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
examples of some effectors
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
summary of the views on the heart
summary
QISS QIQ mneumonic
