3. Secondary messengers 1

What is the primary function of a phosphorylation cascade in signal transduction?

It allows for signal amplification and propagation through sequential kinase activation.

Which type of protein kinase is most common in intracellular signalling?

Serine/Threonine kinases.

What molecule is bound to a G protein when it is in its active state?

GTP (Guanosine triphosphate).

How is the signal switched off in GTP-binding protein pathways?

The protein's intrinsic GTPase activity hydrolyses bound GTP into GDP.

In the adrenaline signalling cascade, what is the direct effect of activating the G_{s} protein?

It activates the enzyme adenylate cyclase.

What is the relationship between the \alpha-subunit and the \beta\gamma-complex in an inactive G protein?

They are bound together as an inactive heterotrimer with GDP attached to the \alpha-subunit.

What is the function of a Guanine nucleotide Exchange Factor (GEF) in G protein signalling?

It facilitates the exchange of GDP for GTP to activate the G protein.

Which second messenger system is activated by G_{s}-coupled receptors?

The adenylate cyclase system, which increases cyclic AMP (cAMP).

Which enzyme is activated by G_{q} proteins to generate IP_{3} and DAG?

Phospholipase C (PLC).

What are the two downstream effects of Phospholipase C activation?

Release of Ca^{2+} from internal stores and activation of Protein Kinase C (PKC).

How do G_{i} proteins affect cellular cAMP levels?

They inhibit adenylate cyclase, leading to a decrease in cAMP levels.

What is the specific molecular mechanism by which Cholera toxin causes permanent activation of G_{s}?

It ADP-ribosylates an arginine residue on the \alpha-subunit, abolishing its GTPase activity.

What is the clinical consequence of the permanent cAMP increase caused by Cholera toxin in the intestine?

Phosphorylation of Cl^{-} channels leading to an efflux of water and electrolytes (diarrhoea).

How does Pertussis toxin interfere with G_{i} protein signalling?

It ADP-ribosylates G_{i} at a cysteine residue, preventing its interaction with the receptor.

What is the initial step in the activation of the JAK-STAT signalling pathway?

Ligand-induced receptor dimerization.

Why is the JAK-STAT pathway considered distinct from Receptor Tyrosine Kinases (RTKs)?

The receptors lack intrinsic tyrosine kinase activity and must associate with Janus kinases (JAKs).

What specific domain do downstream transducers like Grb2 use to recruit to phosphorylated tyrosine residues?

SH2 domains.

What is the role of MEK in the MAPK pathway?

It is a dual-specificity kinase that phosphorylates both serine/threonine and tyrosine residues.

Which small G-protein is responsible for regulating the MAPK/ERK pathway?

Ras.

What is the primary cellular role of Rab proteins?

They are involved in the regulation of vesicular transport.

Which enzyme converts ATP into cyclic AMP?

Adenylate cyclase.

How does soluble adenylate cyclase differ from transmembrane isoforms regarding regulation?

It is not regulated by G proteins but is activated by calcium ions and bicarbonate.

What are the two main types of G proteins that regulate adenylate cyclase activity?

Gs proteins (stimulatory) and Gi proteins (inhibitory).

What occurs when cAMP binds to the regulatory subunits of Protein Kinase A (PKA)?

The catalytic subunits are released and become active.

Which nuclear protein is the main substrate for PKA-mediated transcriptional regulation?

CREB (cAMP response element binding protein).

What is the direct metabolic outcome of CREB activation by PKA?

Expression of genes for catabolic pathways like glycolysis and lipolysis.

Which enzyme catalyses the synthesis of Nitric Oxide from L-arginine?

Nitric oxide synthase (NOS).

Which NOS isoform is constitutive and located in neurons?

nNOS (or NOS I).

What is the intracellular target of Nitric Oxide once it diffuses into smooth muscle cells?

Soluble guanylate cyclase (sGC).

What second messenger is produced by the activation of soluble guanylate cyclase?

cGMP (cyclic guanosine monophosphate).

How does shear stress from blood flow activate eNOS?

It activates G proteins leading to AC/cAMP signaling and eNOS phosphorylation.

Which reactive species is formed when NO reacts with a superoxide anion?

Peroxynitrite (ONOO-).

What is S-nitrosylation in the context of protein regulation?

The covalent modification of cysteine thiol groups by NO to form S-nitrosothiols.

Which system is responsible for the enzymatic reversal of protein S-nitrosylation?

The thioredoxin/thioredoxin reductase system (Trx/TR).

How does Sildenafil (Viagra) prolong smooth muscle relaxation?

It inhibits phosphodiesterase-5 (PDE5), preventing the breakdown of cGMP.

Which NOS isoform is expressed in macrophages as part of the immune response?

iNOS (inducible Nitric Oxide Synthase).

What transcription factor is released by IKK to induce iNOS gene expression?

NF-kB.

Which cytokine pathway involving JAK-STAT stimulates the expression of iNOS?

The IFN-gamma pathway.

What is the role of the Cationic amino acid transporter (CAT) in NO synthesis?

It delivers arginine to caveolae where eNOS is located.

Which intermediate metabolite of adrenaline metabolism is used as a clinical marker for pheochromocytoma?

Vanillylmandelic acid (VMA).

