Biosignaling

Introduction: Why Cells Need to Talk

Cells constantly send and receive molecular messages to coordinate everything from a muscle twitch to insulin release. Signal transduction converts an extracellular signal (a ligand, a voltage change, a photon) into an intracellular response.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

The MCAT tests three receptor classes here: gated ion channels, receptor enzymes (mainly receptor tyrosine kinases), and G protein-coupled receptors (GPCRs). Each uses a distinct mechanism and timescale. Build your model around mechanism first.

General Principles of Signal Transduction

Must know

The universal logic of signaling:

1. Reception — a ligand (or stimulus) activates a receptor, usually at the cell surface.
2. Transduction — the receptor changes activity, triggering intracellular events.
3. Response — a cellular change (gene expression, enzyme activation, ion flux).
4. Termination — the signal shuts off (ligand dissociates, second messengers degrade, receptors desensitize).

A key theme is signal amplification: one receptor can activate many downstream effectors, so tiny hormone concentrations produce large responses.

Quick check: Why is signal amplification physiologically advantageous?

Answer: It allows a very small amount of signal (e.g., nanomolar epinephrine) to produce a massive response (e.g., mobilizing glucose stores), making signaling extremely sensitive and efficient.

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Gated Ion Channels

The Core Idea

Must know

Ion channels are membrane proteins that, when open, let specific ions flow down their electrochemical gradient (concentration gradient + electrical potential). The key question for any channel: what controls whether it is open or closed? The two MCAT answers are voltage and ligand binding.

Know the logic

Each ion has an equilibrium (Nernst) potential — the voltage at which electrical force balances the concentration gradient, so there's no net flow (Na⁺ ≈ +60 mV, K⁺ ≈ −90 mV). When a channel opens, the ion's direction is set by how membrane potential compares to that ion's equilibrium potential. At rest (−70 mV), opening a Na⁺ channel drives Na⁺ in; opening a K⁺ channel drives K⁺ out. (Quantitative Nernst calc is in the Electrochemistry guide.)

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Voltage-Gated Ion Channels

Must know

Voltage-gated channels open or close in response to membrane potential changes. They drive the action potential in neurons and muscle.

Voltage-Gated Na⁺ Channels and the Action Potential

Must know

The action potential phases:

Phase	Event	Channel(s)
Resting ($-70$ mV)	Polarized, most channels closed	Leak K⁺ channels
Depolarization	Rapid rise toward $+40$ mV	Voltage-gated Na⁺ channels open
Repolarization	Returns toward $-70$ mV	Na⁺ channels inactivate; voltage-gated K⁺ channels open
Hyperpolarization	Briefly below rest	Slow K⁺ channel closing
Refractory periods	No (absolute) or harder (relative) firing	Na⁺ channels inactivated/recovering

The critical detail: voltage-gated Na⁺ channels have an activation gate (opens fast on depolarization) and an inactivation gate (closes more slowly, ending Na⁺ influx). This dual-gate design creates the refractory period. Voltage-gated K⁺ channels open slowly and repolarize. The Na⁺/K⁺-ATPase (a pump, not a channel) restores resting gradients using ATP (3 Na⁺ out / 2 K⁺ in).

Quick check: A neuron is in its absolute refractory period. Why can't a second action potential fire?

Answer: Voltage-gated Na⁺ channels are inactivated — the inactivation gate is closed. They cannot reopen by depolarization until they return to the resting (closed) conformation during repolarization.

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Ligand-Gated Ion Channels

Must know

Ligand-gated channels (ionotropic receptors) open directly when a ligand binds — the receptor IS the channel, with no second-messenger cascade. This makes them among the fastest signaling mechanisms (milliseconds).

Nicotinic acetylcholine receptor (nAChR): the canonical example. Acetylcholine (ACh) binding opens a cation pore (Na⁺/K⁺); net Na⁺ influx depolarizes the postsynaptic cell. Found at the neuromuscular junction and autonomic ganglia; nicotine mimics ACh here.

Passage-level

Other ligand-gated channels:

• GABA-A receptor: a Cl⁻ channel; GABA (main inhibitory neurotransmitter) binding lets Cl⁻ in, hyperpolarizing the cell (IPSP). Benzodiazepines and barbiturates enhance it (sedation).
• AMPA receptors: ligand-gated cation channels — fast excitatory signaling.
• NMDA receptors: require both glutamate binding and depolarization to relieve a Mg²⁺ block, making them coincidence detectors — the basis of long-term potentiation (LTP) in learning/memory. They also conduct Ca²⁺.

