Biochemistry Chapter on Receptor Interactions

BIOC 325: Receptors and Signal Transduction

• Course Coordinator: Dr. Ayad Jaffa, DTS 4th floor, email: [email protected]
• Teaching Assistant: (information not available)

Resources

• Signal transduction (3rd edition), 2015, by IJsbrand M. Kramer, ISBN: 9780123948038, eBook ISBN: 9780123948199
• Signal Transduction: Principles, Pathways, and Processes 1st Edition, by Lewis Cantley (Editor), Tony Hunter (Editor), Richard Sever (Editor), Jeremy Thorner (Editor)

BIOC 325 Course: Fall Schedule 2022 (Tentative)

• August 30: Topic 1: Signal Transduction: Definition and Pharmacological Introduction
• September 4: Topic 2: Roles of Structural Domains and Scaffold Proteins in Signal Transduction
• September 6: Topic 3: 7TM G-protein Coupled Receptors, General Features, Effector Systems, and Second Messengers (Part 1): Adenylate Cyclase
• September 11: Effector Systems and Second Messengers (Part 2): PLCB
• September 13: Calcium in Signaling
• September 18: Proteins associated with GPCR (Part 1): GRKs, RGS
• September 20: Proteins associated with GPCR (Part 2): β-Arrestin, Dynamin, Clathrin
• September 25: GPCR Desensitization, Internalization, and Recycling
• September 27: Special GPCRs: Protease-Activated Receptors (PARs) and GPCR Heterodimerization
• October 2: Classical methods to assess signaling and coupling of GPCRS
• October 4: Topic 4: Receptor Tyrosine Kinase (RTKs) and Important Kinases Downstream of RTKs
• October 9: Topic 5: Mitogen-Activated Protein Kinases (MAPKs) and Transactivation of EGFRs by GPCRs
• October 11: Topic 6: Nuclear Receptors
• October 16, October 18, October 23: Article presentations and discussions
• September 28: EXAM 1
• October 26: EXAM 2

Student Evaluation

• Student Presentation: 15%
• Midterm Exam: 35%
• Final Exam: 45%
• Class Participation: 5%

Lecture 1: Introduction into Signal Transduction

• Aims: Define signal transduction, overview steps of signal propagation, receptor types and properties, define receptor agonists and antagonists.

Receptors

• Receptors are structural proteins on the surface or inside of a cell that selectively receive and bind a specific substance (ligand) and elicit a specific response.

Types of Receptors

• Membrane bound receptors: G Protein coupled receptors, Adrenergic receptors, Angiotensin II, Bradykinin, Enzyme receptors, Tyrosine kinase, Ligand gated ion channel receptors, Nicotinic, GABA, glutamate
• Intracellular and nuclear receptors: IP3 receptor (ER), Steroid hormones receptor

G Protein Coupled Receptors (GPCRs)

Enzyme-like Receptors

Ligand-Gated Ion Channel Receptors

Intracellular Receptors

Properties of Receptors

• A. Recognition: Saturability, Reversibility, Stereo-selectivity, Agonist specificity, Tissue specificity
• B. Transduction: Linking to effector systems (either directly or through intermediate signal amplification systems).

What is Signal Transduction?

• The relaying of molecular signals or physical signals from a cell's exterior to its intracellular response mechanisms.

Steps of Cell Signaling

• Initiation of signal
• Transmission of signal
• Nuclear or cytoplasmic events
• Biological effects

Steps of Signal Processing

• Initiation: interaction of ligand with its receptor
• Transmission: receptor transmitting the signal into the cell
• Nuclear or cytoplasmic events
• Biological effects

Signal Transduction

• Involved in multiple biological processes (proliferation, migration, apoptosis)
• Defects in signaling pathways can lead to various diseases (cardiovascular, Alzheimer's, cancer)

Signal Transduction Refer to any process by which a cell converts one kind of signal or stimulus into another.

Signal Transduction Cascade

• Signal reception
• Signal integration
• Signal amplification
• Signal reaches its target (functional response)

Signal Transduction Cascade: Integration, Amplification, and Adaptation

• Integration: Several receptors activate/deactivate the same catalyst. Cross-talk in eukaryotic networks adds another level.
• Amplification: Activation of catalysts (e.g., protein kinases) amplifies the input of a single unit into many target molecules.
• Adaptation: Return of the signaling system to the pre-stimulus levels while stimulus persists. This allows cells to perceive changes in stimulus size rather than the absolute stimulus level (feedback).

