Basic cell signaling 2, Protein and Drug target-17

Question 1

What is a drug?

Answer 1

Agent that interacts with specific target molecules in the body, thereby producing a biological effect

drug molecule interacts with a specific molecule in the biologic system that plays a regulatory role

Question 2

How do drugs interact with receptors?

Drug and receptor relationship

Answer 2

Drugs interact with receptors by means of chemical forces or bonds

ligands (signal molecules) (durgs mimicking the ligands) bind to receptors to trigger a series of proteins, pathway, which lead to a biological response for example releasing a certain hormone

Question 3

What is the drug action: receptor theory?

Answer 3

Many drugs act by binding to receptors where they either provoke/ stimulate or inhibit a biological response

1. the endogenous ligand binds to receptor and produces an effect
2. An agonist drug has an active site of similar shape to the endogenous ligand so binds to the receptor and produces the same effect
3. An antagonist drug is close enough in shape to bind to the receptor but not close enough to produce an effect.

It also takes up receptor space and so prevents the endogenous ligand from binding

Question 4

What are the 4 kinds of protein involved as drug targets?

Answer 4

1. Enzymes
2. Carrier molecules/ Transporters
3. Ion channels
4. Receptors

Question 5

What are Irreversible inhibitors?

Answer 5

Are those that bind very tightly to an enzyme and decrease its activity

examples:
Antibiotic-penicillin
Nerve agents- the deadliest of the classical chemical warfare agents
Organophosphate pesticides

Question 6

Penicillin is an example of an irreversible inhibitor. Explain:

Answer 6

Irreversible inhibitor: bind very tightly to an enzyme to decrease its activity

Penicillin kills susceptible bacteria by specifically inhibiting the transpeptidase that catalyzes the final step in cell wall biosynthesis, the cross linking of peptidoglycan

Question 7

What are reversible inhibitors?

What are the 2 types of reversible inhibitors?

Answer 7

Reversible inhibitors- bind temporarily to an enzyme and prevent its function

Competitive
mimics substrate and competes for active site, prevent the substrates from being transferred into products

Non competitive
alters the conformation of enzyme so active site is no longer fully functional

Question 8

One example of competitive enzyme inhibitors is ACE inhibitors:

What is its function and how does it act as an competitive inhibitor?

Answer 8

The angiotensin converting enzyme (ACE) can convert angiotensin 1 to angiotensin 2

Angiotensin 2 is a peptide hormone that causes vasoconstriction and an increase in blood pressure

ACE inhibitors prevent ACE producing angiotensin 2 and help relax blood vessels and are used for treatment of hypertension

Captopril was the first anti hypertensive drug that acted as ACE inhibitor

Question 9

Tipranavir is an example of an Non-competitive inhibitor

Explain what an non-competitive inhibitor is and how Tipranavir acts as one:

Answer 9

The inhibitor and substrate do not compete for the same binding site

The inhibitor acts at an allosteric site instead of the enzyme’s active site

Tipranavir is a potent non-competitive inhibitor of a protease in HIV

Tipranavir has the ability to inhibit the replication of HIV viruses that are resistant to other protease inhibitors

Question 10

What are enzyme activators?

Give an example:

Answer 10

These molecules are often involved in the allosteric regulation of enzymes in the control of metabolic pathway

These molecules must bind to a site other than the substrate binding site for enzyme reaction to occur

Fructose 2,6-bisphosphate activates phosphofructokinase 1 and increases the rate of glycolysis in response to the hormone glucagon

Question 11

What is the function of carrier molecules/ transporters?

Answer 11

Help transport of molecules across the membranes

Molecules translocated across membranes by binding to specific transporters

Either via facilitated diffusion (passive transport) or pumps (active transport)

Question 12

What is the function of ion channels- voltage gated/ligand gated?

Answer 12

Control movement of ions by the opening and closing of the ion channel

For example:
Cell membrane, the phospholipid bilayers, is a hydrophobic barrier to hydrophilic and charged molecules Ca2+, K+, Na+ or Cl- ions

Ion channels provide a high conducting, hydrophilic pathway across the hydrophobic interior of the membrane for movement of ions across the membrane

Examples:

• Voltage dependent ion channels that respond to mechanical or electrical
• Ligand gated ion channels that respond to chemical stimuli

Question 13

Example of a drug that you can develop to target channel ions, Calcium Antagonists, What is the function of this?

Answer 13

Calcium channel blockers also called calcium antagonists

Prevent calcium from entering cells of the heart and blood vessel walls

Relax and widen blood vessels by affecting the muscle cells in the arterial walls
-resulting in lower blood pressure

e.g Amlodipine

Question 14

Cells contain receptors, What is function of these receptors?

What molecule binds to an receptor?

Answer 14

Proteins called receptors in the cells bind to signaling molecules and initiate a physiological response

• Each target cell responds to limited set of signals
• Different cells have different receptors for the sane signal

A ligand is a signal molecule that binds to a receptor

Question 15

What are agonists? and what effect does it have when it binds to a receptor?

Answer 15

An agonist is a mimetic of the natural ligand and produces a similar biological effect as the natural ligand when it binds to the receptor

Question 16

What are antagonists? and what effect does it have when it binds to a receptor?

Answer 16

Mimic ligands that bind to a receptor and prevent receptor activation by a natural ligand

The efficacy of a full agonist is by definition 100%, a neutral antagonist has 0% efficacy

Question 17

Where are intracellular receptors and cell surface receptors found?

Answer 17

Intracellular receptors- found inside the cell, cytoplasm or nucleus

Cell surface receptors-found on cell membrane

Question 18

What characteristic of a ligand supports its function?

