Enzyme-Coupled Receptors

Question 1

12.Describe fundamental functional domains of enzyme-coupled receptors and name at least two examples of receptor tyrosine kinases.

Answer 1

● Extracellular signal molecule binding domain
● Single transmembrane domain
● Cytosolic domain

● Examples: Insulin receptor, ephrin receptor, nerve growth factor

Question 2

13.Describe how signaling complexes develop and the types of interaction domains facilitating these complexes.

Answer 2

● signal molecule binds to RTKs that causes conformational change and activates kinase.
● Auto-phosphorylation
● Phosphoryl groups serves as docking sites for intracellular signalling proteins

○ Signaling protein activation can occur by docking or through phosphorylation

Question 3

14.Describe how monomeric G proteins differ from GPCR-G proteins and the types of signaling cascades they regulate. Be able to sketch a typical kinase/phosphorylation cascade.

Answer 3

● RTK receives a signal, autophosphorylates and can then recruit an adaptor protein which recruits another enzyme, a GEF, a Ras activating protein, which then activates RAS (by exchanging a gdp for gtp) .
○ RAS contains a lipid anchor and is associated with membrane
○ Resembles alpha subunit of trimeric G proteins
○ Serves as a hub for signal-protein complexes

● Ras activates a MAP kinase kinase kinase (Raf)
○ Which activates MAP kinase kinase (Mek)
○ Which activates MAP kinase (Erk)
○ Which can phosphorylate target proteins and modulate enzymatic activity or gene expression

Question 4

15.Describe the activity of PI3-kinase and type of 2nd ​messenger produced by this enzyme.

Answer 4

● Lipid anchored docking sites
● PI3 kinases dock and become active when attached to Tyrosine Kinase.
● PIP2 phosphorylated to PIP3
●PIP3 is most important binding site for signaling.
○ Not cleaved by Phospholipase C
○ active till de-phosphorylated

Question 5

16.Understand how enzyme-coupled receptors contribute to programmed cell death and how inhibitors of transduction pathways can induced apoptosis.

Answer 5

● Activated Receptor Tyrosine Kinase → activated PI 3-Kinase → production of PIP3 → docking sites for PDK1 and AKT (PDK is a kinase that phosphorylates AKT) and together with mTOR causes activation of AKT → allows it to release from the membrane
○ Released AKT phosphorylates Bad causes release of inhibitory protein and Bad can interact with other protein
○ It’s the release of the apoptosis inhibitory protein that leads to the inhibition of apoptosis
○ Promotes cell signaling
● Without active Akt cells will die
○ Tor is target of Rapamycin; inhibited Tor downregulates Akt

Question 6

17.Know the functional differences between receptor tyrosine kinases and serine/threonine kinases; provide an example of each.

Answer 6

Example of Tyrosine Kinase
● Ser/Thr Kinase is structurally similar to tyrosine kinase
○ Form heterodimers & fast acting

■ Directly phosphorylate SMADs (transcription factors)
● Ex: TGF-Beta superfamily
■ Heterodimers
■ Trans-phosphorylation of one subunit
■ Go on to phosphorylate other factors in this case Smad2/3
■ Smad 2/3 recruits Smad4, translocates to nucleus to regulate genes

■ When thinking of TGFbeta signaling,think heterodimeric receptor Ser/Thr kinase also think fast acting
○ TNFalpha receptor family
■ Trimer receptor
■ Activation of Ikappa-Kinase complex phosphorylates IkappaB leading to its degradation
■ Releases NFKappaB to go to nucleus and induce transcription