A ligand binds its receptor through a number of specific weak non-covalent
bonds by fitting into a specific binding site or "pocket".
In situations where even low concentrations of a ligand will result in binding of most of the cognate receptors, the receptor affinity is considered to be high.
Low receptor affinity occurs when a high concentration of the ligand is required for most receptors to be occupied.
The dissociation constant (Kd,) is the concentration of ligand required to occupy one half of the total available receptors.
This measurement of receptor affinity is often in the range of 10-4 to 10-9 mM.
With prolonged exposure to a ligand (and occupation of the receptor)
cells often become desensitized.
Desensitization of the cell to a ligand depends upon receptor down-regulation by either
1) removal of the receptor from the cell surface (receptor-mediated endocytosis) or
2) alterations to the receptor that lower the affinity for ligand or that render it unable to initiate the changes in cellular function (such as phosphorylation).
Desensitization may lead to tolerance, a phenomenon that results in the loss of medicinal effectiveness of some medicines that are over prescribed.
Receptor binding activates a "preprogrammed" sequence of signal transduction events that make use of previously dormant cellular processes.
G Proteins bound to GTP are active, those bound
to GDP are not.
The two classes of G Proteins are large heterotrimeric G Proteins and small monomeric G Proteins.
In heterotrimeric G proteins (G alpha, beta, gamma), when a messenger binds the G Protein-linked receptor, the receptor changes conformation to allow association of the trimeric G Protein with the receptor.
G-alpha subunit binds the guanine nucleotide (GDP or GTP).
This interaction causes the G alpha subunit to release the GDP, pick up a GTP and detach from the complex.
Depending upon the G protein in question, either the GTP-G alpha complex, the G beta- G gamma complex or both bind target protein(s).
The G alpha will remain an activating messenger until the GTP is hydrolyzed by the G alpha subunit (GTP -> GDP +Pi).
The "inactive" GDP-G alpha will then reassociate with the G-beta:G-gamma complex to rapidly turn down this pathway when the original stimulatory signal is removed.
Large numbers of G proteins provide diversity for signal transduction
Some bind potassium or calcium ion channels in neurotransmitters.
Some activate kinases (enyzmes that phosphorylate).
Some cause either the release or formation of major second messengers such as cyclic AMP (cAMP) and calcium ions.
cyclic AMP is a second messenger used by a major class of G proteins.
cyclic AMP (cAMP) is generated by adenylyl cyclase which is embedded in the plasma membrane with the enzymatic activity in the cytoplasm.
Adenylyl cyclase is activated by binding an activated alpha subunit of the Gs G-protein (GTP-Gs).
Phosphodiesterase continally degrades cAMP so in the absence of the ligand and active G-Protein, cAMP levels are reduced.
Protein kinase A (PKA), a cAMP-dependent kinase, is the main intracellular target of cAMP.
PKA phosphorylates a number of proteins that bear the key short stretch of amino acids, the PKA phosphorylation site (PKA PO4 site).
PKA transfers a phosphate from the ATP to a serine or threonine in the PKA PO4 site.
cAMP activates the catalytic subunits by causing the release of the negative regulatory subunits.
Disruption of G Protein signaling causes several human diseases.
Vibrio cholerae (causes cholera) secretes the cholera toxin which alters salt and fluid in the intestine normally controlled by hormones that activate Gs G-Protein to increase cAMP.
The cholera toxin enzymatically changes Gs so that it is unable to convert GTP to GDP.
Gs can not then be inactivated and cAMP levels remain high causing intestinal cell to secrete salt and water.
Eventually dehydration can lead to death (cholera).
Many G proteins use inositol triphosphate and diacylglycerol
as second messengers G Protein-linked Receptor.
The Gp G-Protein is activated by a ligand binding its G Protein-linked receptor to activate phospholipase C.
Phosphatidylinositol-4,5-bisphosphate (PIP2) is cleaved by phospholipase C into two molecules cytosolic inositol-1,4,5-triphosphate (InsP3) and membrane-bound diacylglycerol (DAG).
The InsP3 receptor, a ligand-gated calcium channel in the endoplasmic reticulum, binds InsP3 and calcium ions are released into the cytosol.
Calcium binds a protein known as calmodulin, and the Ca-calmodulin complex act to activate an number of processes.
DAG remains membrane-bound and activates protein kinase C (PKC).
Although other proteins bind calcium to control activity, most often
binding to the protein calmodulin, to form a calcium-calmodulin
complex, is an intermediate step.
