Flashcards in Drug Target: The Many Types of Drug Receptors Deck (22):
What are the types of drug targets?
Receptors: responsible for transmitting a signal to a cell (voltage gated ion channel, nuclear hormone receptors, ligand gated ion channel, G-protein coupled receptors
Enzymes: catalyze biochemical reactions (soluble enzymes, transmembrane or membrane bound enzymes)
Describe how voltage gated ion channels work
Binding can occur on the intracellular or extracellular part of the ion channel
Binding can be within the pore of the channel itself or on the outside
Where and when the drug binds depends on the ion channel and the drug
Examples: local anesthetics, calcium channel blockers, class I and class III antiarrhythmics
Describe how ligand gated ion channels work
Binding of agonist to the ion channel opens the ion channel
Some receptors require more than one ligand are required (but not all of them)
Receptor binding site normally on the outside of membrane
Examples of agonist: nicotine, acetylcholine, GABAa, AMPA receptors
Describe how nuclear hormone receptors
The ligand crosses the membrane and goes into the nucleus. There's often a dimerization event. The hormone binds to nuclear receptor. Then the DNA binding domain of the ligand binds to the response element on the DNA, usually within the promoter. This causes an increase or decrease in mRNA expression
Describe the structural organization of nuclear receptors?
There are transcription activation function domains (AF 1 and AF2). One of those regions binds with the DNA and the other binds with the ligand. They are sequence specific-ish (consensus sequence: some nucleotide sequences are specific, some are more flexible)
What are examples of nuclear hormones and receptors? ((8)
Estradiol: estrogen receptor
Testosterone: androgen receptor
Progesterone: progesterone receptor
Cortisol: glucocorticoid recept (also mineralocorticoid receptor)
Aldosterone: mineralocorticoid receptor (also glucocorticoid receptor, weakly)
Vitamin A: retinoic acid receptor
Vitamin D: vitamin D receptor
Thyroid hormone: thyroid hormone (T3, T4) receptor
Describe how the estrogen receptor becomes activated
Estrogen binds to the estrogen receptor (EAalpha). There is a conformational change and dimerization. Then there is an increase in mRNA expression and results in proteins causing changes in tissue response
What are transmembrane enzymes?
These are a group of receptors that have an extracellular ligand binding domain and an intracellular enzymatic domain within the same protein
Examples: epidermal growth factor (EGF), insulin receptors, which contain intrinsic tyrosine kinases in the cytoplasmic domain
What is EGFR?
Epidermal growth factor receptor (EGFR) is a transmembrane receptor tyrosine kinase which is needed for the growth and differentiation of epithelial cells. Overexpression of EGFR is found in epithelial cancers. Ligands are epidermal growth factor (EGF) or transforming growth factor alpha (TGFalpha)
What happens when EGF binds to EGFR?
The binding of ligands causes EGFR dimerization and tyrosine kinase activity, resulting in autophosphorylation of several EGFR tyrosine residues. This activates several signal transduction cascades which results in DNA synthesis and cell proliferation. This completely changes the properties of the tyrosine; phospho-tyrosine can bind to proteins with an SH2 domain
Describe the mechanism of EGFR
The phospho-tyrosin binds to the SH2 domain of the Grb2 adaptor. The Grb2 adaptor acts on RAS, which activates the mitogen-activated protein kinase (MAPK) pathway. The kinase results in phosphorylation of a number of transcription factors in the nucleus, including the cAMP response element-binding protein (CREB). CREB acts on CRE, which stimulates DNA synthesis and cell proliferation
What are one of, if not, the most important groups of receptors?
G protein coupled receptors (GPCR)
They are a target for approximately 45% of drugs on the market
Several important endogenous ligands, including adrenalin, acetylcholine, serotonin, noradrenaline, dopamine, histamine, ADP, glutamate, peptide hormones
What are the three components of GPCRs?
Receptor on the outside of the membrane brings the ligand
G-protein on the inside of the membrane senses single from ligand receptor interaction
Effector protein: binds G protein and produces or inhibits production of a second messenger
Made of a large protein that spans the membrane
GPCRs are composed of 7 alpha helices that cross the membrane so they are also called 7 transmembrane helix receptors (7-TM)
Ligand receptor is in the extracellular side and its exact position depends on the ligand
A signalling protein called the G protein is on the intracellular side
The G protein is bound to the intracellular side of the membrane and bind GDP and GTP (hence the name)
G protein has alpha, beta and gamma subunits from receptor ligand complex into the cell through a biochemical cascade
How are GPCRs activated?
A single receptor can activate multiple G proteins resulting in amplification of the signal
The drug-receptor complex results in a conformational change that allows binding to the G-protein. Binding of the receptor reduces the G-protein's affinity for GDP and increasing its affinity for GTP. GTP results in dissociation of the alpha subunit from the beta/gamma subunits. Beta/gamma dissociate and interact with respective effector proteins. The alpha subunit hydrolyzes GTP into GDP, which results in reallocation of alpha and beta/gamma subunits, eliminating the signal
What are the different types of G-proteins?
G alpha s: Activates adenylate cyclase, opens calcium channels, leading out of the cell (second messenger: cAMP produced)
G alpha i: Inhibits adenylate cyclase and opens K channels (second messenger: cAMP inhibited)
G omicron: activates receptors that inhibit calcium ion channels leading out of the cell
G alpha q: activates phospholipase C beta (second messenger: DAG and IP3 produced)
Describe the activation of adenylate cyclase via G alpha S
The membrane bound adenylate cyclase (AC) binds G alpha s (s for stimulatory); this allows production of cAMP
cAMP (second messenger) binds to and activates protein kinase A (PKA)
PKA then phosphorylates many target proteins that amplifies the signal again and produces many coordinated responses
Phosphodiesterase (PDE) extinguishes the signal from cAMP by hydrolysing it into 5'AMP which does not bind to PKA
What are the effects of adenylate cyclase activity?
Stimulation of PKA through G alpha s initiates a cascade of effects that liberate energy sources from storage and prevent storage of energy sources (increase lipolysis, decreased glycogen synthesis, increase glucose)
Describe the activity of G alpha i
Membrane bound adenylate cyclase (AC) binds to G alpha i (i for inhibitory)
AC is inhibited and no cAMP is produced
PKA remains inactive
Together G alpha i and the beta/gamma subunits produce an overall inhibitory effect
Describe the activity of phospholipase C beta
The membrane bound phospholipase C beta (PLC beta) binds to G alpha q and PLC beta is activated
PLC beta hydrolyzes phosphotidylinositol diphosphate (PIP2) to second messengers diacylglycerol (DAG) and inositol triphosphate (IP3). DAG remains membrane bound. IP3 dissociates into the cytosol and opens Ca ion channels in the ER (increase in intracellular calcium). Ca binds to calmodulin and stimulates PDE, which hydrolyzes cAMP into 5'AMP, which counteracts the signal from G alpha s
What does the activation of PKA from G alpha s result in (with regards to calcium)?
PKA phosphorylates Ca ATPase, which pumps Ca back into the endoplasmic reticulum thereby decreasing intracellular Ca and increasing Ca stores.