10,000 glucose-1-phosphate units.

• Per receptor: 100 × 250 = 2.5 × 10⁴ active PKA.
• × 10 = 2.5 × 10⁵ phosphorylase kinase; × 100 = 2.5 × 10⁷ phosphorylase.
• × 10,000 = 2.5 × 10¹¹ glucose-1-phosphate molecules from one hormone–receptor event.
• In moles: 2.5 × 10¹¹ / 6.022 × 10²³ = 4.15 × 10⁻¹³ mol ≈ 0.415 pmol of G1P (≈ 0.11 ng) per receptor activated. (Illustrative step factors; the order of magnitude, 10⁶–10⁸ before the final glycogen step, is the examinable point.)

Example 3 — Scatchard analysis of an insulin-binding assay. Binding data at equilibrium on adipocyte membranes: at F = 2 nM, B = 0.40 nM; at F = 10 nM, B = 1.00 nM.

• Point 1: B/F = 0.40/2 = 0.200 → 0.200 = (Bmax − 0.40)/Kd.
• Point 2: B/F = 1.00/10 = 0.100 → 0.100 = (Bmax − 1.00)/Kd.
• Subtract: 0.100 = (1.00 − 0.40)/Kd = 0.60/Kd → Kd = 6.0 nM.
• Substitute: 0.200 × 6.0 = Bmax − 0.40 → 1.20 = Bmax − 0.40 → Bmax = 1.60 nM of sites.
• Check with point 2: (1.60 − 1.00)/6.0 = 0.100 ✓.
• If the membrane preparation was 0.5 mg protein per mL of a 1 mL assay, sites = 1.60 nmol/L × 0.001 L = 1.60 pmol → 3.2 pmol per mg protein.

Example 4 — Interpreting a thyroid panel (clinical reasoning). A 22-year-old reports TSH 11.5 mIU/L (reference about 0.4–4.0), free T₄ 7.0 pmol/L (reference about 9–20), anti-TPO antibody positive.

• TSH high, free T₄ low → the pituitary is driving a failing gland → primary hypothyroidism, autoimmune (Hashimoto) given anti-TPO.
• Reasoning: feedback is intact, so the defect is below the pituitary. Had both been low, the lesion would be pituitary/hypothalamic (secondary/tertiary).
• Treatment logic: levothyroxine (T₄) replacement; dose titrated by TSH after 6–8 weeks because T₄'s half-life is about 7 days (≈5 half-lives to steady state). Reference intervals differ by assay platform and by trimester in pregnancy — always use the laboratory's own range.

Example 5 — Second-messenger concentration change for Ca²⁺. Resting cytosolic [Ca²⁺] = 100 nM; after IP₃ release it reaches 1.0 µM in a cell of volume 1 pL (10⁻¹⁵ m³ = 10⁻¹² L).

• Δ[Ca²⁺] = 1,000 − 100 = 900 nM = 9.0 × 10⁻⁷ M.
• Moles released into the cytosol = 9.0 × 10⁻⁷ mol/L × 10⁻¹² L = 9.0 × 10⁻¹⁹ mol.
• Ions = 9.0 × 10⁻¹⁹ × 6.022 × 10²³ = ≈ 5.4 × 10⁵ Ca²⁺ ions — about half a million ions give a 10-fold signal, which is why the gradient (10,000-fold across the membrane) must be maintained by ATP-driven pumps and why most of the released Ca²⁺ is actually buffered by proteins.

Where it is used

• Diagnostics industry: immunoassay (ELISA, CLIA, ECLIA) panels for TSH/free T₄, insulin, C-peptide, HbA1c-linked workups, cortisol, HCG, FSH/LH, PTH, vitamin D — the volume business of Thyrocare, Dr Lal PathLabs, Metropolis, SRL, Agappe and Transasia; roles as assay development scientist, QC analyst, application specialist.
• Pharmaceutical R&D and biosimilars: recombinant human insulin and analogues (Biocon, Wockhardt), EPO, GCSF, HCG, hGH; receptor-targeted drugs — β-blockers, salbutamol (β₂ agonist), losartan (angiotensin receptor), sildenafil (PDE5), metformin and DPP-4/SGLT2 agents in diabetes; kinase inhibitors in oncology (imatinib on BCR-ABL, erlotinib on EGFR).
• Clinical biochemistry and hospital labs: interpreting dynamic function tests — oral glucose tolerance test, dexamethasone suppression, ACTH (Synacthen) stimulation, TRH stimulation, HOMA-IR for insulin resistance — in NABL/ISO 15189 accredited laboratories.
• Agriculture, dairy and veterinary biotechnology: reproductive hormone protocols (GnRH, PGF2α) in cattle breeding at NDDB and state livestock boards; plant growth regulators acting through analogous receptor–kinase logic.
• Research and academia: CSIR labs (IICB, CCMB, CDRI, NIN), DBT institutes (NII, NIBMG, inStem), THSTI and medical college departments working on diabetes, obesity, thyroid disorders and GPCR pharmacology; signalling assays — cAMP ELISA, Ca²⁺ imaging with Fura-2, western blotting for phospho-Akt/phospho-ERK, luciferase reporter assays for CRE/GRE.
• CRO and bioanalytical services: Syngene, Sai Life, Aragen — receptor binding assays, PK/PD of hormone biologics, bioequivalence of insulin formulations under CDSCO's New Drugs and Clinical Trials Rules, 2019.