Ligands

• Are substances that bind to receptors to trigger a biological activity.
• They're signal-triggering molecules binding to a site on a target protein to transfer information
• Types: Peptides (short proteins), small molecules (neurotransmitters, hormones, pharmaceutical drugs, toxins), lipids, photons/light particles

Ligand-Receptor Interaction: Law of Mass Action

• Ligand-receptor interaction follows simple mass-action relationships.
• When a ligand combines with a receptor, it does so at a rate dependent on the concentration of both.
• Formulas for rate of association (k₁), rate of dissociation (k₂), Dissociation Constant (KD), and Affinity Constant (KA) are given.
• KD represents the ligand concentration required to occupy 50% of receptor binding sites.

Law of Mass Action (Saturation Binding Curve)

• Activation of membrane receptors and target cell responses is proportional to the degree of receptor occupancy. KD values indicate affinity of binding. Drug A or B is associated with a particular KD in figures.

Characterizing Receptors and Ligand-Receptor Interactions

• Radioimmunoassay (RIA)
• Dose-response experiments
• Use of receptor antagonists (competitive, non-competitive)

Characterizing Receptors Using Radioimmunoassays (RIA)

• Direct measure of receptor number/density and ligand affinity/selectivity utilizing radio-labeled ligands (125I, 35S, or 3H).
• Selecting a proper radio-ligand is essential, especially in agonist vs. antagonist contexts. Higher affinity generally favours antagonists.
• Saturation binding curves are theoretical (occur during steady state/equilibrium).

Characterizing Receptors Using Radioimmunoassays (RIA): Important Notes

• The amount of ligand bound is affected by the number of receptors, ligand concentration, and ligand affinity.

Characterizing Receptors Using Dose-Response Experiments

• Measures the functional response of a ligand/drug, which is indirectly assessing receptor binding.
• Can be conducted in vitro, in vivo or ex vivo. Graphs shown demonstrating dose-response curves presented in linear and log scale.

Ligands: Agonists, Antagonists, and Partial Agonists

• Agonists: Bind to the active site of receptors and lead to a specific effect.
• Antagonists: Prevent agonist-mediated responses by obstructing binding and eliminating the normal response.
• Partial agonists: Have moderate binding or effect alone, yet exhibit antagonistic effects in the presence of agonists.

Agonists/Antagonists

• an agonist drug structure is similar to an endogenous ligand, it so binds to same receptor and produces similar effects shown in figures with agonist/antagonist plots.

Agonists and Partial Agonists

• Not all agonists produce the same maximal response at the same receptor. Figure shows comparative graphs of A, B, C which are agonist, with C as partial agonist in figure
• The response of the drug (i.e. maximal response) is related to different affinities of the receptors with various agonists as shown by the plots

Receptor Antagonists

• Antagonists halt agonist-mediated responses by preventing receptor binding and any typical effect. Two types of antagonists exist.
• Competitive: Reversibly binds to the same site as endogenous ligand blocking it. The effect can be overcome by more agonist. There is no change in the maximal response (E_max ) or the affinity but the EC₅₀ (concentrations that produce 50% of its effect) is changed
• Non-competitive: Binds to an allosteric site (different binding site to agonist), does not prevent formation of the L–R complex, impairs response triggering, and cannot be reversed. E_max and B_max are reduced and the EC₅₀ values are not affected in figure.

Examples of Agonists and their Receptors

• Neurotransmitters, neuropeptides, ions (e.g., Na+, Ca2+)
• Hormones (e.g., Growth Hormone)

Second Messengers

• Molecules activated after ligand binding (first messenger) to propagate the signal.
• Released/activated within a cell to initiate biological processes (proliferation, differentiation, migration, survival, apoptosis).
• Examples include cAMP, cGMP, IP3, DAG, and Ca2+.

Test your knowledge on essential concepts from the chapter covering receptor interactions in biochemistry. This quiz includes questions on GPCRs, ligand-receptor dynamics, and the overall assessment structure of the course. Prepare for a deeper understanding of signal transduction and receptor functionality.