17.7: 17.7 Signal transduction (2023)

A. G protein via PKA (protein kinase A)-mediated signal transduction

GTP binding protein(G protein) transduce extracellular signals stimulating productionsecond messengerMolecules in cells. Allosteric changes in membrane receptors occur when hormones or other effector (signaling) molecules bind to their membrane receptors.cytoplasmicThe cytoplasmic domain of the receptor increases the affinity of the cytoplasmic domain of the receptor for G proteins on the surface of the endoplasmic membrane. G protein istrimerIt consists of \(\alpha\), \(\beta\) and \(\gamma\) subunits, embedded in the cytoplasmic surface of the sensitive cell membrane. G protein-mediated signal transduction is illustrated in the seven steps shown on the next page.

Receptors change shape by binding to their effector signaling molecules (steps 1, 2). In this conformation, the receptor recognizes and binds to a G-protein trimer on the cytoplasmic surface of the plasma membrane (step 3). GTP replaces GDP when the trimer binds to the receptorsubunit \(\alpha \)Protein G (step 4).

After the conformational change,subunit \(\alpha \)Separated from subunits \(\beta\) and \(\gamma\) (step 5). In this picture, the GTP-\(\alpha\) subunit can now bind to the transmembrane enzyme,cyclaza adenilat(step 6). Finally, the initial extracellular chemical signal istransductionIntracellular responses involving second messenger molecules (step 7). In this case, the second messenger iscamp. well knownfight or flightThe response of superior animal hepatocytes to epinephrine is a good example of a cAMP-mediated cellular response. After adrenaline binds to its receptor, the G protein binds to the cytoplasmic side of the receptor and then binds to adenylate cyclase. cAMP binds and activatesprotein kinase A(protein kinase A), triggeringamplification cascadethe answer. Some details of the G protein-mediated signal amplification cascade are detailed in the box on the next page.

After activation of adenylyl cyclase (steps 1 and 2 in the diagram), cAMP is synthesized and binds to two of the four subunits of adenylyl cyclase.inactive PKA(Step 3). The conformational change dissociates the tetramer into two cAMP-bound and two inert subunitsPKA activeSubunits (step 4). eachPKA activePhosphorylation and activation catalyzed by enzymes calledphosphorylase inhibitor(paso 5).

Korak 6, cataliziran fosforilazom kinazomglycogen phosphorylasephosphorylation. Finally, at the end of this issuephosphorylation cascade, now activeglycogen phosphorylaseIt catalyzes the hydrolysis of glycogen to glucose-1-phosphate (step 7). This results in rapid recycling of free glucose from hepatocytes into the circulation. Recall how this works by reviewing the conversion of glucose 1-phosphate (G-1-P) to G-6-P in glycolysis and its fate in gluconeogenesis. Of course, the increase in circulating glucose gives the body energy.fight or flightTo decide.

317 G protein activation of protein kinase A and afight or flightrespondent

In addition to activating glycogen-degrading enzymes, cAMP-activated PKA mediates the cellular response to various effectors, leading to a phosphorylation cascade.

• Activates enzymes that catalyze glycogen synthesis.
• activationlipaseHydrolyzes fatty acids into triglycerides.
• microtubule sets.
• Disassembly of microtubules.
• Mitogenesis (replicase activation).
• Activation of transcription factors increases/decreases gene expression.

Of course, when the organism no longer needs a cellular response, it must stop producing signaling molecules (hormones or other effectors). As its levels fall, the effector molecule dissociates from the receptor and the response ceases. All this is possible because the binding of the signal to its receiver is freely reversible! The links below provide animations for G protein-based signaling.

316 G protein signal transduction

B. Signal transduction by PKC

Many reactions involving G proteins are initiated by the activation of integral membranes.cyclaza adenilat. Different G protein-mediated signaling pathways generate other second messengers.protein kinase C(PKC) play an important role in the activation of these second messengers and subsequent onesphosphorylation cascadeAmong them, the activation of a few enzyme molecules in a cell will lead to the activation of more enzymes. likeprotein kinase A,PKCmediated signal transductionto increaseThe first molecular response of the cell to effectors. G proteins act in a similar way to PKAPKC signal transduction. Responses may involve different effectors at different cells...even within the same cell using different effector signals. However, PKC and PKA signaling differ in that additional steps are required for the activation of PKC as well as the generation of two intracellular messenger molecules. The event that caused the activation.PKCAs shown below.

