Cell Signalling Pathway Flashcards
Describe hormones
They are secreted by endocrine glands and are carried by the blood to their target tissues, they can act over long distances and have onsets between minutes to hours to days
Describe neurotransmitters
These are secreted by neurons and conduct nerve impulses across a synapse, they work over short distances and over very short time frames
Describe local chemical mediators
They are produced by specialised and non-specialised cells and affect other cells in the local environment, they’re distances and time frames are between hormones and neurotransmitters
Give an example of a hormone, neurotransmitter and local chemical mediator
Hormone: insulin
Neurotransmitter: acetylcholine
Local chemical mediator: histamines
What are the 3 types of extracellular messengers?
Hormones, neurotransmitters and local chemical mediators
Give some examples of chemical structures that extracellular messengers can be as
Peptides, amino acids and fatty acids
Nerve growth factors, histamine and prostaglandin are all examples of…
Local chemical mediators
What is a ligand?
Any binding molecule
What makes the ligand-receptor interaction specific?
The specific shape of the binding site formed by the proteins of the receptor
What type of bonding occurs between a receptor and a ligand?
Non-covalent bonds, making the interaction reversible
What is different about a receptor-ligand interaction compared to an enzyme-substrate interaction?
The chemistry of a receptor doesn’t change i.e there is no chemical reaction where as in an enzyme there is a chemical reaction
What happens when an extracellular chemical messenger binds to its receptor?
The receptor experiences a change in conformation which is transmitted to the cell (transduction) and results in a physiological response
What does the superfamily of a receptor depend on?
Its molecular structure and the transduction mechanism that links the receptor to the physiological response
What are the 4 superfamilies of receptors?
Ligand-gated ion channels
G-protein-coupled receptors
Kinase-linked receptors
Nuclear receptors (DNA binding)
Describe ligand-gated ion channels
These are the fastest type of receptor
When bound, they allow the flow of ions into or out of a cell
Results in either depolarisation or hyperpolarisation in order to produce the physiological response
Describe GPCRs
This is where a g-protein is attached to the receptor, when the receptor is activated, the g-protein has its own set of pathways that it follows
This can also result in an enzyme being activated to synthesis a different messenger
Describe kinase-linked receptors
This is where the ligand binds to a receptor with an enzyme inside the receptor which becomes activated leading to protein phosphorylation and protein synthesis in order to have the cellular effects
Describe nuclear receptors
Once the messenger is bound to the receptor, it moves inside the nucleus where it stimulated gene transcription and protein synthesis which is responsible for the cellular effects
Where are ligand-gated receptors found at chemical synapses?
On the post-synaptic membrane where they alter the membrane potential in order to produce an electrical signal
Describe the structure of ligand-gated receptors
Each receptor is made up of multiple subunits each with 4 transmembrane domains (M1 M2 M3 and M4) which each contain helices of a large hydrophilic domain. The subunits surround the central pore which allows the flow of ions
What is the time scale of ligand-gated ion channels?
Milliseconds
How many subunits does the nicotinic acetylcholine receptor consist of and what are they?
5, alpha, alpha, beta, gamma, delta
In a nicotinic acetylcholine receptor, which domain from each subunit has the kink halfway down which influences the activation of the receptor?
M2
What is required for the nicotinic acetylcholine receptor to be activated?
The binding of 2 acetyl choline molecules on the 2 binding sites on the alpha subunits
What is different about the M2 domain in each subunit of the ligand-gated ion channels in nicotinic acetylcholine receptors?
It is kinked halfway down which is responsible for constricting the channel in the resting state
They are also rich in negative amino acids which facilitate the passage of Na+ ions
What happens when 2 acetylcholine molecules bind to the binding sites of the nicotinic receptors?
The alpha subunits have a conformational change which signals the other subunits to change conformation leading to the channel opening
Where are GABAa receptors found?
In the hypothalamus and the amygdala in the brain
How many subunits make up the GABAa receptor and what types are they?
5: 2 alpha, 2 beta and a gamma
What is the inhibitory molecule of GABAa?
GABA
What type of receptor if GABAa?
Ligand-gated ion channel
What happens when GABAa is activated?
The M2 helices of each subunit change shape resulting in the channel being opened
What in the M2 helices of GABAa receptors attracts the Cl- ions to flow through?
