S3) Intracellular Signalling Pathways Flashcards
What are G Proteins and what do they do?
- G-protein coupled receptors are a superfamily of receptors with an enormous diversity of cellular functions
- They alter the activities of effectors
Describe the structure of G proteins
- Heterotrimeric i.e. consist of three subunits: α, β, γ
- The β and γ subunits bind tightly to each other and function as a single unit
Describe the properties of the α-subunit of the G protein
- The α-subunit has a guanine nucleotide binding site
- The α-subunit possesses intrinsic GTPase activity as this site binds GTP and then slowly hydrolyses it to GDP
Describe the activity/appearance of the G protein under basal resting conditions
- G-protein is present at the inner face of the plasma membrane in its heterotrimeric form
- GDP is bound to the α subunit
Explain the effects of ligand binding on the GPCR
- The ligand binds with the GPCR and activates it
- The activated receptor has a high affinity for the G-protein
- GDP is later released and exchanged for GTP by the α-subunit
What role does the GPCR take on when GDP is exchanged for GTP?
The receptor acts as a guanine nucleotide exchange factor (GEF)
Describe the affinity of the GPCR for the G protein after the guanine nucleutide exchange
- The binding of GTP to the α-subunit decreases the affinity of the receptor for the G-α and Gβγ subunits
- Thus, both α and βγ subunits are released and can each can interact with effectors
Explain how the GPCR cycle is reactivated after subunit interaction with effectors
- Effector interaction is then terminated by α-subunit’s intrinsic GTPase activity which hydrolyses GTP → GDP
- Affinity of the Gα-subunit for the Gβγ-subunit then increases
- Gαβγ hetero-trimer is reformed & awaits reactivation
State the structure and function of Gs/Gαs
- Structure: Gs carries the β-adrenoceptor for adrenaline and the enzyme adenylyl cyclase
- Function: stimulates adenylyl cyclase
State the structure and function of Gi/Gαi
- Structure: an inhibitory version of G protein for the activation of adenylyl cyclase
- Function: inhibits adenylyl cyclase
Outline the mechanism of action of Gq
- Gq proteins preferentially interact with the membrane-bound enzyme phospholipase C
- This causes hydrolysis of a minor plasma membrane phospholipid (PIP2) and generates 2 messengers: IP3 and DAG
For the Gs, Gi and Gq proteins, compare and contrast the following:
- Endogenous signalling molecule
- Receptor
- Effector
- Physiological response
State the structure and function of adenylyl cyclase
- Structure: an integral plasma membrane protein
-
Function: hydrolyses cellular ATP to generate cAMP
1) G coupled protein receptor activated by agonist (ligand)
2) A receptor (YBA) binds to G protein
3) G protien undergoes GDP-GTP protein exchange
4) this then activates adenylyl cyclase converting ATP -> CAMP
5) CAMP can cause cellular changes
6) cAMP stimulates protein Kinase A
7) phosphorylation of proteins
State the activation and inhibition of adenylyl cyclase
- Activation: via Gs – noradrenaline/adrenaline at β-adrenoceptors
- Inhibition: via Gi – noradrenaline/adrenaline at α2-adrenoceptors
What does cAMP do?
Cyclic AMP interacts with protein kinase A (PKA)
What does PKA do?
PKA phosphorylates a variety of cellular proteins to increase/decrease their levels of activity:
- Glycogenolysis and gluconeogenesis in the liver
- Lipolysis in adipose tissue
- Smooth muscle changes
- Inotropic and chronotropic effects in the heart
- low conc of CAMP - R binds to C and its activated
- high conc of CAMP -2 CAMP binds to one R and weaken subunits, releasing C which will change activity/stability of protein
State the structure and function of Phospholipase C
- Structure: an effector enzyme in the cell signalling pathway involving the hydrolysis of PIP2
-
Function: generates secondary messengers – IP3, DAG
1) agonist binds to GPCRs
2) interacts with G proteins containing Alpha Q subunits
3) GDP -> GTP
4) GTP interacts with PLC and activates it
5) PLC cleaves PIP2
6) PIP2 -> IP3
7) iP3 interacts with receptors in ER and allows Ca to move out into cytoplasm down conc gradient
8) DAG is the co-product of cleaving PIP2 which stays in the membrane. Interacts with PKC (phosphorylates proteins)
9) sometimes ca interacts with PKC
What is the role of IP3 ?
IP3 interacts with intracellular receptors on the ER to release Ca2+ from its so it can enter the cytoplasm e.g. activation of Ca2+- sensitive protein kinases
What is the role of DAG?
DAG interacts with a family of protein kinases (PKC), which are activated by this second messenger
The various intracellular signalling pathways involving GPCRs can be summarised using the mnemonic QIS.
What does this mean?
Q = PLC pathway
I = Inhibitory AC pathway
S = Stimulatory AC pathway
The various types of adrenergic receptors can be summarised using the mnemonic QISS.
What does this mean?
Q - α1
I - α2
S - β1
S - β2
(Q = PLC pathway, I = Inhibitory AC pathway, S = Stimulatory AC pathway)
The various types of muscarinic receptors can be summarised using the mnemonic QIQ
What does this mean?
Q - m1
I - m2
Q - m3
(Q = PLC pathway, I = Inhibitory AC pathway, S = Stimulatory AC pathway)
examples of some effectors
name 3 other versions of effectors
- ion channels
- voltage operated Ca channels (VOCCS)
- G - protein regulated inwardly - rectifying K channels (GIRKs)
give examples of other GCPRs that interact with receptors to activate adenylyl cyclase
B-adrenoreceptors
D1- dopamine recpetors
H2 - histamine receptors
inhibition of adenylyl cyclase
different types of GPCR preferentially interact with Gi type G proteins
to facilitate GDP-GTP exchange
releasing active alpha i components
and inhibit enzyme
examples of different GPCRs that inhibit adenylyl cyclase
a2-adrenoreceptors
D2 - dopamine receptors
u-opioid receptors
difference between PKC and PKA
both phosphorylate key proteins
difference:
have different selection of proteins to phosphorylate
summary of the views on the heart
calculating heart beat using 300 rule
- regular rhythm
300/ number of big boxes (5 small boxes) between beats
calculating heart fare for irregular heart rate
count the number of beats in 6 seconds
x10
= heart beats per min
types of rhythm based on intervals
PR interval should be 0.12-0.2 s (3-5 small boxes)
QRS interval <0.12s and <3small boxes if its widened then depolarisation in ventricle isnt spreading via bundle of his and purkunjie fibres so it takes longer
example of a signalling pathway
inotropy in the heart
1) adrenaline/noradrenaline interact with ventricular B1 adrenoreceptors
2) this leads to activation of pKA
3) this phospohorylates ca channels
4) this allows more Ca to enter the next time the membrane depolarises
5) greater change in ca and enhanced contractility
6) more contraction next time
smooth muscle example
1) noradrenaline released and intercats with a1 adreno receptors
2) = vasoconstriction
3) activates Ca2+/PKC signal transduction pathways (overall increases ca conc in cell and more contraction of smoothe muscle)
signal amplification
small change to cause a massive cellular response
eg B-adrenoreceptor -> Gs protein -> adenyl cyclase => the production of adenyl cyclase causes a big physiological change
summary
QISS QIQ mneumonic
