Unit 8

Question 1

cAMP

Answer 1

• Formed by ATP by a cyclisation reaction: removes 2 phosphate groups & joins the “free” and of the remaining phosphate group to the sugar part of ATP molecule
• Conversion of cAMP to AMP performed by a phosphodiesterase enzyme that reduces one of the ester bonds
• Effect of cAMP-dependent protein kinase

Question 2

Glycogen

Answer 2

• Polymer of glucose
• Major storage form of glucose
  1.) Glucose coupled to UDP
  2.) Glucose incorporated into polymer by glycogen synthase
  1.) When glucose needed glycogen phosphorylase removes the sugar as glucose-1-phosphate\
  2.) Then converted to glucose-6-phosphate & incorporated into the glycolyte pathway in muscle cells

Question 3

Adrenaline

Answer 3

• Binds to its receptor inducing a conformational change that activates the receptor
• Leads to activation of α subunit through conformational change which activates adenylyl cyclase
• cAMP levels rise in cell

Question 4

PKA

Answer 4

• Protein kinase A
• Binding of PKA to cAMP activates the enzyme
• Targets
  a.) Glycogen synthase: phosphorylated form is inactive & prevents glycogen production
• Can be restored to active form by actions of a phosphatase called protein phosphatase 1
  b.) Phosphoryl kinase
• Once this kinase is phosphorylated, it is activated. Activation also reversible by protein phosphatase 1
• Then phosphorylates glycogen phosphorylase. This enzyme then initiates glucose removal from glycogen
• Glycogen phosphorylase activation can be reversed by action of protein phosphorylase 1

Question 5

CaM

Answer 5

• Calmodulin
• Binds to Ca2+
• In Ca2+-bound state it binds to numerous CaM kinases & other proteins that elicit diverse responses
  a.)
  1. CaM·Ca2+ activates myosin light chain kinase (MLCK) (a CaM kinase)
  2. MLCK phosphorylates the light chain of myosin which then binds to actin to initiate muscle contraction.
  b.)
  1. CaM·Ca2+ activates CaMKII (a CaM kinase)
  2. CamKII enters the nucleus through the pores and phosphorylates CREB.
  3. CREB dimerizes and binds to DNA at CRE sequences, and recruits p300 and CBP to activate gene transcription.
  c.)
  1. CaM·Ca2+ activates calcium-calmodulin dependent phosphodiesterase 1
  (PDE1), highly expressed in brain, heart and lung.
  2. PDE1 converts cAMP (produced by adenylyl cyclase) to AMP,
  downregulating cAMP-dependent signaling pathways.

Question 6

Ras

Answer 6

• Small GTP binding protein
• Inactive Ras can associate with Sos, so it can uptake GTP
• Activated Ras binds to complex of kinase Raf & 14-3-3 protein (Raf kinase activity inhibitor)
• Binding relieves 14-3-3 inhibition & allows Raf phosphorylation & subsequent activation
• When Ras hydrolyses its GTP to GDP & Pi, it releases activated Raf

Question 7

GRB2

Answer 7

• An SH2 domain-containing protein (binds to phosphotyrosine residues)
• Mediates attachment of Sos to activated EGF receptor
• GRB2-Sos complex binds to activated receptor through the SH2 domain of GRB2

Question 8

EGF

Answer 8

• Epidermal growth factor
• Tyrosine Receptor kinase activity
• Binds to its receptor, initiating a cascade of events leading to the activation of the small GTP binding protein Ras
  1.) Aggregation of EGF receptors
  2.) Autophosphorylation of the receptors on tyrosine residues on the cytosolic tail

Question 9

Sos

Answer 9

• Nucleotide exchange factor for Ras, activates the GTPase
• GRB2-Sos complex binds to activated receptor through the SH2 domain of GRB2
• Inactive Ras can then associate with Sos
• Sos then activates Ras by releasing its GDP & assisting in its uptake of GTP

Question 10

Raf

Answer 10

• Activated Ras binds to complex of kinase Raf & 14-3-3 protein (Raf kinase activity inhibitor)
• Binding relieves 14-3-3 inhibition & allows Raf phosphorylation & subsequent activation
• When Ras hydrolyses its GTP to GDP & Pi, it releases activated Raf
• Activated Raf then phosphorylates & activates downstream kinase MEK

Question 11

MEK

Answer 11

• Activated Raf then phosphorylates & activates downstream kinase MEK
• Activated MEK then phosphorylates & activates downstream kinases called MAP kinases (have over 160 known targets)

Question 12

Insulin receptor

Answer 12

• Heterotetramer, composed of 2 α & 2 β subunits
• α subunits are extracellular & form binding pocket for insulin
• Binding of insulin changes α subunits conformation. Because α & β subunits are covalentaly linked through disulfide bonds, β subunits undergo conformational change
• Activates its tyrosine kinase activity & it autophosphorylates
• Autophosphorylated receptor binds to adaptor proteins (insulin receptor substrates (IRS1 & IRS2)). Proteins are also phosphorylated by receptor

Question 13

IRS 1 & 2

Answer 13

• Autophosphorylated receptor binds to adaptor proteins (insulin receptor substrates (IRS1 & IRS2)). Proteins are also phosphorylated by receptor
• IRS 1 can activate several pathways include Ras by binding to GRB2-Sos & a phosphoinostide pathway by binding to lipid kinase PBR

Question 14

Delta

Answer 14

• Ligand for Notch receptor
• Transmembrane protein

Question 15

Notch

Answer 15

• Receptor for the Delta ligand
• Transmembrane protein
• When interact with Delta, Notch cleaved by 2 proteases
  a.) ADAM10 - cleaves within membrane-spanning region & releases a fragment of Notch intracellular domain