Quick check: Why is the NMDA receptor a "coincidence detector"?

Answer: It requires simultaneous presynaptic glutamate release AND postsynaptic depolarization to displace the Mg²⁺ block. Neither alone opens the channel, so it activates only when two signals occur together.

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Receptor Enzymes

Receptor Tyrosine Kinases (RTKs)

Must know

Receptor tyrosine kinases are transmembrane receptors whose intracellular domain has intrinsic kinase activity — they phosphorylate tyrosine residues (using ATP) rather than moving ions, initiating a phosphorylation cascade.

Must know the mechanism:

1. Ligand binding (usually a growth factor — EGF, PDGF, insulin).
2. Dimerization: two monomers come together.
3. Autophosphorylation: each monomer phosphorylates tyrosines on the other.
4. Docking: phospho-tyrosines recruit downstream signaling proteins (e.g., SH2-domain proteins).

Know the logic

Insulin receptor: the high-yield RTK. It's a pre-formed dimer (held by disulfide bonds). Insulin binding → autophosphorylation → activation of PI3K → Akt signaling. Net effects: increased glucose uptake (GLUT4 translocation in muscle/fat), glycogen, lipid, and protein synthesis.

Know the logic

RAS–MAPK pathway: another major RTK output:

$\text{RTK} \to \text{Grb2/SOS} \to \text{RAS (GDP} \to \text{GTP)} \to \text{RAF} \to \text{MEK} \to \text{ERK} \to \text{gene expression}$

RAS is a small GTPase: active with GTP, inactive after hydrolyzing GTP to GDP. Mutations locking RAS in the GTP-bound (ON) state are common in cancers (RAS is a proto-oncogene).

Quick check: A drug blocks dimerization of the EGF receptor. What downstream effect?

Answer: Without dimerization, autophosphorylation can't occur, so RAS–MAPK and other growth cascades aren't activated. Cell proliferation falls — the basis for anti-cancer drugs targeting growth-factor receptors.

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G Protein-Coupled Receptors

The GPCR Architecture and Logic

Must know

GPCRs are the largest family of cell-surface receptors, sharing a 7-transmembrane structure. They signal through heterotrimeric G proteins (subunits Gα, Gβ, Gγ) that act as molecular switches: inactive Gα binds GDP, active Gα binds GTP.

The GTP switch cycle:

1. Ligand binds GPCR → receptor acts as a GEF, causing Gα to swap GDP for GTP.
2. Gα-GTP dissociates from Gβγ; both can activate downstream effectors.
3. Gα's intrinsic GTPase activity slowly hydrolyzes GTP → GDP, shutting itself off (signal termination).
4. Gα-GDP reassociates with Gβγ, ready for the next cycle.

Passage-level

Anything blocking GTPase locks signaling ON — e.g., cholera toxin (modifies Gαs) and pertussis toxin (modifies Gαi); see Common Confusions.

The Three Major G Protein Pathways

Must know

These three pathways and their second messengers are core:

Gₛ pathway (cAMP/PKA): Gαs activates adenylyl cyclase (AC), converting ATP → cAMP, the classic second messenger.

$\ce{ATP ->[\text{adenylyl cyclase}] cAMP + PP_i}$

cAMP activates protein kinase A (PKA), which phosphorylates Ser/Thr residues on many targets.

Classic example — epinephrine: binds a β-adrenergic receptor (Gαs) → ↑cAMP → ↑PKA → activates glycogen breakdown while inhibiting glycogen synthase. One second messenger turns one pathway on and the opposing one off. Termination: phosphodiesterase (PDE) degrades cAMP → 5′-AMP; caffeine inhibits PDE, prolonging signaling.

Gᵢ pathway: Gαi inhibits adenylyl cyclase → ↓cAMP → ↓PKA (opposite of Gαs). Examples: μ-opioid and α₂-adrenergic receptors.

Gq pathway (IP₃/DAG/Ca²⁺): Gαq activates phospholipase C (PLC), which cleaves membrane PIP₂ into two second messengers:

$\ce{PIP2 ->[\text{PLC}] IP3 + DAG}$

• IP₃: soluble; opens Ca²⁺ channels on the ER, releasing Ca²⁺ into the cytosol.
• DAG: stays in the membrane; with Ca²⁺, activates protein kinase C (PKC).
• Cytosolic Ca²⁺ also binds calmodulin → activates CaM kinases.

