Unit 3 - Cell Signalling

Question 1

what is the difference between a protein kinase and phosphatase?

Answer 1

kinase - phosphorylates - switches proteins on - active form

phosphatase - dephosphorylates (removes a protein) - switches off - inactive form

Question 2

what is the effect of protein kinase to the proteins?

Answer 2

conformational changes in proteins causes post-translational modifications in different parts of the protein
- phosphoserine/ phosphothreonine/ phosphotyrosine

Question 3

what are the two main classes of kinases?

Answer 3

serine/ threonine kinases: 
- PKA, PKB, PKC
- MAPK
- cyclin dependent kinases
tyrosine kinases:
- growth factors
- oncogenes

Question 4

what is a protein kinase cascade?

Answer 4

receptor kinases is activated by hormone and becomes autophosphorylated and then series of phosphorylations which amplifies the amount of protein at each level
- e.g. MAPKKK - MAPKK - MAPK

Question 5

what is the difference in calcium concentration in ECF and cytosol?

Answer 5

ECF = 1mM
cytosol = 100 nM
(thousandfold)

Question 6

what causes release of calcium from endoplasmic reticulum?

Answer 6

neurotransmitter acts on voltage gated sodium channel
depolarisation causes sodium influx
triggers opening of voltage gated calcium channels on ER
OR
production of IP3 activates release of calcium

Question 7

what protein does calcium bind to and what effect does this protein have?

Answer 7

calcium binds to calmodulin
calmodulin activates CAM kinase which triggers a protein cascade and targets different proteins such as:
- myosin light chain kinase = smooth muscle contraction
- nitrogen oxide synthase = release of nitrogen oxide

Question 8

give five examples of 2nd messenger molecules

Answer 8

cAMP
cGMP
PIP3
DAG
IP3

Question 9

what is the effector of cAMP and describe what it activates

Answer 9

adenylate cyclase (catalyses conversion of ATP to cAMP)
cAMP activates PKA
(PKA inactivates myosin light chain kinase or activates glycogen phosphorylase)

Question 10

what is the effector of cGMP and describe what it activates

Answer 10

guanylate cyclase (catalyses conversion of GTP to cGMP)
cGMP activates PKG
(PKG activates myosin phosphatase)

Question 11

what is the effector of PIP3 and describe what it activates

Answer 11

PI3K (catalyses conversion of PIP2 to PIP3)
PIP3 activates PDK1 to activate PKB
(PKB activates mTOR leading to cascade of signalling for growth and proliferation)

Question 12

what is the effector of DAG/ IP3 and describe what it activates

Answer 12

phosholipase C (catalyses hydrolysis of PIP2 to DAG/IP3)
IP3 binds to calcium channels on ER to release more calcium
DAG activates PKC (calcium also activates PKC) which activates PIP2 to produce more IP3 - amplify release of calcium

Question 13

describe the structure of G proteins

Answer 13

7 transmembrane alpha helices with 3 subunits: alpha/ beta/ gamma

Question 14

describe the mechanism of G proteins

Answer 14

ligand binds to GPCR - alpha subunit exchanges GDP fror GTP

- alpha subunit dissociates and regulates effector which relays signal by 2nd messenger molecules

Question 15

how is GTP hydrolysed and what is the significance of this?

Answer 15

hydrolysed by GTPase back into GDP
rate of hydrolysis determines the length of time protein in active form
(can be modulated by RGS and GAP proteins)

Question 16

name three classes of G proteins

Answer 16

Gas - activates adenylate cyclase
Gai - inhibits adenylate cyclase
Gaq - activates phospholipase C

Question 17

describe the mechanism of enzyme- linked receptors e.g. RTK - receptor tyrosine kinases

Answer 17

agonist binds to domain on receptor
undergoes dimerisation reaction
phosphorylates each other
act as docking site for signal proteins

Question 18

how is cGMP/cAMP/ PIP3 degraded?

Answer 18

cGMP/cAMP degraded by phosphodiesterase

PIP3 degraded by PTEN

Question 19

how are GPCR regulated?

Answer 19

binding of arrestin blocks G protein binding

Question 20

how does acetylcholine induce nitric oxide production?

Answer 20

Ach binds to G protein which uses class Gaq to activate phospholipase C - catalyses hydrolysis of PIP2
production of IP3 causes release of calcium activating calmodulin - stimulates NOS

Question 21

what is the effect of nitrogen oxide?

Answer 21

activates guanylate cyclase which catalyses conversion of GTP to cGMP
cGMP activates PKG which activates myosin light chain phosphatase
- smooth muscle relaxation and vasoconstriction

Question 22

how does insulin induced signalling promote glucose uptake?

Answer 22

insulin binds to RTK forms a docking site for IRS protein
IRS activates PI3K to convert PIP2 to PIP3
PIP3 activates PDK1 to PKB which stimulates more GLUT4 incorporated into membrane

Question 23

how does glucagon promote glycogenolysis?

Answer 23

glucagon activates Gas - adenylate cyclase - PKA

PKA phosphorylates phosphorylase kinase which activates glycogen phosphorylase (glycogen - glucose 1P-glucose6P-glucose)

Question 24

how does PKA promote gluconeogenesis?

Answer 24

phosphorylates PFK2 (substrate cycle) and activates FBPase2 which converts Fru 2,6 BP to Fru 6P 
fru 6P - glc 6P to glucose