signal transduction

• when bound to GTP, alpha subunit disassociates from other G coupled protein subunits and activates the signalling down stream
• an inactive g protein coupled receptor associates with g proteins bound to GDP, when receptor is activated, the GDP is replaced with GTP
•  Two receptors are brought together (dimerise) when they bind the same dimerised signaling molecule. The close proximity of the two receptors activates kinase activity in the intracellular part of the receptors.
• ion-channel-coupled receptors are embedded in cell membranes and change shape upon binding of a signalling molecule.
• second messengers in signal transduction pathways

  cAMP, calcium, IP3, DAG
• primary requirement of signalling

  can be turned on and off
• metabolic enzymes


  cause altered metabolism
• gene regulatory proteins

  SSTF and cause altered gene expression
• cytoskeletal proteins

  alter cell shape or movement
• signal transduction pathway

  signal, receptor, second messengers and intracellular signalling proteins, target effector proteins, cellular response
• receptors are on the cell surface if signalling molecule is unable to cross the plasma membrane
• receptors are inside the cell if signalling molecule can diffuse across the membrane
• close range signalling

  autocrine and paracrine
• autocrine signalling


  cell also produces signal
• paracrine signalling


  close range and only produced by one cell binding onto another cell
• long range signalling

  endocrine, hormone travels in blood stream to target cells
• second messengers


  small diffusible molecules that relay signal inside the cell, affect a wide variety of enzymes, ion channels, and transporters, and are often coupled to phosphyorylation cascades that amplify the signal
• phosphorylation regulates the activity of enzymes in signalling pathways
• altered protein synthesis from tranduction

  needs SSTF, can bind to DNA to enhance, facilitates initiation complex, and is a slow response
• ion channel coupled receptors


  important fro neuronal signalling, binding of ligand opens ion channel, regulates the entry of calcium, potassium, or sodium
• enzyme coupled receptors


  receptor has enzymatic activity or direct association with an enzyme
• inactive GPCR


  associates with g protein (3 subunits), alpha subunit is bound to GDP
• activated GPCR

  alpha subunit binds to GTP, disassociates from beta and gamma, and activates second messengers