Lecture 2

Law of Mass Action - the rate of a reaction is dependent on the concentration of the reagents (drug and receptor)

Rate of Association - how fast the drug and receptor bind

Rate of Dissociation - how fast the drug and receptor unbind

Equilibrium - individual receptors can bind and unbind but the net receptors are unchanged

Kd - the equilibrium dissociation constant and is the rate of the dissociation constant (Koff) and association constant (Kon) at equilibrium

• Kd = Koff/Kon, it is also the concentration needed to occupy half of the receptors

• Drugs with low Kd have a high affinity and bind well to receptors

The Hill-Lanqmuir equation makes a sigmoid curve

• It shows that there are a maximum amount of receptors that can bind and once that is reached the maximum drug effect can no longer be increased

• Also illustrates the sensitivity of a drug as a drug with a higher affinity will need a lower concentration and the curve will shift to the left as less receptors will need to be bound

Mutual affinity of a drug and its receptor determines the selectivity of drug effects and is independent for each drug receptor pair

Receptor - proteins that respond to an external stimulus that cause a change inside the cell

Channels - form a pore for passive ion movement

Transporters - actively transport molecules independent of a concentration gradient

Intracellular Receptors - inside of cells and are lipid soluble to cross the plasma membrane, most commonly are steroids  

• Steroids bind to the ligand binding domain, displacing the heat shock protein revealing the DNA recognition domain to influence transcription of target genes

• The transcription and translation processes make them slow acting receptors

• Often associated with cancer drugs

G-protein coupled receptors

• Largest membrane receptors

• Proteins embedded in the cell membrane and bind to a ligand

• Most common drug target receptor

• Ligand binds to extracellular cell part and activate signal cascade mediated by G-proteins

• Are single polypeptides, folded 7 times called 7 transmembrane receptor with an extracellular loop allowing for binding

GPCR Binding

• Drug Binding causes a conformational change in the GPCR triggering an interaction between the receptor and a nearby G protein inside the cell

• G proteins are specialized proteins that bind GTP and GDP, they are heterotrimic proteins with three subunits

• An activated GCPR swaps its GDP for an active GTP causing the G protein to dissociate into alpha and beta-gamma subunits allowing them to interact with other signalling molecules

• GTPase is a part of the alpha subunit as a molecular timer that hydrolyzes GTP back to GDP to stop the signalling and reset the inactive G-protein 

Alpha Subunits

1. Gs - stimulates the cell ( via adenylate cyclase), receptors coupled to Gs increase the activity of AC, rpoducing cyclic AMP (cAMP) which activates cAMP dependent protein kinases

2. Gi - inhibits the cell (via adenylate cyclase) by suppressing the activity of AC

3. Gq - modulates the cell (via phospholipase C), receptors coupled to Gq increase the activity of PLC producing IP3 (inositol) from the breakdown of PIP2, PLC can cleave off part of the membrane to act, the IP3 triggers the release of intracellular Ca ions which can influence various signalling pathways, other PIP2 breakdown byproducts lead to the activation of protein kinase C and target substrate

G beta gamma subunits can influence the activity of various proteins

Tyrosine Kinase Receptors

• Embedded in the membrane and bind to ligand

• Activation is caused by the dimerization of receptors when a ligand is present, activating the receptors by causing them to autophosphorylate

• Drugs that inhibit or stimulate the receptor activation will influence the signalling mechanisms

Ion Channels

• Fastest mechanism depending on electrical signals

• Controlled by a distinct stimuli such as ligand binding or voltage 

• Drugs that target ion channels alter the response to the stimuli, or block the flow of ions

• ligand -gated channels are selective for either positive or negative ions, open and close in response to small signalling molecules and have large effects in the brain

• voltage -gated ion channels are specific to amino acids, respond rapidly and drive action potentials for neurotransmitters

Active Transporters

• Specialized proteins that assist the movement of various molecules against a concentration gradient across a membrane

• Example is dopamine, its level is important and uses a transporter to control its concentration, cocaine blocks the dopamine transporter leading to the accumulation of dopamine in the synapse