Practice 13 structured short- and long-answer medical questions on Signal Transduction and Second Messengers. Review model answers for exam revision.
Answer: ### 1. Cellular Responses to Environmental Factors Cells constantly interact with and respond to their environment to maintain homeostasis. Key environmental factors include: * **Chemical Signals**: Hormones, neurotransmitters, and signaling molecules that bind to specific receptors. * **Physical Stimuli**: Temperature, pressure, and light. * **Nutrient Availability**: Impacts cell growth, metabolism, and survival. * **Pathogen Presence**: Detection of infections to activate immune responses. ### 2. Signal Transduction: Definition and Key Components Signal transduction is the series of molecular events involved in transmitting a signal from the exterior to the interior of a cell. **Key Components:** * **Receptors**: Proteins located on the cell membrane or within the cytosol that bind signal initiators. * **Signal Initiators**: Molecules like hormones or ligands that trigger the pathway. * **Target Molecules**: Downstream components (proteins or genes) affected by the cascade. * **Signal Mediators**: Intracellular molecules that relay and amplify the signal. * **Action**: The final outcome, such as changes in gene expression or enzyme activity. ### 3. Protein Phosphorylation #### Protein Kinases and Reversible Phosphorylation Protein kinases are enzymes that phosphorylate proteins by adding a phosphate group (from ATP) to serine, threonine, or tyrosine residues. This covalent modification alters protein activity, localization, or interactions. **Regulated Functions:** * Cell growth and proliferation. * Differentiation and viability (apoptosis). * Homeostasis and effector functions (e.g., cytokine production). #### The Phosphorylation Process * **Protein Kinases**: Protein-OH + ATP → Protein-O-P + ADP. * **Protein Phosphatases**: Remove phosphate groups via hydrolysis (Protein-O-P → Protein-OH + Pi). #### Regulation * **Calcium/Calmodulin**: Activates specific kinases. * **Cyclic-AMP (cAMP)**: Activates Protein Kinase A (PKA). ### 4. Second Messengers Second messengers are intracellular molecules that transmit and amplify signals from receptors to target molecules. **Categories:**
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Answer: ### 5. The cAMP Signaling Pathway #### cAMP Dynamics * **Synthesis**: Adenylate Cyclase converts ATP to cAMP. * **Degradation**: Phosphodiesterase (PDE) hydrolyzes cAMP to AMP. * **Protein Kinase A (PKA) **: A tetramer (R₂C₂). Binding of 4 cAMP molecules to the regulatory (R) subunits releases active catalytic ( C) subunits. ### 6. G Protein-Coupled Receptors (GPCRs) GPCRs are the largest family of cell surface receptors, characterized by seven transmembrane α-helices. #### G Protein Structure * **Heterotrimeric**: Composed of α, β, and γ subunits. * **α Subunit**: Binds GDP (inactive) or GTP (active). It possesses intrinsic GTPase activity. * **Small GTPases**: Related proteins like Ras (growth), Rab (vesicle transport), and Rho (cytoskeleton). ### 7. Sequence of Events in cAMP Signaling #### Activation Phase
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Answer: ### 8. Pathogen Influence on G Protein Signaling * **Cholera Toxin (*Vibrio cholerae*)**: Catalyzes ADP-ribosylation of the Gαs subunit. This inhibits GTPase activity, keeping Gαs permanently active. This leads to constitutive Adenylate Cyclase activity, high cAMP levels, and massive efflux of water and electrolytes into the intestinal lumen.