What is the typical concentration of free cytosolic Ca^{2+} in a resting cell?

Approximately 0.1 \mu mol/L.

Which Ca^{2+} pump is responsible for transporting calcium from the cytosol into the lumen of the endoplasmic reticulum?

SERCA (Sarco/Endoplasmic Reticulum Ca^{2+}-ATPase).

What is the stoichiometry of the Na+/Ca2+ exchanger (NCX) in the plasma membrane?

It moves 3 Na^{+} into the cell for every 1 Ca^{2+} extruded.

Which Ca^{2+}-binding protein acts as a buffer specifically within the endoplasmic reticulum?

Calreticulin.

What is the structural homologue of Calmodulin in skeletal muscle?

Troponin C.

How does the Ca^{2+}/Calmodulin complex regulate smooth muscle contraction?

It modulates the activity of the actin-myosin complex.

What defines 'paracrine' signalling?

A cell secretes a molecule that acts locally on neighbouring target cells.

Which natriuretic peptide receptor lacks intrinsic guanylyl cyclase activity and acts as a clearance receptor?

NPR-C.

What is the role of Protein Kinase G (PKG) in the regulation of Ca^{2+} in the heart?

It phosphorylates phospholamban (PLN) to regulate the SERCA pump.

Which G protein pathway activated by the Calcium-Sensing Receptor (CaSR) leads to changes in the cytoskeleton via RhoA?

The G_{12/13} pathway.

What clinical condition is specifically associated with the secretion of Parathyroid Hormone-related Protein (PTHrP) by tumours?

Hypercalcaemia.

What is the rate-limiting enzyme in the synthesis of catecholamines from tyrosine?

Tyrosine hydroxylase (TH).

Which enzyme converts dopamine into norepinephrine inside synaptic vesicles?

Dopamine \beta-hydroxylase (DBH).

In which tissue is norepinephrine primarily converted to epinephrine by PNMT?

The adrenal medulla.

What is the primary intracellular location of the degradation enzyme monoamine oxidase (MAO)?

The mitochondria.

Which adrenergic receptor subtype is coupled to G_{i} and inhibits cAMP production?

\alpha_{2}-adrenergic receptors.

What is the hierarchy of agonist sensitivity for \beta-adrenergic receptors?

Isoproterenol > epinephrine > norepinephrine.

How does adrenaline inhibit insulin secretion?

By acting through \alpha-adrenergic receptors in the pancreas.

What is the effect of adrenaline on glycogen metabolism in the liver?

It activates glycogen phosphorylase (glycogenolysis) and inhibits glycogen synthase (glycogenesis).

Which intracellular receptor acts as a master regulator of xenobiotic detoxification in the liver?

Pregnane X Receptor (PXR).

What DNA-binding partner is required for PXR to activate its target genes?

RXR (Retinoid X Receptor).

Which metabolic product of mutated IDH enzymes acts as an 'oncometabolite' by altering the epigenome?

D-2-hydroxyglutarate (D-2HG).

How does D-2HG interfere with epigenetic regulation?

It competitively inhibits \alpha-ketoglutarate-dependent dioxygenases like TET enzymes.

What is the function of TET (Ten-Eleven Translocation) enzymes in the cell?

They are involved in DNA demethylation.

What general reaction occurs during Phase I of procarcinogen biotransformation?

Hydroxylation, primarily catalysed by cytochrome P450 monooxygenases.

What is the chemical goal of Phase II biotransformation (conjugation)?

To increase the polarity of a substance to facilitate its excretion in urine.

In chemical carcinogenesis, what is the fundamental difference between an initiator and a promoter?

Initiators cause direct, irreversible DNA mutations, while promoters stimulate proliferation of mutated cells without being mutagens.

Which enzyme converts Tyrosine to DOPA during melanin synthesis in the skin?

Tyrosine hydroxylase (regulated by MSH).

Which small GTPase is involved in nuclear transport processes?

Ran.

What is the relationship between cAMP and the phosphorylation of Cl^{-} channels in the context of Cholera?

cAMP activates PKA, which then phosphorylates the channels to trigger electrolyte efflux.

Which lipid mediator is known to induce Ca^{2+} release from the endoplasmic reticulum?

Inositol 1,4,5-trisphosphate (IP_{3}).

Which enzyme breaks down cGMP into 5'-GMP to terminate the signal?

Phosphodiesterase (PDE).

How does Nitric Oxide affect platelet function?

It inhibits platelet aggregation and adhesion.

Which NOS isoform is 'inducible' and does not depend on high Ca^{2+} levels for its activity?

iNOS (NOS II).

What is the stoichiometry of Na^{+} to Ca^{2+} in the NCX exchanger?

3 Na^{+} ions are exchanged for 1 Ca^{2+} ion.

Which natriuretic peptide is primarily known as 'C-type'?

CNP.

Which catecholamine is synthesized from norepinephrine via the enzyme PNMT?

Epinephrine (Adrenaline).

Which G protein subunit possesses the intrinsic GTPase activity?

The \alpha-subunit (G\alpha).

What is the role of IRF-1 in immune signalling?

It is a transcription factor that stimulates the expression of iNOS.

Which phase II conjugation molecule is commonly used to detoxify carcinogens?

Glucuronic acid (or glutathione/acetate).