Introduction to Signal Transduction (College Board AP® Biology) : Study Guide

Signal transduction pathways

• A signal transduction pathway is the sequence of events that links receipt of a cell signal with a cellular response

  • The signal is provided by a chemical signaling molecule known as a ligand; ligands bind to receptors on or in specific target cells

  • The response might involve changes to cell processes, such as changes in gene expression

• Events that occur during a signal transduction pathway include:

  • protein modification, e.g. the binding of cAMP can activate enzymes

  • phosphorylation cascades, e.g. an enzyme may be activated by the addition of a phosphate group, setting off an additional cascade of chemical reactions

Steps in a signal transduction pathway

• Signal transduction pathways involve:

  1. reception of a signal

  2. transduction

  3. a cellular response

1. Reception

• Cells receive a signal when a ligand binds to the ligand-binding domain of a receptor protein

  • Ligands can be peptides, small chemicals, or proteins

  • G protein-coupled receptors are an example of a receptor protein in eukaryotes

• Receptor proteins are specific to one type of ligand

  • If a cell does not have the specific receptor for a ligand then that ligand will have no effect on the cell

• Receptors can be located on the cell surface membrane, or within the cytosol of the target cell

  • Hydrophilic ligands cannot cross the cell surface membrane, so they function by binding to extracellular membrane receptors which then relay the signal across the membrane

  • Hydrophobic ligands can pass between the phospholipids of the cell surface membrane and bind directly to intracellular receptors

• Binding of a ligand triggers a conformational change in the receptor, which then triggers transduction

  • The binding of ligands to extracellular receptors triggers a conformational change in the the intracellular domain of the receptor

  • The binding of ligands to ligand-gated ion channels can cause channel proteins to open or close; this alters the passage of substances across the cell membrane, resulting in a chemical change inside the cell

2. Transduction

• Transduction is the series of chemical changes that occurs inside a cell which result in an eventual cellular response

• Examples of chemical changes include:

  • activation of kinase enzymes, which transfer phosphate groups to other molecules in order to activate them

  • activation of phosphatase enzymes, which remove phosphate groups from other molecules in order to deactivate them

  • the production of secondary messenger molecules

    • These are molecules that relay and amplify the intracellular signal after the binding of an extracellular ligand

    • E.g. adenylyl cyclase activation results in the production of the secondary messenger cyclic AMP (cAMP)

• These chemical changes form part of signaling cascades; these are sequences of chemical reactions where one change triggers a second change, which triggers a third, and so on

• Signaling cascades can amplify signals, meaning that a small signal can result in a significant cellular response

3. Response

• The signalling cascade that occurs during transduction eventually brings about a specific response inside the cell

• Examples of cellular responses include:

  • regulation of gene expression through control of transcription or translation

  • changes in metabolic activity

  • secretion of molecules, e.g. enzymes or hormones

  • cell death

  • cell growth