pcb 3134 exam 3

multicellular organisms coordinate the activities of specialized cells by

synthesizing and releasing regulatory chemical messengers into their environment

intercellular signaling molecular are categorized based on

the physical distance between their site of production and their target tissues

endocrine signals

produced fa from target tissues and travel through the circulatory system (bloodstream) to reach target cells throughout the body

paracrine signals

diffusible local mediators that act over a short range on nearby target cells

juxtacrine signals

require direct physical contact between the membranes of sending and receiving cells

autocrine signals

act directly on the same cell that synthesizes and secretes them

primary messenger or ligand

when a signaling molecule reaches its target cell, it acts as this by binding to a specific receptor located either on the plasma membrane or inside the cell

signal transduction

the process by which ligand binding induces a cascade of intracellular changes that alters cell behavior or gene expression

receptor-ligand interactions resemble

enzyme-substrate interactions, relying on noncovalent bonds within a specialized binding pocket containing specific amino acid side chains

as ligand concentration increases,

the system reaches saturation when most receptors are occupied

receptor affinitig

is quantified by the dissociation constant (K_d), which is defined as the ligand concentration at which 50% of the total receptors are bound

agonists

natural or synthetic molecules that bind to a receptor and activate its signaling cascade

antagonists

synthetic or natural compounds that bind to a receptor without triggering a response, physically blocking the binding of natural ligands

signaling molecules fall into two main chemical classes

1. hydrophilic ligands

2. hydrophobic ligands

hydrophilic ligands

• include proteins, small peptides, amino acid derivatives, and nucleotides

• because they can’t cross the hydrophobic lipid bilayer, they bind exclusively to cell-surface transmembrane receptors

hydrophobic ligands

include lipid-soluble compounds (such as steroid hormones) that diffuse across the plasma membrane to bind intracellular cytosolic or nuclear receptors, directly regulating gene transcription

cells use four major categories of signal transduction pathways

• ligand-gated ion channels\

• G protein-coupled receptors (GPCRs)

• receptor kinases

• nuclear receptors

signal amplifications

cells elicit large physiological responses from small quantities of extracellular ligands; at each step, an activated signaling intermediate stimulates the generation of multiple downstream molecules

G protein-coupled receptors (GPCRs)

membrane receptors that undergo a conformational change upon ligand binding to activate a G protein

GPCR structure

all share a conserved structure consisting of seven transmembrane a-helices traversing the plasma membrane, with an extracellular ligand-binding domain, and a cytosolic G protein-binding domain

heterotrimeric G protein molecular switch

• large ones consist of three distinct subunits: G_a, G_B, and G_y

• they act as molecular switches whose state depends on whether GTP (guanosine triphosphate) or GDP (guanosine diphosphate) is bound of the G_a subunit

heterotrimeric G protein molecular switch steps

1. resting state: the GPCR is unbound, and G_a is bound to GDP in a complex with G_By

2. ligand binding and exchange: ligand binding induces a conformational change in the GPCR, causing it to associate with the G protein; G_a releases GDP and acquires a GTP molecule

3. subunit separation G_a-GTP detaches from the G_By subunit

4. effector regulation G_a-GTP or free G_By interacts with specific target enzymes or ion channels

5. GTP hydrolysis: the intrinsic GTPase activity of G_a hydrolyzes GTP back to GDP

6. reassociation: inactive G_a-GDP rebinds G_By, terminating signal transduction

G_By subunit signaling example

while G_a frequently drives downstream signaling, G_By complexes can also directly regulate target proteins

for example, when acetylcholine (ACh) binds muscarinic ACh receptors in cardiac tissue, the released G_By subunit binds directly to membrane potassium (K⁺) channels, opening them and allowing K⁺ to flow out of the cell

adenylyl cyclase & cyclic AMP (cAMP) pathway (G_sa)

• synthesis: cyclic AMP (cAMP) is synthesized from cytosolic ATP by the enzyme adenylyl cyclase (this is activate when bound to an active

• degradation: intracellular cAMP is hydrolyzed to AMP by the enzyme phosphodiesterase

• protein kinase A (PKA) activation

  • PKA consists of two regulatory subunits and two catalytic subunits

  • in the absence of cAMP regulatory subunits bind and inhibit catalytic subunits

  • binding of cAMP to regulatory subunits induces a conformational change that causes active catalytic subunits to detach