Here are the details of the steps leading up toPKCactivation. Effector signaling molecules bind to their receptors, activating integral membranes.phospholipase Cheavy.phospholipase CCatalyzes the formation of cytoplasm.inositol triphosphate(intellectual property₃) and bound by a membranediacilglicerol(forehead), two other intracellular secondary messenger molecules. IP3 interacts with receptors on the smooth endoplasmic reticulum, resulting in the release of sequestered Ca++ ions into the cytoplasm. Finally,football²⁺Activation of ions and DAGprotein kinase C(PKC) then initiates an amplification cascade of phosphorylation that leads to a specific cellular response.

318 G protein activates protein kinase C and phospholipase C

Actions mediated by protein kinase C include:

• Release of neurotransmitters.
• Hormone secretion (somatotropin, luteinizing hormone, testosterone) causes cell growth, division and differentiation.
• Hidroliza glycogena, sinteza masti.

additionally independentphospholipase CEffects include:

• Decomposition of liver glycogen.
• Pancreatic amylase secretion.
• platelet aggregation.

PKA and PKC areserin-treonin kinazaThey place phosphates on serines or threonines on the target polypeptide. Next, let's consider tyrosine kinases.

C. Signal transduction mediated by receptor tyrosine kinases

The intracellular activity of these receptors is located in the cytoplasmic domain of the receptors themselves. When bound to their effectors, receptor kinases catalyze the phosphorylation of specific tyrosine amino acids on target proteins. while studying the plotnerve growth factor(nerve growth factor) gepidermal rasta factor(epidermal rasta factor) to stimulate the growth and differentiation of nerves and skin, discovered by Stanley Cohen and Rita Levi-Montalciniepidermal growth factor receptor, Firstassociated with enzymes tyrosine inhibitor...and received the Nobel Prize for Physiology or Medicine in 1986! See the animation of receptor kinase signaling in the link below (description is provided in the following paragraphs).

Kinase 319 receptor signaling

Monomeric membrane receptor kinases form dimers after binding to effector ligands, which now contain sulfhydryl groups.SH₂proteinbound to each monomer. This activates the kinase domain of the receptor. After multiple cross-phosphorylation of receptor monomers, SH₂The protein is removed, allowing the receptor to interact with other cytoplasmic proteins to continue the response pathway. A characteristic response to EGF and NGF signaling is cell proliferation. Not surprisingly, mutations associated with cancer cells are often found in signaling pathways that lead to cell proliferation (growth and division). oncogenic, oroncogeneIt was actually first discovered in viruses, but J. Michael Bishop and Harold Varmus won the Nobel Prize in Physiology or Medicine in 1964 for showing that cells were, in fact, the origin of the chicken retrovirus (chicken sarcoma virus). Rous).oncogeneIt turned out to be a mutation in the gene for a protein in the mitogenic signaling pathway. Under normal conditions, mitogenic chemical signals (such asepidermal rasta factor) binds to its receptor and induces the target cell to start dividing.

It isRussianActivation of protein-mediated phosphorylation cascades leads tocan(mitogen-activated protein）antidoteis an example of a signaling pathway that plays a central role in many receptor kinase signaling pathways. The Ras gene is one of the first discovered oncogenes whose mutations cause uncontrolled cell division, known as cancer. In fact, Ras gene/protein activity may be responsible for up to 30% of all cancers!

320 RAS oncogenes, their normal mitotic function and cancer

MAP kinase phosphorylationtranscription factorand other nuclear proteins that influence gene activity and lead to cell proliferation and differentiation, as shown below.

D. Evolution of signal transmission

We see that signal transduction often requires some signaling molecule to interact with some receptor on the cell surface to amplify the response in a series of enzymatic reactions (usually phosphorylation) to activate (or inactivate) the target protein. Amplification cascades can take individual interactions between receptors and effectors and amplify their effects in cells by several orders of magnitude, making signaling systems fast and efficient. Cellular and systemic (organism) responses to the same chemical signal are extensive and complex. Different cell types can have receptors for the same effectors, but react differently. For example, epinephrine targets cells such as the liver and blood vessels, each of which has a different effect. In fact, adrenaline is also a neurotransmitter. It is clear that as organisms evolve, they become more sophisticated in response to the demands of their environment, adapting by exploiting existing signaling systems to serve new pathways. Just as the same signaling event can lead to different response pathways in different cells, evolution allows the involvement of different signaling pathways.diaphony. This is a situation where two different signal transmission paths intersect in the same station. In one example, cAMP produced at the upstream end of the PKA signaling pathway can activate (or, where appropriate, inhibit) enzymes in the MAP kinase pathway. These effects cause changes in the levels of active or inactive transcription factors, so that two (or more) signals can be used to regulate gene expression. We're just beginning to understand what doesn't seem quite linear.