Answer 18

The ligands of intracellular receptors are small, hydrophobic molecules since they must be able to cross the cell membrane in order to reach their receptors

for example:
the primary receptors for hydrophobic steroid hormones, such as the sex hormones, vitamin D are intracellular

Question 19

What is the function of cell surface receptors?

Answer 19

Membrane-anchored proteins that bind to ligands on the outside surface of the cell

In this type of signaling, the ligand does not need to cross the cell membrane. So, the chemical structures of ligands are more diverse

Question 20

What are the differences between a ligand in the intracellular receptor and a cell surface receptor?

Answer 20

Ligand in intracellular receptor is small and hydrophobic to be able to go across the membrane to reach the receptor

whereas, cell surface receptor ligands are more diverse because it does not need to go across a membrane

Question 21

How do receptors form channels?

Answer 21

By multi-pass transmembrane proteins

opening or closing of channel triggered by binding of ligand

-Ions move through channel

Question 22

What are the 3 main types of cell surface receptors?

Answer 22

1. Ion-channel-linked receptors
   - receptor forms a channel by multi-pass transmembrane proteins
   - opening or closing of channel triggered by binding of ligand
   - ions move through channel
2. G Protein-Linked Receptors
   - seven pass transmembrane proteins
   - ligand receptor interaction mediated by trimeric GTP binding protein (G Protein)
3. Enzyme-Linked Receptors
   - Single-pass transmembrane proteins
   - Are themselves enzymes or directly activate enzymes
   - Mainly protein kinases

Question 23

What are G protein couples receptors and its function? (GPCRs)

Answer 23

The largest and most diverse group of cell membrane receptors in eukaryotes

GPCRs play a diverse role in regulating a lot of functions and cellular response in the body

due to this they are highly sought after in drug target

fact: researchers estimate that between one-third to one-half of all marketed drugs act by binding to GPCRs

Question 24

Describe the structure of GCPRs (G Protein Coupled Receptors):

Answer 24

7 membrane-spanning regions with their N-terminus on the exoplasmic face and C-terminus on the cytosolic face

-trimeric= 3 subunits

G proteins are heterotrimeric which is made up of 3 different subunits: an alpha, a beta and a gamma subunit

G proteins have the ability to bind the nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP)

Question 25

Examples of ligands that bind to GCPRs:

Answer 25

Hormones
Neurotransmitters
Sensory signals

Question 26

How is a GCPR activated?

Answer 26

When a ligand binds to a GPCR, a change in the conformation of the GCPR activates the G protein, and GTP physically replaces the GDP bound to the alpha subunit

As a result, the G protein subunits dissociate into 2 parts:

1. GTP-bound alpha subunit
2. A beta-gamma dimer

G proteins no lounger bound to the GCPR

Question 27

How long will GTP proteins remain active?

Answer 27

G proteins remain active as long as their alpha subunits are joined with GTP

When a ligand separates from the receptor, GTP is hydrolyzed back to GDP

The subunits once again resume the form of an inactive hetero-trimer, and the entire G protein associates with the now inactive GCPR

Question 28

What is a kinase?

Answer 28

Protein kinases transfer phosphate groups onto particular amino acid side chains that have a hydroxyl (OH) group from high energy donor molecule, e.g ATP

Question 29

What are the most common amino acids Kinases act on?

Answer 29

Most kinases act on both serine threonine others act on tyrosine and some act on all 3 amino acids

Question 30

The adding of a phosphate group on proteins are known as……?

Answer 30

The adding of a phosphate group on proteins are known as phosphorylation

On switch: Proteins are phosphorylated by enzymes called protein kinases and thus are activated

Off switch: Protein phosphatases are enzymes that can rapidly remove phosphate groups from proteins (dephosphorylation) and thus inactivate protein kinases

Question 31

What are the 3 subunits in G proteins?

Answer 31

3 different subunits: an alpha, a beta and a gamma subunit

Question 32

What are Enzyme-linked receptors?

Answer 32

Cell-surface receptors with intracellular domains that are associated with an enzyme that can catalyze a reaction

Question 33

What are RTKs?

Answer 33

Receptor Tyrosine Kinases (RTKs) are a class of enzyme linked receptors, transfers phosphate groups from ATP to the tyrosine residues of a protein in a cell

functions as an “on” and “off: switch in many cellular functions

Question 34

Tyrosine kinase

Answer 34

represent a major portion of oncoproteins

Question 35

Other classes of Enzyme linked receptors:

Answer 35

1. Receptor Ser/Thr kinases
2. Receptor guanylyl cyclase
3. Tyrosine kinase associated receptors

Question 36

Describe the structure of RTKs receptor tyrosine kinases:

Answer 36

1. 1 transmembrane (TM) domain
2. Extracellular ligand-binding domain
3. Cytosolic tail that contains cytosolic residue

In the inactive state, the membrane proteins exist as monomers

Question 37

How does the ligand bind to the receptor tyrosine kinase?

What occurs after the binding?

Answer 37

The binding of ligands to RTKs results in the 2 receptors aggregate (join) to form a cross-link dimers

Phosphorylation of RTK
The RTK dimers cause the auto-phosphorylation of the C terminal tyrosine residues and activate RTK

Question 38

Summary of the RTK (receptor tyrosine kinase) process:

Answer 38

1. (Binding, joining, cross-link dimers)
   The binding of ligands to RTKs results in the 2 receptors aggregate to form cross-link dimers
2. (phosphorylation and activation)
   RTK dimers cause the auto-phosphorylation of the c terminal in tyrosine residues and activate RTK
3. (signal transduction)
   Activation of relay proteins by RTK, RTK triggers many cellular signal transduction pathways
4. Response is carried out