Receptors and Signalling - more?

Question 1

What are the principles of Receptor Signalling?

Answer 1

Receptors mediate the actions of pharmacological agonists and antagonists.

Receptors determine the quantitative relationship between dose or concentration of the drug and the pharmacological effects.

Receptors can cause signal amplification and thus receptors can induce a biological response at very low concentrations of ligand/drug

Question 2

What are Cell Surface Receptors?

Answer 2

Act as signal transducers (converts an extracellular signal into an intracellular response. Intracellular signaling proteins trasmit message from the receptor to target proteins, alteration in activity/function of these target proteins leads to a cellular/physiological response)

Question 3

What are G-protein Coupled Receptors?

Answer 3

metabotropic, a ligand binding to this receptor activates a G-protein which then activates or inhibits an enzyme or an ion channel

Regulated by GTPase-activating protein (helps activate GTPase) and Guanine nucleotide exchange factors (catalyses exchange of GDP for GTP)

Question 4

What is the Structure of G-Protein Coupled Receptors?

Answer 4

Single polypeptide chain that contains 7 transmembrane domains, this transmembrane region is coupled with a G-protein.

All are glycoproteins, some function as dimers

The 3rd intracellular loop and C-terminal tail are highly variable in length and sequence. Responsible for heterotrimeric G-protein interactions within the cell, and contains the site of phosphorylation (which kinases can use to regulate GPCRs)

Question 5

What is the Gs family?

Answer 5

GsPCR binds to beta adrenergic receptors (beta1 in heart, beta2 in smooth muscle).
Stimulates adenylyl cyclase which catalyses conversion of ATP to cAMP.

Increased cAMP stimulates PKA activation. PKA phosphorylates target proteins

Question 6

What is the Gi family?

Answer 6

GiPCR binds to alpha2 adrenergic receptor in presynaptic neurons.
Inhibits adenylyl cyclase which catalyses conversion of ATP to cAMP.

Reduced cAMP causes inhibition of PKA activation.

Question 7

What is the Gq family?

Answer 7

GqPCR binds to alpha1 adrenergic receptor
Stimulates phospholipase C, which cleaves PIP2 in the cell membrane into IP3 and DAG.

IP3 opens calcium channels on ER, causing efflux into the cytoplasm.

DAG activates protein kinase C (PKC) which phosphorylates target proteins

Question 8

How does GPCR signalling work?

Answer 8

When a ligand binds to the extracellular portion of the GPCR (at either the N-terminus or within transmembrane region), this causes a conformational change resulting in the exchange of GDP from the alpha-subunit for GTP.

This actives the (heterotrimeric?) G-protein, causing the alpha-subunit bound to GTP to dissociate from the beta-gamma-subunits.

The alpha-subunit interacts with effector proteins causing downstream effects with inhibition or production of second messengers, such as opening an ion channel or regulating enzyme activity.

Second messengers can activate or inhibit its target protein, causing the target protein response

Question 9

What are Heterotrimeric G-proteins?

Answer 9

Heterotrimeric G-proteins couple the GPCR to an effector protein. G-proteins can also be monomeric.

Made up of alpha, beta and gamma subunits. In it’s inactive state, GDP is bound to the alpha subunit.
The alpha subunit has intrinsic GTPase activity, so can hydrolyse GTP to GDP (and Pi) to inactivate.

Question 10

What are Ligand Gated Ion Channels?

Answer 10

Transmembrane proteins that open to allow ions to pass through the membrane in response to the binding of a ligand to receptor. Common ligands include neurotransmitters. Involved in the rapid transmission of a signal (ms).

Important for communication between neurons and other cells. Occur at synapses, receptors are located at post-synaptic cell, neurotransmitters released from presynaptic cell

Question 11

What is the Structure of Ligand Gated Ion Channels?

Answer 11

Made up of at least 2 domains, a transmembrane domain which includes the ion pore, and an extracellular domain which includes the ligand binding location.

Question 12

What is the Classification of Ligand Gated Ion Channels?

Answer 12

Classified into 3 superfamilies which lack evolutionary relationship; cys-loop receptors, ionotropic glutamate receptors, ATP-gated channels. The prototypic ligand-gated ion channel is the nicotinic acetylcholine receptor (a cys-loop receptor)

Question 13

What are Nicotinic Acetylcholine Receptors?