When calcium ions are present, two bind each globular end (4 in total), the helical arm region then changes conformation (the active complex) and then the wraps around the
calmodulin-binding site of target proteins.
These are often protein kinases and protein phosphatases which vary depending upon the target cell (different cells have different responses).
Fertilization of animal eggs reveals an important example of calcium-mediated
signal transduction after a receptor-ligand interaction.
Initially the sperm binds the egg’s surface at the membrane and within 30 seconds, a wave of calcium release spreads from the site of sperm contact.
Two main events in fertilization rely on calcium release.
1) Calcium stimulates the fusion of the cortical granules with the egg’s plasma membrane to alter the coat surrounding the egg to help prevent the binding of another sperm cell to the egg (slow block to polyspermy).
2) Calcium initiates egg activation, the resumption of metabolic processes.
Receptor tyrosine kinases can initiate a Ras/MAP kinase signal transduction
Phosphotyrosine-containing sites bind SH2-containing proteins which result in Ras, a small monomeric G protein, becoming activated.
Once the epidermal growth factor receptor (EGFR) is autophosphorylated in response to the EGF ligand, a complex of GRB2 (SH2 domain-containing) and Sos (guanine-nucleotide release protein: GNRP) binds the receptor.
Sos is thus activated to cause Ras to release GDP that allows Ras to bind a new GTP and become active.
Once active, Ras triggers a cascade (the Ras pathway) which includes the mitogen-activated protein kinases (MAPKs).
Note: a mitogen is a growth factor signal.
Receptor tyrosine kinases activate a variety ofsignaling pathways
RTK can activate a form of phophlipase C and phosphatidylinositol-3- kinase (PI3K).
Disruption of growth factor signaling through receptor tyrosine kinases
can have dramatic effects on embryonic development.
The fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) function in both embryonic and adult signaling.
FGFRs are important in the development of mesoderm, the embryonic tissue that eventually becomes muscle, cartilage, bone and blood cells.
A mutant receptor that, due to dimerization with normal versions of FGFR, has a dominant inhibitory effect upon the normal activity is a dominant negative (dn) mutation.
A dn mutant version of FGFR mRNA injected into frog eggs cause the failure of mesodermal tissue to develop and produces tadpoles with heads but no bodies.
In humans, defects in FGFRs lead to achondroplsia (dwarfism) and thanatophoric dysplasia severe bone abnormalities (fatal in infancy).
Serine/threonine kinase receptors phosphorylate both serine and threonine
residues (not tyrosine) and act to transduce other types of growth factor
Transforming Growth Factor Beta (TGFß) binding to receptor results in the clustering of type I and type II TGFß receptors.
The type I receptors are phosophorylated by type II receptors.
Activated Type I receptors phosphorylate specific receptor-mediated SMADs.
The activated receptor-mediated SMADs bind to co-SMADs and enter the nucleus to interact with DNA binding proteins to regulate gene expression.
Hormonal signals can be classified by the distance that they travel
to reach their target cells.
An endocrine hormone travels through the circulatory system and a paracrine hormone acts only upon near by cells.
A paracrine hormone is roughly equal to a growth factor.
Endocrine tissues secrete directly into the blood-stream and exocrine tissues into ducts for transport of the secretions to other parts of the body.
The pancreas has both endocrine (insulin and glucagon) and paracrine (digestive enzymes) functions.
Once in the circulatory system, the endocrine hormones will eventually reach their target tissue(s) such as heart and liver (epinephrine) or liver and skeletal muscles (insulin).
In the target tissue, intracellular effects, such as the activation of the cAMP pathway can control a number of cell functions.
One example, is by epinephrine binding to the beta-adrenergic receptor, activation of PKA to cause the stimulation of glycogen breakdown.
Insulin activates a wide range of intracellular
effects by the phosphorylation by the insulin receptor complex of its substrate
This, in turn, leads to activation of 1) the Ras-MAPK pathway and 2) the PI3K/akt kinase pathway that activates (and inactivates) a number of proteins.
Chemical properties of animal hormones
There are four (4) categories of endocrine hormones.
1) amino acid derivatives (epinephrine)
2) peptides (antidiuretic hormone [vasopressin])
3) proteins (insulin)
4) lipid-like hormones including steroids (testosterone)
Paracrine hormones include histamine (a histine derivative) and the prostaglandins (arachidonic acid derivatives).
Notes prepared from Becker's World of the Cell, 8th edition
Hardin, Bertoni & Kleinsmith, 2012
Figures copyright of Pearson Education Inc.
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