Social cells 3: G protein-coupled and enzyme linked receptors

Ability of cells to receive and act on signals fundemental to life, all physiological signals are mediated through proteins

What is cell signalling?

Proteins recieving signals are called receptors

signals that are converted to a cellular response = signal transduction

cell signalling is specific: precise binding between the signal and the receptor - lock and key theory. it is also sensitive: receptors work at really low concentrations of 10^-6 - 10^-9 molar in the body (can be just as effective if we have only a few)

what is receptor activation?

receptors are either active state or inactive state dependent on their conformation (i.e. their shape)

what are ligand-gated ion channels

work really quickly (in milliseconds)

involved in homeostasis, fast synaptic events, muscle contraction

ligand binds to receptor → conformation change allows transit of ions across membrane

no intermediate biochemical processes involved

what is this showing

types and speed of recpeots

G protein coupled receptors

your nose works through G-protein coupled receptors - allows you to smell very quickly, work in seconds

over a 1/3 of medicines you prescribe act via GPCRs

more than 800 GPCRs in humans identified. the types involved in olfaction, regulation of immune responses, inflammation, homeostasis, autonomic nervous system, neurotransmission, growth and metastasis of tumours

receptor structure: 7 membrane-spanning a-helices

sits within the bilipid membrane, crosses the membrane 7 times. closely associated with the recetor is the g protein - composed of 3 parts = trimeric G protein (not attached yet) → alpha, beta and gamma. Attached to the g protein is a GDP molecule (guanine diphosphate), a way of transferring energy in the cell - at rest it is at a low energy state. Finally you have a signalling molecule/drug which interacts with the receptor outside the cell. When that happens it changes the shape of the receptor, then allows the G protein to connect to the receptor to couple → GDP molecule gets replaced with GTP molecule = higher energy state. THat enables G protein to split into 2 which is beta-gamma component and the alpha-GTP component interacts elsewhere (like to an enzyme) to trigger biological effect. Inside the alpha unit is a phosphatase enzyme that takes away phosphate group to become alpha-GDP, that can then recombine wit hthe G protein. If the signalling molecule gets off the receptor, the receptor reverts back to its original shape, and the G protein dissociates from receptor again and system stops.

summarise G protein coupled receptors

apomorphine is a drug you can inject to make the dog sick to vomit out the poison

Signal transducing molecules – i.e. they transfer extracellular signals into a cell, leading to a cellular response.

Inactive G proteins are trimers consisting of 3 subunits (α, β and γ) with a GDP molecule bound to the α subunit

G proteins are tethered to the cell membrane by the α and γ subunits, but are freely diffusible in the plane of the membrane

(what i basically wrote above)

1. Ligand binds to the GPCR at the ligand binding site and causes a

conformational change in the receptor

2. This results in coupling of the receptor with a G Protein trimer; coupling

causes a conformational change in the G Protein

3. GDP on the α-subunit of the G protein dissociates and is replaced by

intracellular GTP

4. The α subunit-GTP complex and the βγ-complex dissociate from the

receptor and from each other. Both the α subunit-GTP complex and the βγ-

complex are free to diffuse in the membrane and interact with enzymes or

ion channels leading to signalling within the cell

5. GTP is hydrolysed to GDP by GTPase activity of the α-subunit

6. The resulting α subunit-GDP complex reunites with

the βγ-complex and signalling stops

what are the 4 main types of G protein?

usually changes on the alpha end

Gα_s, Gα_i, Gα_q, Gα_12/13

Diff a subunits show selectivity with respect to receptors and secondary messenger systems with which they interact

