Chapter-23-Signal Transduction

23.1 Chemical Signals and Cellular Receptors

  • Cells produce signals via surface display molecules or by releasing chemical signals.

  • Chemical signaling regulates and coordinates diverse cell and tissue activities in multicellular organisms.

Types of Chemical Signals

Short- Versus Long-Range Signals

  • Paracrine signals: Short-range, diffusible molecules acting on nearby cells.

  • Endocrine signals: Long-range signals carried by the circulatory system to distant target tissues.

Classification of Signaling Molecules

  • Autocrine signals: Act on the same cell that produces them.

  • Juxtacrine signals: Require direct cell-cell contact to transmit signals.

Receptors and Ligands

  • Receptors: Proteins on target cells that bind to messengers (ligands).

  • Ligands: Molecules that bind to receptors, triggering signal transduction events within the cell.

  • Binding occurs at a specific site, with amino acid side chains forming chemical bonds.

Signal Transduction

  • Process: Ligand binding to a receptor activates a series of intracellular events leading to a cellular response (e.g., changes in gene expression).

  • Stages:

    1. Reception: Detection of the signaling molecule by the receptor.

    2. Transduction: Binding alters the receptor, activating a signaling pathway.

    3. Response: The transduced signal invokes a specific action in the target cell.

Receptor Binding Dynamics

Quantitative Interaction

  • Similar to enzyme-substrate dynamics; when receptors are occupied by ligands, saturation can occur.

  • Dissociation Constant (Kd): Indicates receptor affinity for the ligand; lower Kd signifies higher affinity.

Agonists and Antagonists

  • Agonists: Synthetic ligands that activate receptors more effectively than natural ligands.

  • Antagonists: Block receptor activation by binding without triggering a response.

Signal Termination

  • Cells reduce signaling by lowering ligand concentration or receptor sensitivity.

  • Receptor Desensitization: Prolonged occupancy leads to reduced cell responsiveness.

  • Mechanisms include endocytosis of receptors and biochemical changes reducing binding affinity.

Coreceptors and Their Role

  • Coreceptors: Assist primary receptors in ligand binding (e.g., heparan sulfate proteoglycans like glypicans).

  • Example: HIV targets immune cells through the CD4 protein and CCR5 coreceptor.

Signal Amplification

  • A cell can amplify responses from minimal ligand quantities, as each signaling step enhances the signal's effect.

  • Example: Epinephrine induces the breakdown of glycogen into glucose, amplifying the response.

G Protein-Coupled Receptors (GPCRs)

Structure and Activation

  • Ligand binding changes receptor conformation, activating associated G proteins.

  • G proteins: Guanine nucleotide-binding proteins that mediate signal transduction via binding to specific target proteins.

Regulation of GPCRs

  • Phosphorylation by GRKs and β-arrestin binding can inhibit GPCR activity, an example of negative feedback.

Second Messengers

  • Molecules like cyclic AMP (cAMP) help propagate signals and amplify cellular responses.

  • cAMP is produced from ATP by the enzyme adenylyl cyclase, activated by G proteins.

Cellular Responses and Phosphorylation

Protein Phosphorylation

  • Kinases: Transfer phosphate groups to proteins, often cascading through signaling pathways.

  • Phosphatases: Remove phosphate groups, thus reversing kinases' actions.

Example Functions Regulated by cAMP

  1. Glycogen degradation: Muscle and liver cells under epinephrine.

  2. Fatty acid production: Adipose tissue due to epinephrine.

  3. Heart rate increase: Cardiovascular response to epinephrine.

  4. Water reabsorption: Kidney response mediated by antidiuretic hormone.

Coordination of Signaling Pathways

  • Cells utilize core signaling pathways for diverse functions, including gene regulation or cytoplasmic activities, ensuring specificity and efficiency in responses.