They are lined with positive amino acids
Why does the influx of Cl- from GABAa receptors cause a sedative effect?
The Cl- hyperpolarise the cell membrane making it harder for the action potential to be produced
Give some examples of ligands that bind to DNA-binding receptors (nuclear receptors)
Steroid hormones, thyroid hormone, retinoic acid and vitamin D
What happens when a ligand binds to a DNA binding receptor?
The ligand-receptor complex binds to the DNA in the nucleus and stimulates the transcription of specific target genes, The mRNA produced is then translated resulting in more proteins
What are mineralcorticoids?
Steroid hormones which are ligands for DNA-binding receptors found in the kidneys, they result in the synthesis of transport proteins involved in renal tubular function
What is oestradiol?
Steroid hormones which are secreted by the ovaries and bind to the oestrogen receptor (DNA-binding) and stimulate the synthesis of proteins which are important for endometrial thickening and mucus secretion
Describe the transport of steroid hormones
They are lipid-derived molecules which are carried by the blood but easily traverse the plasma membrane to reach target cells
Describe the mechanism of action of steroid hormones
They bind with high affinity to the binding domain on the intracellular receptor. The ligand-receptor complex then binds to specific sites on DNA (HRE) which are upstream of the gene which is to be expressed. This forms another complex which change the shape of the chromatin structure resulting in the gene being exposed, RNA polymerase then transcribes this gene and the production of mRNA which is then translated into proteins
What are hormone response elements? (HRE)
A specific part of the DNA which is able to bind to ligand-receptor complexes and stimulate gene transcription
What is the time frame for steroid hormones?
Minutes to hours to days- proteins need to be made
Describe the structure of growth factor receptors (tyrosine kinase receptor)
They have a very large N-terminal ligand-binding domain (extracellular) and a single alpha helical transmembrane segment. The C-terminal is intracellular and has a protein kinase active site which can phosphorylate the amino acid tyrosine found on the protein substrates by binding to the protein and ATP
Describe the structure of insulin receptors (tyrosine kinase receptor)
It is made from 4 subunits, the 2 alpha extracellular subunits are linked by a disulphide bridge and form the binding site for insulin. Disulphide bridges link these to the 2 beta subunits which cross the membrane and have the tyrosine activity.
Describe how insulin receptors work
Insulin binds to the extracellular binding site of the 2 alpha subunits. ATP is used to phosphorylate the tyrosine kinase domains on the inside of the cell on the beta subunits. Substrates then bind to these same binding sites where they use the phosphate group which leads to other pathways
What do protein kinases do?
They catalyse the phosphorylation of amino acids
How many amino acids can be phosphorylated and what bonds is formed?
3, covalent phosphoester bond
Why does phosphorylation of amino acids often lead to changes in the proteins biological activity?
The phosphate group that is added is large and bulky and results in steric and ionic hinderance
Describe the tyrosine kinase receptor pathway
Ligand binds to the extracellular part causing a conformational change and activation of the tyrosine kinase in the C terminal inside the cell. The receptors inside dimerise and auto phosphorylate each other until all the tyrosine residues are phosphorylated forming binding sites for target proteins. When the target proteins bind via the SH2 domain, they become phosphorylated and activated
What is the SH2-domain on target proteins of tyrosine kinase receptors?
The region on the protein that binds to the phosphotyrosines on the intracellular regions of the receptors
Describe how growth factor signals increase cell growth
These signals are the ligands for tyrosine kinase receptors, by activating the receptors a kinase cascade occurs until the target proteins which are also transcription factors are activated, these act like steroid hormones (ligands for DNA binding receptors) and result in proteins being synthesised which are used for cell growth
Describe the structure of GPCRs
A single protein with both extracellular and intracellular domains and a 7 transmembrane segment.
Where does ligand binding occur on GPCRs?
In a pocket in the membrane on one or more of the 7TM segments
What happens when a ligand binds to a GPCR?
The receptor protein changes conformation resulting in the C terminus binds to and activates the G protein
Where is the beta-adrenergic receptor found?
