free essays
Free essays

Cell Communication



1.Describe each step in the epinephrine signaling pathway beginningwith epinephrine and finishing with the breakdown of glycogen.Explain how a futile cycle is prevented in this signaling pathway.&nbsp

Signaltransduction coordinates cellular activities and is essential to themulticellular organisms. The endocrine system produces hormones asintracellular messengers. There are four types of chemicals that canserve as an extracellular signaling molecule and one example of it isthe epinephrine. The hormone epinephrine is also called theadrenaline. Epinephrine acts as a signal by binding to a receptor. Itis a kind of second-messengers which amplifies signals and integratesresponses among cell types.

Epinephrineacts on muscle tissue and raise the levels of glucose demand. It isrelease to respond from stress. It sets off a chain of events when itbinds to specific receptor which activates a number of stimulatory Gproteins. This increase in the enzyme activity involvesgluconeogenesis and glycogen breakdown. Activation of stimulatory Gproteins also activates adenylate cyclase. The cAMP is then producedand activates a cAMP-dependent protein kinase.

Thisreaction activates phosphorylase kinase synthesis and slowingglycogen synthesis. Glycogen is broken down since phosphorylasekinase activates Glycogen phosphorylase.

Secondmessengers amplify and transmit signals received at the cell surfacewithin the cells. The cell regulates its concentration by rapidlyproducing and activating second messengers then also rapidlyinactivating and degrading it when it is not needed. Counterbalancingactivities function at each step of the cascade makes the systemreturns to a resting state when the stimulus is removed. Insulintriggers the feedback response to ensure that the amount of glucosedoes not reach at excessive amount.

2.Describes the structure of a G-protein coupled receptor in detail.What parts of the receptor allow for the incredible diversity ofsignals GPCRs can be activated by? What is the important about theC-terminus of the receptor and describes events that occur when aGPCR is activated (the critical initial steps)

Gprotein is a mobile protein that is exposed when ligand binding onthe extracellular side of the receptor changes the shape of itscytoplasmic region. It is partially inserted into the lipid bilayerso that the receptors are exposed on the cytoplasmic surface of themembrane. It is a special protein that binds the energy-rich moleculeGTP. A G protein-coupled receptor is a plasma membrane receptor. Gprotein is active when the GTP is bound to it while inactive when GDPis bound to it thus acts like a switch in the plasma membranereceptor. It has the similar structure to G protein coupled receptor.A G protein-coupled receptor is a plasma membrane receptor. It has 7alpha helixes and there are loops between helixes to form bindingsites for the signaling molecules.

Likethe G protein, GPCR also associate the tri metric shape whichconsisting of the alpha, beta and gamma subunits. After the ligandbinds to the receptor, the G protein is activated then dissociatesinto an alpha and a beta-gamma complex. This activity also generatesthe second messengers. The G-alpha subunit activates the enzymeadenylyl cyclase which catalyses the formation of the secondmessenger cyclic AMP (cAMP). G-alpha subunit also activates thephospholipase C which generate two second messengers. The structureof the C terminus tail of the GPCR also served important functionswhich are beyond ligand-binding. It contains Serine or Theorine whichwhen phosphorylated, increase the affinity of the intracellularsurface for the binding of proteins called beta arrestins.

3.Describe three mechanisms in detail that are responsible for proteinkinase A to be self-regulatory. List the steps for each mechanism.

Proteinkinase A is a class of enzyme which modifies a protein when aphosphate is attached to it. Protein kinase A is activated by cAMPand regulates the activity of key enzymes in many essential metabolicpathways due to phosphorylation. Mostly it is found in the cytoplasmand in the nucleus. Regulation of PKA in the cell is modulatedprimarily of its phosphortransferase activity.

Oneof the mechanisms of protein kinase A is its activation. Itsactivation is terminated when it dephosphorylates enzymes such as theglycogen metabolism phosphoprotein phosphatase-1. Some of thephosphatases are self-regulated by phosphorylation. An example ofthis is the phosphoprotein phosphatase-1 which is regulated byinsulin which is stimulated by the protein kinase a. In theactivation of PKA, two molecules of cAMP are binded to two Rsubunits. The regulatory subunits are move out of the active sitesand the complex dissociates.

