L3- G protein- coupled and enzyme-linked receptors

What is signal transduction?

When signals received by receptors are converted to a cellular response

Cell signalling is specific and sensitive. What does this mean?

Specific- precise binding between signal molecule and receptor

Sensitive- receptors detect concentrations of 10⁻⁶ – 10⁻⁹ molar (dilute)

What are the two states of receptors, and what does this depend on?

Active or inactive state, dependent on conformation (shape)

What are the 4 types of receptors?

• Ligand-gated ion channels

• G protein-coupled receptors

• Kinase-linked receptors

• Nuclear receptors

Explain how ligand-gated ion channels work, including the speed of the process and when these channels are used

Ligand binds to the receptor causing a conformational change, allowing the transit of ions across the membrane

• Ligand binding and channel opening in milliseconds

• Involved in homeostasis and fast synaptic events e.g. muscle contraction

How does propofol work as a ligand- gated ion channel?

used to induce anaesthesia, binds to GABA receptor in brain. Activation opens the ion channel allowing chloride ions into the cell (inhibitory effect)

Give an example of nuclear receptors, and the relative time scale they take to activate

Steroid hormones

Hours

What is the relative speed of G protein coupled receptors, in time scale and in comparison to ion channels?

Seconds

Slower than ion channels

What is the structure of a G protein coupled receptor?

• 7 membrane- spanning alpha helices

• GPCR (receptor) interacts with a G protein. The G protein is made of 3 subunits: (α, β and γ)- heterotrimeric (3 different parts)

Describe the structure of an inactive G protein

• The G protein is made of 3 subunits: (α, β and γ)- heterotrimeric (3 different parts)

Inactive: 3 subunits are together, with GDP attached to α subunit

How is a G protein tethered to the cell membrane?

By the α and γ subunits

Describe the mechanism of activation of a G protein coupled receptor

• Signalling molecule interacts with receptor causing a conformational change

• Receptor changes shape and binds to the G protein

• GDP dissociates from α of G protein and is replaced with GTP

• The α-GTP and βγ components dissoiate from receptor and from each other. They are free to diffuse in the membrane and interact with enzymes/ ion channels 9signalling within the cell)

• GTP hydrolysed to GDP by GTPase

• α-GDP complex reassociated with βγ and signalling stops

How many GPCRs are there in comparison to G proteins?

Many GPCRs, only a few G proteins

What are the 4 main classes of G protein?

Gαₛ, Gαᵢ/ₒ, Gαq/₁₁, Gα₁₂/₁₃

Which G protein stimulates adenylyl cyclase? What happens to cAMP?

Gαₛ, which produces cAMP. cAMP is a secondary messenger that activates many enzymes

Which G protein inhibits adenylyl cyclase?

Gαᵢ/ₒ

Which G protein activates phospholipase C? What else is produced?

Gαq/₁₁. Inositol triphosphate (IP₃) and diacylglycerol (DAG)

IP₃ releases intracellular Ca²⁺

DAG activates protein kinase C

What does the G protein Gα₁₂/₁₃ do?

Interacts with pathways involving Rho-family small GTPases

What are enzyme-linked receptors activated by?

Wide range of mediators- growth factors, cytokines, hormones

Describe the structure of an enzyme- linked receptor

• Cross the cell membrane once

• Large extracellular ligand-binding domain

• Intracellular domain linked to catalytic activity

Describe the mechanism of activation for an enzyme linked receptor (Kinase-linked e.g. receptor tyrosine kinase (RTK))

• A ligand binds to the receptor (dimerization- two becomes one)

• This becomes an active intracellular kinase enzyme

• Tyrosine residues are phosphorylated.

• These phosphorylated residues act as docking sites for relay proteins (these proteins become activated)

• A signalling cascade produces a cellular response, the signal is amplified each time

• Terminated by protein tyrosine phosphatases

What do RTKs (a type of enzyme linked receptor commonly signal through)?

Ras and P13-kinase pathways, which are important in cell division, growth and differentiation

How is the Jak-Stat pathway activated and what is it important in?

Activated by many cytokines, and important in inflammation