It is embedded in the plasma membrane
Describe the structure of the beta-adrenergic receptor
it has 2 N-linked glycosylation sites near the N terminus and 2 oligosaccharides. Each region in the 7TM contains 20-28 amino acids which form the pocket binding site for the ligand (M3 M5 and M6 are important for this)
What is the natural ligand for the beta-adrenergic receptor?
Adrenaline
Which parts of adrenaline interact with which parts of the 7TM on the receptor?
NH3+ forms an ionic bond with the COO- on M3
OH forms a H-bond with M6
Benzene ring forms a hydrophobic bond with M6
2 OHs form H-bonds with M5
Which part of the beta-adrenergic receptor binds to the G protein?
The C terminus
What are the 3 subunits that make up G protein?
alpha, beta and gamma
When is GDP bound to the G protein and where is it bound to?
It is bound to the alpha subunit, it binds when it is in the resting state
Which part of the G protein binds to the 7TM?
The alpha subunit
Describe the G protein interaction with the 7TM for beta adrenergic receptors
When a ligand binds to the 7TM, the g protein binding site is exposed, the G protein binds and changes shape resulting in the GDP dissociating and GTP binds in its place, the alpha subunit is now active. The alpha-GTP dissociates from the beta-gamma subunits. The alpha-GTP binds to adenyl cyclase and cAMP is then synthesised from ATP, this binds to PKA which causes protein kinase cascade resulting in a physiological response. After the alpha-GTP binds to adenyl cyclase, GTP is hydrolysed to GDP, the alpha subunit then reassociates with the beta-gamma subunits
What happens in a cascade of protein kinases?
A protein kinase phosphorylates and activates the next protein kinase which then does the same thing
What are the advantages of protein kinase cascades?
They amplify the response, each protein that is phosphorylated can phosphorylate more than one protein each time
They achieve a diverse response, each protein kinase can phosphorylate different target proteins
what is the second messenger in GPCRs?
A messenger that is synthesised from the activation of the receptor and has intracellular effects
Why is it important for the second messenger to be removed after it has stimulated the cascade of events?
So that the receptor responds to future stimuli
What are antibodies?
A special type of receptor that form half of the immune response which destroys foreign organisms and molecules
Where do cells of the immune system originate from?
Bone marrow
What are the different types of cells that make up the immune system?
(Lymphocytes)
T cells: carry out cell-mediated immune response to invasion
B cells: carry out the humoral immune response (antibodies)
What are antigens?
‘Foreign’ proteins which B cells respond to with antibodies
Describe the structure of an antibody (or immunoglobulin)
4 polypeptides, 2 heavy chains and 2 light chains which form a ‘Y’ shaped molecule
What is the role of the specific amino acid sequence found at the end of each antibody?
The specificity allows it to bind to specific antigens
Where is the specific amino acid sequence for each antibody found?
At the ends of the antibody, it includes both heavy and light chains
What is done in order to cleave off the variable region of an antibody to produce a fragment (fab) antigen?
Treated with a protease
What is the role of the constant region on an antibody?
It determines the function and mechanism used to destroy the antigen
How many classes of antibodies are there and what are they?
5, IgM, IgG, Iga, IgD, IgE
What determines the classification of the antibodies?
The constant region structure and immune function
What do IgA antibodies do?
Prevent the colonization of mucosal areas by pathogens
What do IgD antibodies do?
They function as an antigen receptor on B cells
What do IgE antibodies fo?
Bind to allergens and trigger histamine release from mast cells
What fo IgG antibodies do?
Provide the majority of the antibody-based immunity against invading pathogens
What happens when an antigen binds to the antibody on a B cell?
The B cell is stimulated to divide and secretes large amounts of the same antibody in a soluble form
What is VDJ recombination?
The process where B cells randomly select and combine parts of a gene to make a new antibody gene for each B cell
What do V D and J stand for?
Variable, diversity and joining
Which parts of the antibody are VDJ each found in?
VJ are found in both heavy and light chains, D is only found in the heavy chain
What is hypermutation of an antibody gene?
When B cells divide rapidly, there is a high rate of gene mutation which are in the form of point mutations at ‘hotspots’ in the DNA
They alter the shape of the variable region of the antibody so that it becomes better at binding to the antigen (increase in affinity)
What is the epitope?
The part of the antigen that binds to a specific antigen receptor on the surface of a B cell.
