A drug research team has synthesized a novel oral drug that acts as an agonist at multiple adrenergic receptors. When administered in animals, it has been shown to produce urinary retention at therapeutic doses with the absence of other manifestations of adrenergic stimulation. The researchers are interested in understanding signal transduction and molecular mechanisms behind the action of the novel drug. Which of the following receptors would most likely transduce signals across the plasma membrane following the administration of this novel drug?

GqPCRs (Gq protein-coupled receptors)

GoPCRs (Go protein-coupled receptors)

GsPCRs (Gs protein-coupled receptors)

GtPCRs (Gt protein-coupled receptors)

GiPCRs (Gi protein-coupled receptors)

A 47-year-old woman presents to a local medical shelter while on a mission trip with her church to help rebuild homes after a hurricane. She has been experiencing severe nausea, vomiting, and diarrhea for the last 2 days and was feeling too fatigued to walk this morning. On presentation, her temperature is 99.2°F (37.3°C), blood pressure is 95/62 mmHg, pulse is 121/min, and respirations are 17/min. Physical exam reveals decreased skin turgor, and a stool sample reveals off-white watery stools. Gram stain reveals a gram-negative, comma-shaped organism that produces a toxin. Which of the following is consistent with the action of the toxin most likely involved in the development of this patient's symptoms?

Increased adenylyl cyclase activity

Increased membrane permeability

Cleavage of junctional proteins

Activation of receptor tyrosine kinase

A 25-year-old man presents to the physician with 2 days of profuse, watery diarrhea. He denies seeing blood or mucus in the stools. On further questioning, he reveals that he eats a well-balanced diet and generally prepares his meals at home. He remembers having some shellfish from a street vendor 3 days ago. He takes no medications. His past medical history is unremarkable. Which of the following mechanisms most likely accounts for this patient’s illness?

Tyrosine kinase phosphorylation

Tyrosine kinase dephosphorylation

Osmotic effect of intestinal contents

Inflammation of the gastrointestinal wall

A stool sample was taken from a 19-year-old male who presented with profuse watery diarrhea. He recently returned from a trip to Central America. A microbiologist identified the causative agent as a gram-negative, oxidase-positive, comma-shaped bacteria that is able to grow well in a pH > 8. Which of the following is a mechanism of action of the toxin produced by this bacteria?

Overactivation of adenylate cyclase by activation of Gs subunit by ADP-ribosylation

Overactivation of adenylate cyclase by inhibition of Gi subunit by ADP-ribosylation

Inactivation of the 60S ribosomal subunit by cleaving an adenine from the 28S rRNA

Overactivation of guanylate cyclase

Degradation of cell membranes by hydrolysis of the phospholipids

An investigator is studying the principles of cell-to-cell signaling of the autonomic nervous system. It is found that the adrenal medulla has receptors that, when activated, result in the immediate opening of Na+, Ca2+, and K+ channels, which subsequently leads to the secretion of epinephrine and norepinephrine. These receptors are structurally most similar to which of the following receptors?

NM receptors of the quadriceps femoris muscle

Alpha 1 receptors of the bladder neck

D2 receptors of the basal ganglia

An investigator is studying the structure of the amino-terminal of the Huntingtin protein using x-ray crystallography. The terminal region is determined to have an α-helix conformation. Which of the following forces is most likely responsible for maintaining this conformation?

Electrostatic side chain attraction

An 11-month-old boy is brought to a pediatrician by his parents with a recurrent cough, which he has had since the age of 2 months. He has required 3 hospitalizations for severe wheezing episodes. His mother also mentions that he often has diarrhea. The boy’s detailed history reveals that he required hospitalization for meconium ileus during the neonatal period. Upon physical examination, his temperature is 37.0°C (98.6ºF), pulse rate is 104/min, respiratory rate is 40/min, and blood pressure is 55/33 mm Hg. An examination of the boy’s respiratory system reveals the presence of bilateral wheezing and scattered crepitations. An examination of his cardiovascular system does not reveal any abnormality. His length is 67.3 cm (26.5 in) and weight is 15 kg (33 lbs). His sweat chloride level is 74 mmol/L. His genetic evaluation confirms that he has an autosomal recessive disorder resulting in a dysfunctional membrane-bound protein. Which of the following best describes the mechanism associated with the most common mutation that causes this disorder?

Defective maturation and early degradation of the protein

Decreased chloride transport through the protein

Disordered regulation of the protein

Decreased transcription of the protein due to splicing defect

Complete absence of the protein

A research team discovers a novel bacterial toxin that causes severe hypotension in infected patients. In vitro studies show the toxin ADP-ribosylates a specific amino acid on Gq alpha subunits, preventing their activation by GPCRs. Patients develop hypotension despite elevated levels of vasopressin, angiotensin II, and endothelin-1. Synthesize the pathophysiological mechanism explaining why multiple vasopressor hormones fail to maintain blood pressure in these patients.

