Signal transduction pathways are processes that occur inside cells when a signal is received.
The process involves three main stages: Reception, Transduction, and Response.
A signal molecule, or ligand, binds to a receptor protein inside the cell to initiate the pathway.
The ultimate goal of these pathways is a cellular response such as gene activation, protein production, enzyme activation, or changes like cell division or death.
Basic Signal Transduction Pathway (Model 1)
Reception: Occurs inside the cell when the ligand binds to a compatible receptor protein.
Transduction: Movement of the signal through the cell to where the response will occur; often involves amplification.
Amplification: A process where the signal is made larger; a single relay protein can activate multiple relay proteins, each causing a response.
Response: Possible responses include protein production, gene activation, opening of protein channels, or cell division/death.
Phosphorylation Cascade (Model 2)
Initiated when a ligand attaches to an active site in a receptor protein in the cell membrane.
Phosphorylation: The addition of a phosphate group to a protein to change its shape for proper function.
Occurs in steps 4, 5, and 7 of the model.
Phosphate groups are added from ATP, which is converted to ADP.
Kinases: Enzymes that perform phosphorylation.
Steps in Model 2:
Reception: Step 1
Transduction: Steps 2-6
Response: Step 7 - Transcription of a DNA segment is started.
Amplification in Phosphorylation Cascade
Each kinase can activate more than one of the next kinase enzymes in the sequence.
Amplification allows a single ligand to cause responses in several cellular locations, enabling rapid response to stimuli.
Deactivation
To stop the response, activated relay proteins must be deactivated.
Protein Phosphatases: Enzymes that remove phosphate groups to deactivate proteins, returning the cell to inactive status.
Evolutionary Implications
Similarities in signal transduction pathways across species can indicate evolutionary relatedness.
Species using the same ligands, receptors, and relay proteins are likely closely related.
Conclusion
Signal transduction pathways are crucial mechanisms in cellular communication and response.
Despite variation across species, the fundamental steps of reception, transduction, and response are consistent, providing insights into cellular functions and evolutionary biology.