Signal Transduction Pathway Visualizer

Chemical messengers that initiate signaling. Examples include:

• Hormones: Epinephrine, insulin, growth hormone
• Neurotransmitters: Acetylcholine, dopamine, serotonin
• Growth factors: EGF, NGF, PDGF
• Cytokines: Interleukins, interferons

Function: Bind to specific receptors to trigger conformational changes

Catecholamine hormone and neurotransmitter

• Structure: Catechol ring with ethylamine side chain
• Source: Adrenal medulla and some neurons
• Receptors: α and β adrenergic receptors
• Release: Stress, exercise, low blood sugar

Function: Triggers fight-or-flight response

Lipophilic signaling molecules derived from cholesterol

• Examples: Cortisol, testosterone, estrogen, aldosterone
• Properties: Can pass through plasma membrane
• Transport: Carried by binding proteins in blood
• Mechanism: Bind intracellular receptors

Function: Direct regulation of gene transcription

Cyclic adenosine monophosphate - a crucial second messenger

• Production: Synthesized from ATP by adenylyl cyclase
• Targets: Activates protein kinase A (PKA)
• Visualization: Yellow particles spraying from adenylyl cyclase
• Amplification: One activated receptor can generate hundreds of cAMP molecules

Function: Rapidly spreads signal throughout the cell

Energy molecules shown in GPCR and epinephrine pathways

• ATP: Substrate for cAMP synthesis (shown moving to adenylyl cyclase)
• ADP: Byproduct after cAMP formation
• Visualization: ATP moves in, ADP moves out

Function: Provides energy and building blocks for second messenger

Seven-transmembrane domain receptors - largest receptor family

• Structure: 7 alpha-helices spanning the membrane
• Examples: β-adrenergic, muscarinic, rhodopsin
• Mechanism: Ligand binding causes conformational change
• Partners: Activates heterotrimeric G proteins

Function: Transmits signal across membrane via G protein activation

GPCR subfamily that binds catecholamines

• Types: β1 (heart), β2 (lungs, muscle), β3 (adipose)
• Structure: 7 transmembrane helices
• G protein: Couples to Gs (stimulatory)
• Regulation: Desensitized by phosphorylation

Function: Mediates sympathetic nervous system responses

Intracellular receptors for steroid hormones

• Location: Cytoplasm or nucleus
• Examples: Glucocorticoid receptor, estrogen receptor, androgen receptor
• Domains: Ligand-binding, DNA-binding, transactivation
• Partners: Heat shock proteins (HSP90) keep inactive

Function: Act as ligand-activated transcription factors

Heterotrimeric GTP-binding proteins (Gα, Gβ, Gγ subunits)

• Inactive state: GDP bound to Gα, all subunits together at membrane
• Active state: GTP bound to Gα, subunits dissociate
• Membrane coupling: Gα can decouple from membrane when GTP-bound
• Lipid anchors: Gα (palmitoyl), Gγ (prenyl) - Gβγ remains membrane-bound
• Movement: Active Gα diffuses through cytoplasm to target
• Types: Gs (stimulatory), Gi (inhibitory), Gq (phospholipase C)
• GTPase activity: Self-inactivating through GTP hydrolysis (~10-20 seconds)

Function: Transduces signal from GPCR to effector enzymes

Integral membrane enzyme that produces cAMP

• Structure: 12 transmembrane helices, 2 cytoplasmic catalytic domains (C1 & C2)
• Location: Spans the plasma membrane
• Activation: Gs protein α subunit binds to cytoplasmic domains
• Catalytic site: Forms at interface of C1 and C2 domains
• Reaction: ATP + Mg²⁺ → cAMP + pyrophosphate
• Isoforms: 9 in mammals with tissue-specific expression
• Regulation: Can be inhibited by Gi proteins

Function: Amplifies signal by producing hundreds of cAMP molecules while Gα-GTP is bound

cAMP-dependent protein kinase

• Visualization: Green rounded rectangles that activate when contacted by cAMP
• Activation: cAMP binding causes PKA to become active (green)
• Movement: Active PKA moves to target locations
• In GPCR: Activates generic cellular response
• In Epinephrine: Triggers heart, lung, and glycogen responses

Function: Transmits signal from cAMP to cellular responses

Systemic responses to epinephrine signaling shown in the pathway

• Heart: Increased rate (visualized as beating heart)
• Airways: Bronchodilation (shown as widening airways)
• Glycogen: Breakdown to glucose (glycogenolysis animation)

Response: Prepare body for emergency action

Generic cellular response in GPCR/cAMP pathway

• Visualization: Response light that activates
• Mechanism: PKA phosphorylates target proteins
• Amplification: One ligand → thousands of responses

Response: Various cellular changes depending on cell type

Process shown in steroid hormone pathway

• DNA: Target genes with hormone-responsive sequences
• mRNA: Transcribed messenger RNA molecules
• Ribosomes: Translate mRNA into proteins
• New Proteins: Cellular response products

Response: Long-term cellular changes through new protein synthesis

The plasma membrane shown in all pathways

• Visualization: Red heads and dark tails
• Function: Barrier between extracellular space and cytosol
• Permeability: Steroid hormones can pass through

Function: Platform for membrane receptors; selective barrier

Control center shown in steroid hormone pathway

• Visualization: Large purple circle with pores
• Nuclear Pores: White circles allowing hormone-receptor entry
• Contents: DNA for gene transcription

Function: Site of gene transcription in steroid signaling

Messenger RNA produced in steroid pathway

• Visualization: Red wavy strands
• Production: Transcribed from DNA in nucleus
• Movement: Exits nucleus through nuclear pores

Function: Carries genetic information to ribosomes

Protein synthesis machinery in steroid pathway

• Visualization: Purple spheres (large and small subunits)
• Location: Cytosol, bind to mRNA
• Function: Translate mRNA into proteins

Function: Site of protein synthesis

Final products in steroid hormone pathway

• Visualization: Orange folded structures
• Production: Made by ribosomes from mRNA
• Result: New cellular functions

Function: Execute long-term cellular responses