G-protein-receptor.html: 11_07aGProtLinkReceptor.jpg
G-protein-linked receptor proteins form binding sites for signal molecules, such as epinephrine and neurotransmitters. The signal is then relayed to G-proteins.

G-protein.html: 11_07aGProtein.jpg

1. The G-protein is inactive when it is attached to a GDP (guanosine diphosphate) molecule.
2. A signal molecule binds to the receptor, which changes shape and binds to the inactive G-protein. A GTP molecule displaces the GDP, and activates the G-protein.
3. The activated G-protein binds to another enzyme and activates it.
4. The G-protein hydrolyzes the GTP and returns to an inactive state.

Tyrosine.html: 11_07bTyrosineKinases.jpg

1. Receptor tyrosine kinases contain multiple tyrosines and are inactive in the monomer state.
2. Binding of a signal molecule, such as insulin or growth factor, causes 2 monomers to form a dimer.
3. ATP donates a phosphate to each of the tyrosines.
4. Relay proteins bind to the phosphorylated tyrosines, and trigger different transduction pathways.

Viagra.html: 11_01ViagraBoundToEnzyme_U.jpg
Nitric oxide (NO) promotes the relaxation of smooth muscles surrounding blood vessels, increasing blood flow. This action is mediated by cGMP as a second messenger, which is converted to GMP by the enzyme PDE5. The E.D. drug Viagra inhibits the hydrolysis of cGMP to GMP, thus prolonging the vasodilation and hence erection.

acetylcholine.html: ../ch48/48_17ChemicalSynapse.jpg
Acetylcholine is a neurotransmitter that diffuses over short distances of a synaptic cleft between cells to transmit a nerve signal.

calcium.html: 11_11CalciumIonConcentrat_L.jpg
The Ca²⁺ concentration in the cytosol is usually much lower than in the extracellular fluid and ER. This gradient is maintained by protein pumps.

calcium2.html: 11_12CalciumSignalPath_L.jpg
Calcium ions (Ca²⁺) and inositol trisphosphate (IP₃) function as second messengers in many signal transduction pathways, initiated by the binding of a signal molecule to a G–protein–linked receptor, or to a receptor tyrosine kinase.

camp.html: 11_09CyclicAMP.jpg
Cyclic AMP (cAMP) is made from ATP by adenyl cyclase, an enzyme embedded in the plasma membrane. Many G-proteins trigger the formation of cAMP, which then acts as a second messenger.

camp2.html: 11_10cAMPSecondMessenger_L.jpg
A first messenger such as epinephrine may phosphorylate and activate the G-protein. Thus in turn activates adenyl cyclase to convert ATP to cAMP. cAMP acts as a second messenger by activating other proteins in a phosphorylation cascade.

cell_signaling.html: 11_06CellSignalingOver_3-L.jpg
Overview of cell signaling. When reception occurs at the plasma membrane, the transduction stage is usually a pathway of several steps, with each molecule in the pathway bringing about a change in the next molecule. The last molecule in the pathway triggers the cell's response.

epinephrine.html: 11_13CytoplasmicResponse.jpg
Signal amplification. Epinephrine acts through a G–protein–linked receptor to activate a succession of relay molecules, including the second messenger cAMP. This amplifies the signal: one receptor protein can activate about 100 molecules of G protein. Each enzyme in the pathway can act on many molecules of the next molecule in the cascade, resulting in a 100 million-fold amplification in the response.

insulin.html: 11_insulin_TyrosineKinase.jpg
Insulin binds to the α subunits of the tyrosine kinase, resulting in a conformational change. The kinase phosphorylates itself, then phosphorylates a variety of intracellular targets. animation

intracellular.html: 11_08IntracellReceptor_5-L.jpg
Steroid hormones such as testosterone are made from the lipid cholesterol. The hydrophobic steroids can cross the lipid bilayer of the plasma membrane to bind cytoplasmic proteins and eventually stimulate transcription in the nucleus.

ion_channels.html: 11_07cIonChannelReceptors.jpg
Ion Channel Receptors Ion channel receptors respond to ligands such as synaptic neurotransmitters to open sodium gates and relay nerve impulses.

1. The gates is closed.
2. A ligand binds to the receptor and the gate opens, Na and K ions flow through the channel.
3. The ligand dissociates from the cell, closing the gate.

junction.html: 11_03DirectCommunicationA.jpg

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Paracrine signaling. A cell acts on nearby target cells by secreting local regulators (a growth factor, for example) into the extracellular fluid.		Synaptic signaling. A nerve cell releases neurotransmitters such as acetylcholine into a synapse, stimulating the target cell.

long-distance_signaling.html: 11_04CellCommunicationC.jpg
Long-distance signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones such as thyroid hormones may reach many body cells.

nuclear.html: 11_14NuclearResponse_L.jpg
A growth factor triggers a phosphorylation cascade. The last kinase in the cascade enters the nucleus and activates a transcription factor. This in turn stimulates a gene to synthesize an mRNA molecule, which then directs the synthesis of a protein in the cytoplasm.

phosphorylation_cascade.html: 11_08PhosphorylatCascade.jpg
Transduction: in a phosphorylation cascade, a series of molecules in a signal pathway are phosphorylated and activated in turn, each molecule adding a phosphate group to the next one. The phosphorylation usually results in conformation change.

specificity.html: 11_15CellSignalSpecific.jpg
The specificity of cell signaling.
Different combinations of membrane receptors and cytoplasmic proteins allow a diversity of responses among different cells to the same signal molecule.