G Protein	Effector	Second Messenger	Kinase	Example
Gαs	↑ Adenylyl cyclase	↑ cAMP	PKA	β-adrenergic, glucagon
Gαi	↓ Adenylyl cyclase	↓ cAMP	↓ PKA	μ-opioid, α₂-adrenergic
Gαq	↑ Phospholipase C	↑ IP₃ + DAG + Ca²⁺	PKC, CaM kinase	α₁-adrenergic, oxytocin

Quick check: Epinephrine binds an α₁-adrenergic receptor (Gαq) in smooth muscle. Trace the path to increased cytosolic Ca²⁺.

Answer: Epinephrine → α₁ receptor → Gαq → PLC cleaves PIP₂ → IP₃ → binds ER receptor → Ca²⁺ released into cytosol → Ca²⁺/calmodulin → smooth muscle contraction.

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Signal Amplification, Integration, and Termination

Amplification

Know the logic

The cascade design of GPCR and RTK pathways amplifies signals enormously: one active GPCR activates hundreds of G proteins, each adenylyl cyclase makes many cAMP, each PKA phosphorylates many substrates. Modest gain at each step compounds.

Receptor Desensitization and Down-Regulation

Know the logic

Cells avoid overstimulation by desensitization — GPCRs phosphorylated by GRKs recruit β-arrestin, which blocks G protein coupling and triggers receptor endocytosis (down-regulation: fewer surface receptors with prolonged ligand). This explains drug tolerance (e.g., chronic opioid use down-regulates μ-opioid receptors).

Quick check: A patient on high-dose β-agonist asthma therapy for months notices it works less well. Why?

Answer: Prolonged exposure causes GRK phosphorylation of the β₂-adrenergic receptor, β-arrestin recruitment, and internalization (down-regulation). Fewer surface receptors mean a smaller cAMP response — desensitization/tolerance.

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Common Confusions & Tricks

1. Ionotropic vs. metabotropic: Ionotropic = ion channel IS the receptor (fast, direct, ms). Metabotropic = receptor → G proteins → second messengers (slower, amplified). Focus on speed and whether ion flow is direct.

2. Gs vs. Gi: Gαs → Stimulates AC → ↑cAMP. Gαi → Inhibits AC → ↓cAMP. Mnemonic: "Stimulatory = Sky-high cAMP; Inhibitory = cAMP goes down."

3. IP₃ vs. DAG — location matters: IP₃ is water-soluble and travels to the ER; DAG is lipid-soluble and stays in the membrane. The second messenger that releases Ca²⁺ from internal stores is IP₃.

4. Cholera vs. pertussis toxin: Both lock G proteins, but oppositely and on different subunits. Cholera → Gαs stuck active → ↑↑cAMP in gut → watery diarrhea. Pertussis → Gαi can't be activated → AC unopposed → ↑cAMP.

5. RTK dimerization vs. GPCR: RTKs must dimerize to activate (insulin receptor is the constitutively dimerized exception). GPCRs are monomeric and don't require dimerization.

6. Refractory period and gates: The absolute refractory period is caused by Na⁺ channel inactivation, not just closure. A closed channel can be reopened by depolarization; an inactivated one cannot until it resets.

7. GTPase = OFF, GEF = ON: RAS and Gα follow the same logic. Activated receptor = GEF (GDP → GTP, ON). Intrinsic GTPase (sped by GAPs) does GTP → GDP (OFF). Oncogenic RAS disables GTPase → stuck ON.

8. "Second messenger" identity: Classic second messengers are cAMP, cGMP, IP₃, DAG, Ca²⁺. Note that DAG and IP₃ both come from one PIP₂ cleavage — one Gαq event makes two messengers.

9. Epinephrine acts on multiple receptors: Epinephrine hits Gαs (β), Gαi (α₂), or Gαq (α₁) depending on tissue. Seemingly contradictory effects reflect different receptor subtypes.

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Key Signaling Relationships

Mechanistic schemes to know (not equations to plug into). The quantitative Nernst equation lives in the Electrochemistry guide.

Relationship	Meaning	When to Use
$\ce{ATP ->[\text{adenylyl cyclase}] cAMP + PP_i}$	Gαs activates adenylyl cyclase; cAMP is the second messenger	Tracing Gαs signaling; why PDE inhibition raises cAMP
$\ce{PIP2 ->[\text{PLC}] IP3 + DAG}$	Gαq activates PLC; two second messengers from one lipid	Tracing Gαq signaling; Ca²⁺ release and PKC
$\text{Driving force} = V_m - E_{ion}$	Sign tells direction of net ion flux when a channel opens	Determining whether an ion flows in or out