Interview questions

• Q: Why can the same hormone, epinephrine, cause both glycogen breakdown in liver and vasodilation in skeletal muscle vessels? A: Because the response is set by the receptor subtype and the G protein it couples to — β₂–Gs–cAMP in liver and muscle metabolism, versus smooth-muscle relaxation in some vessels and α₁–Gq–IP₃ vasoconstriction elsewhere. Specificity lies in the target cell, not the messenger.
• Q: What exactly is a second messenger, and name five. A: A small intracellular molecule whose concentration changes in response to receptor activation, relaying and amplifying the signal: cAMP, cGMP, IP₃, DAG, Ca²⁺ (and PIP₃).
• Q: How is the cAMP signal switched off? A: The Gα subunit hydrolyses its bound GTP (helped by RGS proteins) and reassociates with βγ; phosphodiesterase converts cAMP to 5′-AMP; protein phosphatase-1 reverses PKA's phosphorylations; and the receptor is phosphorylated and internalised.
• Q: How does cholera toxin cause diarrhoea at the molecular level? A: It ADP-ribosylates Gsα and abolishes its GTPase activity, so adenylate cyclase stays on; the sustained high cAMP in intestinal epithelium opens CFTR, driving Cl⁻ and water into the lumen.
• Q: Walk through insulin signalling to GLUT4. A: Insulin binds the α₂β₂ receptor → β-subunit autophosphorylation → IRS-1 tyrosine phosphorylation → PI3K makes PIP₃ → PDK1 activates Akt → Akt phosphorylates AS160/Rab-GAP machinery so GLUT4 vesicles fuse with the plasma membrane in muscle and adipose tissue.
• Q: Insulin and glucagon both act on liver glycogen enzymes. How are their opposite effects achieved? A: Glucagon raises cAMP and PKA activity, phosphorylating phosphorylase kinase (on) and glycogen synthase (off); insulin lowers cAMP via PDE-3B, activates PP1 which dephosphorylates both enzymes, and inhibits GSK-3 — so the same two enzymes are moved in the opposite direction.
• Q: Why do steroid hormone effects take hours while adrenaline acts in seconds? A: Steroids work by altering gene transcription, so the response needs mRNA and protein synthesis; adrenaline modifies the activity of enzymes already present, by phosphorylation.
• Q: A patient has high TSH with low free T₄. Where is the lesion? A: In the thyroid gland itself — primary hypothyroidism; the intact feedback loop is driving TSH up. If both were low the defect would be pituitary or hypothalamic.
• Q: What is receptor desensitisation and why does it matter clinically? A: Continued stimulation leads to receptor phosphorylation by GRKs, β-arrestin binding, internalisation and reduced receptor number, so the response fades — it explains tolerance to β₂-agonists and the use of pulsatile rather than continuous GnRH.
• Q: Why is a three-tier MAPK cascade used instead of one kinase? A: Each tier amplifies, allows separate regulatory inputs and scaffolding, and converts a graded input into a sharp, switch-like (sigmoidal, near all-or-none) output.
• Q: What is the molecular basis of insulin resistance in type 2 diabetes? A: Mainly post-receptor: serine/threonine phosphorylation of IRS-1 by kinases activated by TNF-α, free fatty acids and mTOR/S6K blocks its tyrosine phosphorylation, so PI3K/Akt signalling and GLUT4 translocation fail despite normal or high insulin.
• Q: How does nitroglycerine relieve angina, in signalling terms? A: It releases NO, which binds the haem of soluble guanylyl cyclase in vascular smooth muscle → cGMP rises → PKG lowers cytosolic Ca²⁺ and activates myosin light-chain phosphatase → vasodilation and reduced cardiac workload.
• Q: Why is calmodulin called a Ca²⁺ sensor rather than a Ca²⁺ buffer? A: Binding Ca²⁺ at its four EF-hand sites changes its conformation so it activates target enzymes (CaM kinases, phosphorylase kinase, NOS, calcineurin); its job is transduction, not merely sequestration.

Common mistakes

• Saying insulin "uses cAMP" — insulin is a receptor tyrosine kinase acting through PI3K/Akt and MAPK; it in fact *lowers* cAMP by activating PDE-3B.
• Confusing the second messenger with the hormone, or calling ATP/GTP second messengers; cAMP is the messenger, ATP is its substrate.
• Mixing up Gs, Gi and Gq — and forgetting that cholera toxin hits Gs while pertussis toxin hits Gi, with the same net result (cAMP up) by opposite mechanisms.
• Writing that IP₃ activates protein kinase C — PKC needs DAG plus Ca²⁺; IP₃ only releases the Ca²⁺ from the ER.
• Claiming steroid hormones act on cell-surface receptors or that peptide hormones enter the cell — the solubility of the hormone dictates the receptor's location.
• Forgetting signal termination and desensitisation in cascade answers; examiners allot marks for how the signal is switched off.
• Interpreting a hormone level without its regulator — a low T₄ means nothing until TSH is known; reporting total instead of free hormone, when only free hormone is active.
• Treating receptor occupancy as proportional to response: spare receptors mean a few per cent occupancy can give a maximal effect, and units/Kd values must be kept in consistent molar terms.

Related topics

Carbohydrate metabolism · Lipid metabolism · Amino acid and protein metabolism · Enzyme kinetics and regulation · Membrane structure and transport · Molecular biology: transcription and gene regulation