Gα_s and Gα_i produce, respectively, stimulation and inhibition

of adenylyl cyclase which produces cyclic AMP. cAMP is a

secondary messenger that activates enzymes involved in many

processes including energy metabolism, cell division and

differentiation, ion transport, ion channels and contractile

proteins in smooth muscle

part of the autonomic nervous system

examples of GPCR

stimulatory vs inhibitory

what is Ga_q

it catalyses phospholipase C to produce inositol triphosphate (IP₃) and diacylglycerol (DAG) -> IP₃ is a secondary messenger which leads to the release of intracellular Ca²⁺ - important role in the action of many hormones -> DAG activates protein kinase C, which plays an important role in many different aspects of cell function

e.g. GPCR linked to Gαq – Histamine (H1) receptor

it is an allergic disease, e.g. hay fever, stimulation of histamine receptorson mast cells leads to release of inflammatory mediators

what is Gα_12/13

these subunits interact with guanine nucleotide exchange

factors (GEFs) for Rho family small GTPases. They play a key

role in signal transduction pathways that regulate various

cellular processes, including cytoskeletal dynamics, cell

migration, proliferation, and survival. Targeting this pathway is

a therapeutic interest for conditions like metastatic cancers and

inflammatory diseases.

• The primary effectors of Gβγ complexes are ion channels,

adenylyl cyclase, phospholipase C and PI3 kinas

what are enzyme-linked receptors?

they are receptors activated by a wide variety of

mediators including growth factors, cytokines and hormones

• Major role in growth & cell division, inflammation,

immune responses

• Main types are receptor tyrosine kinases (RTKs), receptor

serine/threonine kinases and cytokine receptors

• Time course of effect is minutes to hours

tell me more about kinase-linked receptors have a common structure

• Large extracellular binding domain connected via a single

membrane spanning α-helix to the intracellular domain

(tyrosine kinase enzyme)

• Activation of the receptor polypeptide chains usually involved

dimerization followed by autophosphorylation of tyrosine

residues. Relay proteins bind to the phosphorylated tyrosines

and activate downstream signalling

how are kinase-linked receptors activated or inactivated

1. Ligand binds to receptor and this leads to dimerization

2. The association between the 2 intracellular domains creates

and active kinase enzyme

3. Tyrosine residues are phosphorylated

4. The phosphorylated tyrosine residues act as docking sites for

other intracellular relay proteins (often enzymes) which are

then themselves activated

5. A cascade of events ultimately leads to a biological effect

6. The activity of the receptor is terminated by protein tyrosine

phosphatases

the central role of kinase cascades in signal transduction

how do RTKs act through Ras and PI3-Kinase pathways

Ras and PI3-kinase pathways play a key role in cell division,

growth and differentiation

• Ras and PI3-kinase pathways important targets for anti-

cancer therapy

what is the Jak-Stat pathway?

The Jak-Stat pathway is activated by many cytokines and is

important in inflammation

• Oclacitinib is a Janus kinase (JAK) inhibitor - target cytokines

are those that are pro-inflammatory or have a role in allergic

responses/pruritus (also those involved in host defence or

haematopoiesis

what are the key points of GPCRs?

G Protein-coupled receptors are involved in a wide variety of

biological processes, including the action of hormones, energy

metabolism, cell division and differentiation, ion transport, ion

channels and contractile proteins in smooth muscle

• GPCRs act in a seconds timeframe

• All GPCRs have a common structure: 7 membrane-spanning

alpha helices.

• G proteins are comprised of three subunits (α, β, γ). There are

several types of G protein, which interact with different

receptors and control different effectors. Two such important

secondary messenger systems are the adenylyl cyclase and

phospholipase C systems

key points of enzyme-linked receptors

Enzyme-linked receptors are activated by a wide variety of

mediators including growth factors, cytokines and hormones

• Enzyme-linked receptors act in a minutes-hours timeframe

• Kinase-linked receptors have a common structure – a large

extracellular binding domain connected via a single membrane

spanning α-helix to the intracellular domain

• Ligand binding to the receptor leads to dimerization. The

association between the 2 intracellular domains creates and

active kinase enzyme. Tyrosine residues are phosphorylated.

The phosphorylated tyrosine residues act as docking sites for

other intracellular relay proteins (often enzymes) which are

then themselves activated. A cascade of events ultimately

leads to a biological effect. The activity of the receptor is

terminated by protein tyrosine phosphatase

• Receptor tyrosine kinases act through Ras and PI3-kinase

pathways. These pathways play a key role in cell division,

growth and differentiation and are therefore important targets

for anti-cancer therapy

• The Jak-Stat pathway is activated by many cytokines and is

important in inflammation