intracellular signal transduction pathways

Question 1

core concepts

Answer 1

environmental changes must be sensed by the cell
cells respond to environmental changes by activating or inhibiting the activity of specific intracellular signalling pathways to change their biological function/fate
cells can organise pathways efficiently
activity can be amplified
activity can be altered by positive and negative feedbacks
one ligand = different biological outcomes in different cells
one receptor/ligand = activation of more than one pathway

Question 2

environmental changes

Answer 2

sensed by the cell via activation of intracellular (nuclear) or cell surface (GPCr, ligand-gated, enzyme-linked) receptors
response occurs by:
* conformational changes in intracellular signalling molecules
* post-translational modification
* generation of second messengers

Question 3

conformational change

GPCR example

Answer 3

GPCR activation by GDP replacement with GTP to induce a conformational change

Question 4

post-translational modifications

Answer 4

phosphorylation
uniquitylation

Question 5

second messengers

Answer 5

adenylyl cyclase
phospholipase C

Question 6

adenylyl cyclase

role as a second messenger

Answer 6

catalyses the formation of cyclic AMP from ATP releasing a molecule of pyrophosphate
cAMP is only produced via GPCR activation

Question 7

phospholipase C

role as a second messenger

Answer 7

β isoform activated by GPCR
𝛾 isoform activated by kinase-linked receptors

Question 8

protein kinase A pathway

Answer 8

ligand binds to GPCR causing a conformational change, and GDP is replaced by GTP
⍺-subunit dissociates and activates adenylyl cyclase which catayses the formation of cAMP from ATP
cAMP activates protein kinase A which exists in an R2C2 complex
binding of 4 cAMP molecules to the two regulatory subunits causes a conformational change to release the two catalytic subunits which can phosphorylate target proteins

Question 9

PKA target protein one

Answer 9

Ca2+ handling proteins
> muscle contraction and relaxation

Question 10

PKA target protein two

Answer 10

metabolic enzymes
> releases of glucose from glycogen

Question 11

PKA target protein three

Answer 11

PKA activates CREB which activates CBP at the CRE
> gene transcription

Question 12

phospholipase C pathway

Answer 12

PI 4,5-bisphosphate
> diacylglycerol (DAG) >activates protein kinase C
> inositol 1,4,5-triphospate (IP3) > releases Ca2+ from
lumen of the endoplasmic reticulum

Question 13

pathway organisation

Answer 13

components can be pre-assembled within a small 3D area, organised by a large scaffold protein
e.g. MAPK

Question 14

Mitogen - Activated Protein Kinase (MAPK) pathway

Answer 14

extracellular signals > cellular responses
* growth
* differentiation
* stress response
* apoptosis

Question 15

1. receptor activation

MAPK pathway

Answer 15

cell surface receptor (kinase-linked or GPCR) is activated by an extracellular signal (growth factor, cytokine, stress signal)

Question 16

2. Ras activation

MAPK pathway

Answer 16

Ras, a GTPase, is activated by GTP replacing GDP to transmit the signal

Question 17

3. MAPK cascade

MAPK pathway

Answer 17

Ras binds to and activates Raf (MAPKKK)
Raf phosphorylates and activates MEK (MAPKK)
MEK phosphorylates and activates ERK (MAPK)

Question 18

4. target protein activation

MAPK pathway

Answer 18

ERK translocates to the nucleus and phosphorylates targets such as transcription factors which initiates gene expression

Question 19

signal amplification

Answer 19

occurs by:
* increase in the number of available ligands
* increase in the number of available receptors
* use of enzymes
* positive feedback

Question 20

feed back pathways

Answer 20

positive > stimulates its own production
negative > inhibits its own production
alters the amplitude, duration and reversibility of a response

Question 21

positive feedback pathways examples

Answer 21

intracellular calcium signalling
M-phase cyclin/CDK complexes at the G2/M checkpoint

Question 22

intracellular calcium signalling

positive feedback example

Answer 22

binding of four Ip3 molecules activates the Ca2+ channel on the Er to release stored Ca2+, this activates the CRAC channel causing an influx of more Ca2+ ions.

Question 23

one ligand = differing biological outcomes

Answer 23

smooth muscle in bronchi vs blood vessels example