Chapter 8 Cell Signaling Systems

Chapter 8: Cell Signaling Systems

Overview

  • This chapter covers various aspects of cell signaling systems, including key receptors, pathways, and physiological implications important for a thorough understanding of biochemistry and cellular biology in the context of CHEM 441A at CSULB.

8.1 Components of Signaling Pathways

  • Extracellular Messengers: Ligands that bind to receptor proteins, triggering a cellular response.

    • Response Mechanism: Structural changes initiated by ligand binding.

    • Receptor Names: Often reflect the ligand activating them.

8.2 G Protein–Coupled Receptor Signaling

  • Introduction to GPCRs: These structures are involved in essential sensory perceptions like vision, taste, and smell.

  • Structure: Contain seven transmembrane alpha helices, also referred to as serpentine receptors.

  • Glycoproteins: Many GPCRs have carbohydrate functional groups attached to their extracellular domain.

Heterotrimeric G Proteins Signaling: These proteins interact with GPCRs upon their activation, leading to a cascade of intracellular signaling that regulates various physiological responses.

  • Comprised of three subunits: alpha (Gα), beta (Gβ), gamma (Gγ).

  • When activated, G proteins dissociate from the receptor and affect downstream signaling pathways.

GPCR Signal Transduction Systems

  • **Key Components:

    • Adenylate Cyclase (AC)**: Produces cAMP, key signaling molecule.

    • Beta2-Adrenergic Receptor (β2-AR): Binds epinephrine.

  • GAP: GTPase-Activating Protein; stimulates GTP hydrolysis.

  • GEF: Guanine Nucleotide Exchange Factor; promotes GDP-GTP exchange.

Activation Mechanism

  1. Ligand Binding: Causes conformational change in GPCR.

  2. GDP to GTP Exchange: G-protein subunits dissociate into active forms.

  3. Downstream Signaling: Gα-GTP activates or inhibits target proteins.

  4. Termination: Signal is halted by reassociation of Gα with Gβγ and receptor desensitization via phosphorylation.

Sensory Mechanisms Via GPCR

  • Vision: Light activates rhodopsin, leading to cGMP hydrolysis and ion channel closure.

  • Taste: Sweet compounds activate Gqα leading to phospholipase C activation.

8.3 Receptor Tyrosine Kinase (RTK) Signaling

  • Activation: Ligands bind causing receptor dimerization and intrinsic tyrosine kinase activity.

  • Phosphorylation Events: Activated RTKs phosphorylate downstream signaling proteins.

  • Common Examples: Insulin receptors and growth factors like EGF.

EGF Mechanism

  • Step 1: EGF binding leads to dimerization and phosphorylation.

  • Step 2: Phosphorylation of receptor tyrosines enhances downstream signaling through proteins like GRB2.

8.4 Tumor Necrosis Factor Receptor Signaling

  • Role: Controls inflammation and apoptosis. These receptors form oligomeric structures that transmit extracellular signals efficiently.

  • Signaling Process: Similar to other RTK pathways where receptor activation results in downstream signaling cascade via adaptor and signaling proteins.

8.5 Nuclear Receptor Signaling

  • Function: These receptors modulate gene expression following ligand binding.

  • Mechanism: Involves interaction of receptor proteins with DNA in the nucleus and protein-protein interactions crucial for transcription regulation.

Signal Transduction Components

  • First Messengers: Extracellular ligands that initiate signaling by binding to receptors (e.g., insulins, growth factors).

  • Second Messengers: Intracellular molecules that amplify signals (e.g., cAMP, Ca2+).

    • Phosphoinositide Cascade: Involves PIP2 breakdown by phospholipase C, producing diacylglycerol (DAG) and inositol triphosphate (IP3).

Consequences of Receptor Activation

  • Covalent modifications (phosphorylation/dephosphorylation), conformational protein changes, and changes in protein expression rates are observed post-activation.

Practical Implications and Examples

  • Example of NO as a First Messenger: Nitric oxide synthase produces NO from arginine, affecting smooth muscle and vasodilation.

  • Amplification via Second Messengers: For every activated receptor, multiple downstream signaling events are triggered leading to extensive physiological responses.

Poll Questions

  • Example Poll 1: Which of the following is a first messenger?

    • Correct Answer Example: b. insulin.

  • Example Poll 2: Steps of a signaling pathway in order.

    • Correct Answer Example: d. 3,4,1,2.

Conclusion

  • Understanding these signaling mechanisms is crucial for studying cellular processes, diseases like cancer, and therapeutic strategies since many drugs target these pathways.

    • Further study is encouraged in reviewing the ins and outs of specific signaling pathways like GPCR and RTK to understand their therapeutic applications better.