Thebinding of cAMP dissociates the complex in an allosteric mechanism.The catalysis of the subunits occurs and transfers the ATP terminalphosphates to protein substrates. The phosphorylation usually resultsin a change in activity of the substrate. PKA regulation is involvedin different pathways since they are present and act on differentsubstrates. PKA has a direct effect on the activity of a protein inthe direct protein phosphorylation.

Inactivationof protein kinase A occurs when a feedback mechanism is activated.The reduction of the amount of cAMP also reduced the activation ofPKA. PKA also activates phosphodiesterase which converts cAMP to AMP.Since protein kinase A is controlled by cAMP the catalytic subunititself can be down-regulated by phosphorylation. Protein kinase A isself-regulatory due to the process itself.

4.How is the receptor phosphatase, receptor protein tyrosinephosphatase (RPTP) capable of initiating migratory cell behavior andchange gene expression? (Be specific)

Receptorprotein tyrosine phosphatase is a group of enzyme which removesphosphate groups from tyrosine residues that are phosphorylated. Itis created by a reversible reaction controlling the intracellularcommunication. It affects protein stability and regulates enzymeactivity. Maintaining the appropriate level of protein tyrosinephosphorylation is essential to cellular functions since they are thekey regulatory components in signal transduction pathways. Tyrosinephosphorylation is catalyzed by protein tyrosine kinases.

Varioussignals that activate or inactivate the transcription factors whichact on the nucleus regulate the gene expression. Protein tyrosinephosphatase catalyzes the removal of a phosphate group using acysteinyl-phosphate enzyme intermediate. These enzymes are theregulatory components in the signal transduction pathways which areessential in cell cycle control, cell growth, differentiation andtransformation.

ProteinPhosphorylation on tyrosine residues plays a key role in the cellbehavior of the multicellular organism. Protein tyrosine phosphatasecontrol phosphotyrosine levels which are found in both cytoplasmicand transmembrane forms. The biochemical interactions between themlead to a diversity of cellular behavior and can change geneexpression.

Allcell types may express individual protein tyrosine phosphatase ortheir expression may be tissue-specific. The expression of severalprotein tyrosine phosphatases is restricted to hemotopoietic cells,however. The control for phosphortyrosine may also affect the geneexpression by creating overexpression of genes due to the static partof the protein tyrosine phosphatase.

5.What is phospholipase C? How is it activated (be specific)? What isits direct substrate and products? List the two mechanismsphospholipase C can activate the Mater Kinase, Protein Kinase C?

PhospholipaseC is an enzyme that removes the polar head group including thephosphate from the phospholipids. It plays an important role ineukaryotic cell physiology and signal transduction pathways.Receptors that activate this pathway are mainly G protein coupledreceptors coupled to G alpha unit. Examples of these units are5-HT2&nbspserotonergicreceptorsand calcitonin receptors. Phospholipase C activation by cell surfacereceptors triggers a wide variety of cellular responses. Initially,ligand binds to the cell surface receptors creating a chain ofsimilar molecular events that involve heterotrimetric Gnucleotide-binding proteins and phospholipase C enzymes. It leads tothe hydrolysis of plasma membrane regulatory lipid,phosphatidylinositol, and 4,5-biphosphate.

Diacylglycerolor Calcium ions activate the protein kinase C. The source ofdiacylglycerol that activates the protein kinase C can be derivedfrom the hydrolysis phosphatidylinositides. It can also be derivedfrom the use of phospholipase C from the metabolism of thephosphatidylcholine. It can be regulated by more than one secondmessenger. Diacylglycerol derived mainly from the phospholipase Cactivation consist mainly of 1-stearolyl and 2-arachidonyl fatty acidwhich cannot be altered by glucose.

PhospholipaseC cleaves the phospholipid phosphatidylinositol 4,5-bisphosphate intodiacyl glycerol which remains bound to the cell membrane andinositol 1,4,5-triphosphate which is released as a soluble structureinto the cytosol. It then diffuses through the cytosol to bind itsreceptors. This causes the cytosolic concentration of calcium toincrease causing a cascade of intracellular changes. The calcium thenactivates the protein kinase C.


Nelson,David and Cox, Michel. LehningerPrinciple of Biochemistry. 5thedition. ISBN. February 1, 2008