Answer 13

Cation selective Ligand Gated Ion Channels.
Allow sodium ions into the cell, causing a stimulatory effect (produces a nerve impulse or muscle contraction).

Question 14

What are Ionotropic Glutamate Receptors?

Answer 14

Cation selective Ligand Gated Ion Channels.
Produce a stimulatory effect (nerve impulse).

NMDA receptor - selective to calcium and sodium cations
AMPA receptor - selective to sodium
kainate receptor - selective to calcium

Question 15

What are GABA (alpha) Receptors?

Answer 15

Anion selective Ligand Gated Ion Channels. Releases GABA which causes hyperpolarisation and the inhibition of neuronal activity.

Can be target to treat anxiety, hyperactivity, or as anaesthetics

Question 16

What are Enzyme Linked Receptors?

Answer 16

Ligand binding to the receptor activates the enzyme activity of the receptor or an associated enzyme.

The most common is receptor tyrosine kinase (responds to peptide/protein ligands).

Cytokine receptors are associated with intracellular proteins that have tyrosine kinase activity not intrinsic to the receptor

Question 17

What is the structure of Receptor Tyrosine Kinases?

Answer 17

Made up of an N-terminal extracellular binding domain, a single transmembrane alpha-helix and a cytosolic c-terminal domain with protein kinase activity.

Most receptors are a single polypeptide (the insulin receptor is a homodimer) but in binding to the ligand they dimerize, causing a conformational change that activates the kinase. The receptors are trans-autophosphorylated (phosphorylating itself and the receptor it is dimerized with).
Each of the phospho-tyrosines is a specific site which can activate different signaling molecules to relay the signal downstream

Question 18

Describe Recruitment of the Signalling Molecule Phospho-tyrosine

Answer 18

A protein domain is a conserved part of a given protein sequence and tertiary structure that can evolve, function and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded

There are many PTB or SH2 domains on enzymes/proteins that are brought to the receptor and can bind to the phospho-tyrosine

Recruitment can induce a conformational change, lead to the phosphorylation of the proteins or bring the protein to the membrane (near to its substrate/activator/site of action)

Adaptor proteins are important in coupling the receptor to another protein (e.g. the phospho-tyrosine has a SH2 domain which protein A cannot bind to, protein B can bind to SH2, and contains a SH3 domain which protein A can bind to, therefore protein B acts as an adaptor protein)

Question 19

Describe Ras (small monomeric G-protein)

Answer 19

Belongs to the superfamily of low molecular weight G-proteins. When activated Ras relays the message to other downstream signalling pathways, ultimately effecting transcription factors that alter RNA expression in the nucleus

Ras-GEF (Ras-Guanine nucleotide exchange factor) stimulates the removal of GDP and the addition of GTP which activates Ras.
Ras-GAP (Ras GTPase-activating protein) hydrolyses GTP to GDP which inactivates Ras

Question 20

What are Intracellular/Nuclear Receptors?

Answer 20

Either cytoplasmic or nucleus. Binding of ligand activates the receptor and often causes the receptor-ligand complex to enter the nucleus.
The ligand-receptor complex acts as a transcription factor and alters gene expression.

The ligand is often hydrophobic and is able to pass through the plasma membrane

Question 21

Describe Protein Kinases signalling

Answer 21

Can reversibly change the activity or function of proteins by phophorylation (addition or removal of phosphate group bound to an amino acid).

Addition of phosphate group changes shape of protein, affecting binding of ligands to protein, change in shape affecting intrinsic activity, or masking/unmasking binding sites affecting protein-protein interactions

Question 22

What are Second Messengers?

Answer 22

Small diffusible signalling molecules that can diffuse away from the site of synthesis and bind to and activate or inhibit proteins

Cyclic nucleotides – cyclic AMP (synthesised from ATP by adenylyl cyclase) and cyclic GMP (synthesised from GTP by guanylyl cylase)

Gases – nitric oxide synthesised by nitric oxide synthase

Ions – calcium

Question 23

Describe Calcium Ion signalling

Answer 23

From extracellular fluid or from depots within the cell such as ER or mitochondria. These bind to proteins and regulate their function or activity

In muscle cells calcium triggers contraction, in secretory cells calcium triggers secretion, in neurones calcium is essential for neuronal activity and transmission

Calcium can bind proteins and regulate their activity/function