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Cell Signaling Systems Study Notes

8.1 Components of Signaling Pathways

• Extracellular Messenger Molecules (Ligands): Bind to receptor proteins, triggering cellular responses through structural changes in proteins initiated by the binding.

• Receptor Naming: Often reflects the ligand activating it.

Signal Transduction

• Definition: The biochemical mechanism for transmitting extracellular signals across the plasma membrane and throughout the cell.

• Functionality: Converts information into a chemical signal, often concluded with covalent or noncovalent modifications of intracellular target proteins.

• Examples of Outcomes Include:

  • Flux through metabolic pathways

  • Ion flow through the plasma membrane

  • Cell mobility

  • Gene expression

Consequences of Activating a Receptor Protein

• Protein Modifications: Involves covalent alterations such as phosphorylation and dephosphorylation.

• Conformational Changes: High affinity noncovalent binding interactions occur between adaptor proteins.

• Altered Rates of Protein Expression: Can happen at both transcriptional or translational levels.

Cell Signaling Pathway

• Definition: A sequential series of biochemical reactions initiated by the ligand-induced activation of a receptor protein, culminating in a measurable cellular response.

• First Messengers: Extracellular ligands that bind to receptor proteins, initiating the signaling cascade.

• Second Messengers: Intracellular non-protein signaling molecules involved in amplifying the signal.

Hormones and Other First Messengers

• Hormones: Biologically active molecules released into the circulatory systems, interacting with receptors on target cells.

  • Categories:

  • Endocrine: Released by glands directly into the bloodstream (e.g., insulin, estrogen, testosterone).

  • Paracrine: Secreted locally to affect nearby cells (e.g., growth factors, clotting factors).

  • Autocrine: Active only on the same cell that secretes them (e.g., interleukin 1).

NO: A First Messenger

• Activation: Neuronal input activates nitric oxide synthase, converting arginine into NO and citrulline.

• Inhibition of cGMP Phosphodiesterase: Leads to increased cGMP levels via guanylate cyclase activation by NO binding, resulting in protein kinase G activation, which causes muscle relaxation and vasodilation.

  • Example: Sildenafil (Viagra) prolongs NO-mediated vasodilation.

Second Messengers

• Characteristics: Small, nonprotein intracellular molecules that amplify receptor-generated signals.

• Examples Include:

  • Cyclic GMP (cGMP)

  • cAMP

  • Diacylglycerol (DAG)

  • Inositol-1,4,5-triphosphate (IP3)

  • Ca2+

Signal Amplification by Second Messengers

• Process:

  • Receptor activation by first messenger leads to the production of second messengers which amplify the signal.

Phosphoinositide Cascade

• Initialization: Receptor activation leads to the conversion of PIP2 to IP3 and DAG through phospholipase C (PLC).

• Subsequent Actions:

  • IP3: Causes Ca2+ release from the endoplasmic reticulum.

  • DAG: Activates protein kinase C (PKC), involved in further signaling processes.

Classes of Receptor Proteins in Eukaryotes

• G Protein–Coupled Receptors (GPCRs): Involved in sensory perception, contain seven transmembrane alpha helices.

• Ligand-Gated Ion Channels: Control ion flow (e.g., K+, Na+, Ca2+) in response to ligand binding.

• Receptor Tyrosine Kinases (RTKs): Phosphorylate target proteins on tyrosine residues, initiating intracellular signaling.

• Tumor Necrosis Factor (TNF) Receptors: Form receptor trimers and control inflammation/apoptosis.

• Nuclear Receptors: Modulate gene expression by interacting with DNA and other proteins.

8.2 G Protein–Coupled Receptor Signaling

• Role of GPCRs: Most are associated with sensory perceptions (vision, taste, smell).

• Structure: Feature seven transmembrane alpha helices and often glycoprotein characteristics.

Heterotrimeric G Proteins Signaling

• Complex Composition: Consists of G protein subunits (G alpha, G beta, G gamma).

GPCR Signal Transduction Systems

• Key Components:

  • Enzymes (e.g., Adenylate Cyclase) indicate their function (e.g., producing cAMP).

  • Various G protein-coupled receptors (GPCRs) and associated signaling proteins (PDE, GAP, GEF, etc.).

Receptor Agonists and Antagonists

• Agonists: Activate receptor signaling by mimicking natural ligands.

• Antagonists: Bind with high affinity but do not initiate a signaling response.

GPCR Activation Synopsis

• Steps in Activation:

  1. Ligand-induced conformational changes.

  2. GDP-GTP exchange and subunit dissociation, activating downstream signaling.

  3. Termination of the signal through receptor desensitization and phosphorylation by GRK.

Sensory Perception Mechanisms and GPCR

• Examples:

  • Light perception via rhodopsin

  • Odorant detection and signaling through olfactory GPCRs

  • Taste receptor signaling integrated via GPCRs.

Protein Kinase A (PKA) as a Downstream Signaling Protein

• Activation Conditions: Active when cAMP levels are high.

• Functions:

  • Phosphorylation of enzymes involved in glycogen metabolism affecting glucose levels.

Cancer and Ras Mutations

• Mechanism: Mutations in Ras can lead to continuous cell proliferation even without EGF signaled presence, due to defective GTPase activity.

Insulin Signaling Mechanism

• Insulin Structure: Composed of two polypeptide chains connected by disulfide bonds.

• Signaling Pathway: Involves receptor activation, phosphorylation, recruitment of signaling proteins, and eventual glucose uptake and glycogen synthesis.

Final Notes on G Protein Cycle Control

• Regulatory Proteins: GEF activates signaling via GDP-GTP exchange; GAP terminates signaling through hydrolysis.

• Grk function: βARK facilitates the docking of β-arrestin, which further inhibits receptor signaling after its activation.

Conclusion

• Significance of Understanding Signaling: Deciphering these mechanisms is crucial for developing therapeutic strategies for various diseases (e.g., cancer, diabetes).