  • free catalytic subunits phosphorylate serine or threonine residues on target proteins using ATP

phospholipase C, IP₃ and diacylglycerol (DAG) pathway (G_qa)

• activation: ligand binding to a G_q-couped receptor activates G_qa-GTP which binds and activates phospholipase C (PLC_B)

• cleavage: activate PLC_B cleaves plasma membrane PIP₂ (phosphatidylinositol-4,5-biphosphate) into two second messengers:

  • IP₃ (inositol-1,4,5-triphosphate): soluble molecule release into the cytosol

  • DAG (diacylglycerol): hydrophobic molecule that remains in the plasma membrane

• IP₃ function: diffuses to the ER and binds ligand-gated receptor channels, releasing stored calcium ions into the cytosol

• PKC activation: membrane-bound DAG and released calcium ions jointly activate protein kinase C which phosphorylates serine and threonine residues on specific cellular targets

cytosolic calcium ion concentration is kept low relative to the extracellular environment and ER lumen through four transport systems

1. plasma membrane Ca²⁺ ATPases: pump calcium ions out of the cell

2. ER membrane Ca²⁺ ATPases: sequester calcium ions inside the ER lumen

3. sodium-calcium exchangers: use the sodium gradient to pump calcium ions out of the cell

4. mitochondrial transporters: transport excess calcium ions into the mitochondrial matrix

calcium release mechanisms

• IP₃ receptor channels: open upon binding IP₃

• ryanodine receptor channels: calcium-sensitive channels on the ER that open in response of elevated cytosolic calcium ions, causing calcium-induced calcium release

calmodulin

mediates many calcium-regulated processes

• consists of a central arm with two globular hands at each end

• each hand binds two calcium ions for a total of 4 calcium ions per calmodulin

calcium-calmoduling complex

binding four calcium ions to a calmodulin induces a conformational change, forming this complex that wraps around target proteins to regulate their activity

protein kinase-associated receptors function both as

cell-surface receptors and as active enzymes (kinases) that transmit extracellular signals via intracellular phosphorylation cascades

based on the specific amino acid residues they phosphorylate, protein kinase-associated receptors are classified as either

tyrosine kinases or serine/threonine kinases

receptor tyrosine kinases (RTKs) are activated by

various growth factor ligands that regulate cell growth, proliferation, and differentiation, including

• epidermal growth factor (EGF)

• insulin and insulin-like growth factor-1 (IGF-1)

• fibroblast growth factor (FGF)

• nerve growth factor (NGF)

molecular structure of RTKs

they consist of a single polypeptide chain containing a single hydrophobic transmembrane segment

• extracellular portion houses ligand-binding domain

• cytosolic portion contains the tyrosine residues

for RTKs, in some instances the receptor and the tyrosine kinase exist as two distinct proteins where

a nonreceptor tyrosine kinase binds noncovalently to the receptor upon ligand binding

ligand binding and aggregation

ligand binding to the extracellular domain induces receptor aggregation

• in some RTKs, ligands induce physical dimerization

• in other RTKs (ex. EGF receptor), ligand binding causes receptor clustering

autophosphorylation

bringing the cytosolic kinase domain into close physical proximity activates their enzymatic activity; the adjacent receptors cross-phosphorylate specific tyrosine residues on each other’s cytosolic tails

recruitment of SH2 domain proteins

• autophosphorylation creates specific docking sites on the cytosolic tail of the RTK

• cytosolic effector proteins containing specialized recognition domains bind directly to these phosphotyrosine residues and nearby amino acids

• the most prominent of these recognition structures is the SH2 domain which is named for its structural homology to sequences in the Src protein

recruitment of SH2 domain proteins summary

EGF ligand binds → RTK clustering → cross-autophosphorylation on Tyr → recruitment of SH-2 domain proteins

the RAS/MAPK pathway is

the primary signaling cascade utilized by RTKs to drive cell growth, division, and nuclear gene expression

the RAS/MAPK signaling cascade

1. activation of RAS via adapter proteins and GEF

2. the phosphorylation cascade

3. nuclear translocation & gene expression

4. inactivation of Ras

activation of Ras via adapter proteins and GEF

Ras: a small, monomeric G protein anchored to the inner leaflet of the plasma membrane that acts as a molecular switch regulating cell proliferation; Ras is inactive when bound to GDP and active when bound to GTP

GRB2 & Sos