ADP-ribosylation of Gq prevents PLC activation, blocking IP3-mediated calcium release and vascular smooth muscle contraction

The toxin depletes intracellular ATP preventing myosin-actin interaction

The toxin prevents receptor binding of vasopressor hormones through allosteric inhibition

ADP-ribosylation increases cAMP levels causing smooth muscle relaxation

The toxin activates Gi proteins causing excessive vasodilation that overwhelms vasoconstrictor signals

G-protein coupled receptors for USMLE Step 2 CK: High-Yield Biochemistry Lessons

GPCR Structure - The Cell's Mailbox

Single Polypeptide Chain: A serpentine receptor that crosses the plasma membrane 7 times (heptahelical).
Key Domains:
- Extracellular N-terminus: Faces the exterior to bind ligands (e.g., hormones, neurotransmitters).
- Intracellular C-terminus & Loops: Face the cytosol to couple with G-proteins.
Mechanism: Ligand binding alters receptor conformation, activating the associated G-protein.

GPCR with 7 transmembrane domains, N- and C-termini

⭐ GPCRs are the largest family of cell surface receptors and are the target of approximately 30-40% of all modern medicinal drugs.

G-Protein Cycle - The On/Off Switch

Inactive State: Gαβγ heterotrimer is "off" when GDP is bound to the Gα subunit.
Activation ("On"):
- Ligand-bound GPCR acts as a Guanine Nucleotide Exchange Factor (GEF).
- It swaps GDP for GTP on the Gα subunit.
- Gα-GTP dissociates from the βγ dimer; both can now modulate effector proteins.
Inactivation ("Off"):
- Gα's intrinsic GTPase activity hydrolyzes GTP back to GDP ($GTP \rightarrow GDP$).
- The Gα-GDP subunit re-binds the βγ dimer, terminating the signal.

G-protein cycle: activation, dissociation, and inactivation

⭐ High-Yield Toxin: Cholera toxin constitutively activates the Gs protein by ADP-ribosylation, locking it in the GTP-bound "on" state. This causes a dramatic ↑ in cAMP and life-threatening secretory diarrhea.

Major Pathways - The Message Cascades

G-proteins are molecular switches linking receptor activation to intracellular second messengers, amplifying the initial signal.

cAMP Pathway:
- Gs (stimulatory): Activates adenylyl cyclase → converts ATP to cAMP.
- Gi (inhibitory): Inhibits adenylyl cyclase → ↓ cAMP.
- cAMP activates Protein Kinase A (PKA).
Phosphoinositol Pathway (Gq):
- Activates Phospholipase C (PLC).
- PLC cleaves PIP₂ into IP₃ and DAG.
- IP₃: Mobilizes intracellular Ca²⁺ from the endoplasmic reticulum.
- DAG: Activates Protein Kinase C (PKC), along with Ca²⁺.

GPCR signaling with Gs and Gi proteins

⭐ Toxin Targets: Cholera toxin ADP-ribosylates and constitutively activates Gsα, leading to massive ↑cAMP and secretory diarrhea. Pertussis toxin ADP-ribosylates and inactivates inhibitory Giα, also causing a net ↑cAMP, leading to a paroxysmal cough.

Regulation & Disease - Signal Failure

Receptor Desensitization (Homologous)
- G-protein-coupled receptor kinases (GRKs) phosphorylate activated receptor C-terminus.
- β-arrestin binds to phosphorylated sites, blocking further G-protein activation.
- Promotes receptor internalization via clathrin-coated pits, reducing surface receptor density.

GPCR and non-GPCR beta-arrestin mediated desensitization

Toxin-Mediated Signal Disruption
- Cholera Toxin: ADP-ribosylates Gαs → inhibits GTPase → constitutive activation → ↑cAMP.
- Pertussis Toxin: ADP-ribosylates Gαi → traps in GDP-bound state → blocks inhibition of adenylyl cyclase → ↑cAMP.

⭐ Exam Favorite: Cholera toxin causes life-threatening diarrhea by locking Gαs in an "ON" state, leading to massive, unregulated cAMP production and CFTR-mediated chloride and water secretion into the gut lumen.

High‑Yield Points - ⚡ Biggest Takeaways

G-Protein Coupled Receptors (GPCRs) are the largest family of cell surface receptors, defined by seven transmembrane helices.

Activation involves ligand binding causing a conformational change, which allows the G-protein to exchange GDP for GTP.

The activated α-subunit dissociates and modulates effectors like adenylyl cyclase (cAMP) and phospholipase C (IP₃/DAG).

Signal termination is achieved through intrinsic GTPase activity of the α-subunit, which hydrolyzes GTP to GDP.

Represents the largest single class of drug targets.

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