Ch. 14 Part 6 Cell Signaling

Cell Signaling How living organisms control and coordinate their bodies Based on getting a message from one place to another place Outside the cell to inside the cell Inside the cell to outside the cell Two Major mechanisms of signaling Secretion of Chemical Signals Direct cell-cell contact Ex. Embryonic development and lymphocyte attacks Cell membrane plays big role in cell signaling Contains protein “receptors” Stimulus (signal) receptor  transmission of mission “signal transduction” target (effector) response

Signals Signals Single molecules are usually very small and diverse Produced by body Hormones and neurotransmitters Stimuli Light, drugs, pheromones Single molecules are usually very small and diverse Hydrophobic, insoluble molecules get across membrane no problem Steroids Ex. oestrogen Most signal molecules are water soluble need to attach to cell membrane receptors to relay signal Distance travelled by molecules can be small or short Signals can be electrical (nervous system) or chemical (endocrine system) events

Signal Receptor in Cell Membrane Transmembrane proteins “Receptor” Specific shape Only on certain cells When signal molecule attaches to receptor on the outside of the cell: conformational changes occur to protein that spans the membrane Signal from outside is “transduced” Changes to the shape of the receptor protein initiates a “domino” effect Transmits the signal to another protein inside the cell G-Proteins (alpha, beta, gamma) G proteins activate another molecule/protein

How Receptors Alter the Activity of a Cell Activating the release of second messengers Opening an ion channel Changes membrane potential Ex. Nicotine-accepting acetylcholine receptors Acting directly as a membrane-bound enzyme Ex. Insulin receptor Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane Ex. Oestrogen receptor Found in nucleus and directly controls gene expression when combined with hydrophobic oestrogen hormone

G Proteins Protein Family Respond diverse stimuli G Protein Coupled Receptors (GPCRs) Transmembrane protein Guanine nucleotide-binding proteins Intracellular proteins Alpha, beta, and gamma Respond diverse stimuli light, compounds, bioactive lipids, cytokines, hormones, and neurotransmitters Activation puts specific G proteins into play Changes in G-protein structure alters the activity of enzymatic signaling pathways they bind the guanine nucleotides GDP and GTP (that is the “switch” that relays the signal from outside)

Receptor G Protein receives the signal molecule Changes shape to activated alpha G-protein inside the cell membrane “switch” Activated G protein (now with GTP) moves to stimulate another receptor molecule that releases the “second messenger” molecule “amplifies” original signal Key feature of cell signaling

Second Messenger Molecule Amplifies original signal Usually activates enzyme that activates other enzymes Increases amplification of original signal “signaling cascade” Sequence of events triggered by G protein Final enzyme activated brings about required change in cell metabolism Common Second messagner is cAMP Cyclic Adenosine Monophosphate Made from ATP Binds to Kinase Enzymes

Kinases Enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation) Phosphorylation usually results in a functional change of the target protein (substrate) by: changing enzyme activity cellular location association with other proteins

How Second messengers Work

Possible Signaling Pathways in Cells