G protein couples receptors 1/2

What is the process of a drug receptor complex forming and triggering a chain of events called?

signal transduction or receptor signalling

What is the series of biochemical events from a DR complex called?

signalling cascade or signal transduction pathway.

What are the 3 stages of the receptor signalling process?

• signal reception- agonist drug binds to and activates receptor.

• signal transduction- drug receptor complex activates relay proteins and messengers

• cellular response- biological response to original binding triggered.

What are the 4 major signal transduction pathways?

• activation of receptor ion channels- ligand gated receptors

• activation of second messenger pathways via G-protein coupled receptors

• activation of enzyme linked receptors- TK.

• direct activation of gene transcription via intracellular receptors.

What is a receptor superfamily?

group of receptors with similar basic molecular structure that use the same signal transduction pathway.

Which are the largest receptor superfamily in humans?

G-protein coupled receptors

What do G protein coupled receptors mediate responses to?

hormones, neurotransmitters, growth factors, vision

What are the 3 key features and characteristics of GPCRs?

• all share motif of 7 transmembrane a-helices

• couple up to and activate G proteins on agonist binding

• couple to adaptor proteins called B arrestins.

A GPCR consists of a single polypeptide chain with 3 regions. What are they?

• extracellular region- N terminus.

• TM region- 7 TM a-helices

• intracellular region= three loops, a helix and C terminus.

Where are heterotrimeric G proteins localised?

inner leaflet of plasma membrane

What are the 3 subunits of heterotrimeric G proteins?

a, B and y

What does the Ga unit do?

harbours guanine nucleotide binding site which is occupied by GDP in resting state.

Why do Ga and Gy have lipid extensions?

to bind and tether to the G protein complex to plasma membrane

What is the reason for the 2 domains in a Ga subunit?

Ras like domain and a-helical domain have a nucleotide binding pocket.

Ras like domain has GTPase activity and provides binding sites for GBy.

How is the Ga unit dissociated from the GBy dimer?

G proteins interact with GPCR— conformational change — exchange of GDP for GTP.

How are G proteins classified?

based on their Ga subunits

How do most Ga proteins mediate GPCR signalling?

regulating levels of intracellular molecules called second messengers

What do second messengers do?

regulate activity of multiple downstream effector proteins to cause biological response.

How do Ga(s) and Ga(i) effect cAMP?

s activates/i inhibits adenylyl cyclase to change cAMP.

Describe the cAMP signalling pathway.

• Gas-GTP stimulates release of adenylyl cyclase.

• converts ATP to cAMP

• kinase A protein activated

• kinase A mediates phosphorylation of multiple downstream effector targets

• cAMP modulates activity of guanine exchange factors and ion channels.

How are signals amplified in the cAMP signalling pathway?

• one messenger binds to one receptor

• several G proteins activated.

• each G protein activates an adenylate cyclase

• each adenylate cyclase generates hundreds of cAMP

• each cAMP activates protein kinase A

• each protein kinase A phosphorylates hundreds of proteins.

Describe the IP3/Ca2+ signalling pathway.

• Gaq-GTP activates phospholipase in membrane.

• catalyses conversion of PIP2 to IP3

• IP3 acts on ER to trigger release of Ca2+ into cytoplasm

• Ca2+ modulates effector proteins to give cellular response

• DAG activates protein kinase C

• PKC induces phosphorylation of effector proteins.

What is PIP2?

membrane phospholipid

What messengers are formed from PIP2?

IP3 which releases calcium from ER. and DAG which activates protein kinase C

How does the IP3/Ca2+ pathway lead to muscle contraction?

phosphorylation caused by calcium forces cross bridge to thin filament.

What is formed from the GB and Gy subunits?

obligate functional heterodimer

What happens due to lipid modification of the Gy subunit?

attachment and localisation of the GBy dimer to membrane

What happens when GPCR is activated?

dissociation of Ga-GTP to form free GBy dimer

What role does the GBy dimer play in GPCR signalling?

regulating a host of downstream effectors such as voltage gated calcium channels, GPCR kinases.

What controls the temporal and spatial signalling of GPCRs?

desensitisation and internalisation

How is homologous desensitisation achieved?

GRK (G protein couples receptor kinases) docks onto activated GPCR— phosphorylated serine and threonine in receptor—inhibition of activation

How is heterologous desensitisation achieved?

PKA, PKC and other S/T kinases phosphorylate GPCRs.

What happens are homologous/heterologous desnstitisation?

B arrestins bind to GPCR and sterically hinder interactions. This inhibits further activation.

B arrestins couple the GPCRs to clathrin coated pits for receptor internalisations.

Internalised GPCRs trafficked to endosomes.

What are the possibilities when GPCRs are trafficked to endosomes?

• may be dephosphorylates and recycled.

• may be sorted to lysosomes for degradation

• may activate B arrestin dependent signalling cascades.

Give an example of G protein coupled receptors?

norepinephrine and epinephrine are adrenergic receptors.

acetylcholine and carbachol are

Why does rapid attenuation and desensitisation happen after the activation of GPCRs?

prevents uncontrolled signalling

How is desensitisation initiated?

phosphorylation of receptor by GPCR kinases, then GPCR-G proteins are uncoupled. mediated by B- arrestin proteins.

How do B arrestins play a role in the promotion of GPCR signalling?

internalising the receptor and acting as a molecular scaffold to recruit signalling proteins.

Describe the activation of receptor ion channels via signal transduction pathways.

• extracellular messenger binds to receptor

• binding of messenger leads to opening of channel

• ions enter

• ion entry creates cellular response

Describe activation of GPCRs via a signal transduction pathway?

• extracellular messenger binds to receptor

• receptor activated G protein

• G protein activates effector protein

• effector protein produces second messenger

• second messenger activates protein kinase

• protein kinase activates designated protein

• active designates protein brings about response.

Describe activation of tyrosine kinase linked receptors via signal transduction pathways.

• extracellular messenger binds to receptor

• binding of messenger leads to activation of protein kinase enzyme site

• protein kinase activates designated protein

• active protein brings about response.