Molecular biochemistry W12

Question 1

What are GPCRs + what do they mediate responses to

Answer 1

• Largest family of cell surface receptors
  =They mediate responseds to :
  -External signals- (light, smell, tastde)
  -Internal signals( hormones neurotransmitters)

Question 2

Explain the sturtcure of GPCRs

Answer 2

one polypeptide chain that crosses the lipid bilayer 7 times (

Question 3

What is GPCRs also called

Answer 3

Serpentine receptors

Question 4

What does GPCR work with to transmit signals

Answer 4

• G proteins

Question 5

Explain the activation of GPCRs

Answer 5

A single molecular can activate many different GPCRs

Question 6

Explain the 2 ways G protein can bind to the receptor

Answer 6

• Be pre- bound to the receptor
  -Can bind after receptor activation

Question 7

Explain the structure of a G protein

Answer 7

Trimeric protein made of 3 subunits :
-alpha, beta and gamma
alpha and gamma subunits have lipid tails that anchor them to the plasma membrane

Question 8

What is normal G protein bound to

Answer 8

GTP

Question 9

What is inactive G protein bound ton

Answer 9

• GDP and forms a complex with βγ.

Question 10

What subunits in G protein funcion together as one unit

Answer 10

• B and gamma

Question 11

What does the alpha subunit of g protein contain

Answer 11

GTPase activiry that turns GTP into GDP

Question 12

When is alpha subunit active and inactive

Answer 12

• Active when a binds to GTP
  -Inactive when GTP is hydrolysed to GDP

Question 13

Name and explain the 2 alpha subunit domains

Answer 13

• Ras domains - Helps w GTPase activity
  -AH (alpha helical domain) - clamps GDP or GTP in place

Question 14

Explain the process of activation of the GPCRs starting from a ligand

Answer 14

• When an extracellular signal molecule (ligand) binds to a GPCR, the receptor changes shape (conformational change).

This new shape allows the GPCR to bind to a trimeric G-protein and change its conformation as well.

Question 15

GDP Release!

Explain what happens in the G protein after it changes its confirmational chnage after GPCRs bind to a ligand

-

Answer 15

In the G-protein’s α subunit, the AH domain moves outward, opening the nucleotide-binding site.

This causes GDP to dissociate.

GTP then binds to the α subunit, which activates it.

Question 16

Explain the process of what happens once the GTP is boudn to the alpha subunit in the G protein adter GDP dissociation

Answer 16

Activation of Signaling Pathways
Once GTP is bound, the nucleotide-binding site closes, and the α subunit:

Dissociates from the receptor

Separates from the βγ complex

Now, both:

GTP-bound α subunit

βγ complex
→ Can activate downstream signaling molecules.

Question 17

As long as the ligand stays boudn to the GPCr , what does this ensure+ lead to

Answer 17

-That the recpeotr stays active , leading to the activation of multiple G proteins

Question 18

Explain specifcally the 3 components the GPCRs contai

Answer 18

7- transmembrane helices
An extracellular ligand-binding dite
-An intracellular G protein binding site

Question 19

Explain how the inactive G protein is restored at the end of the mechanisk of the ligand binding

Answer 19

Then, the α subunit recombines with the βγ complex, restoring the inactive trimeric G-protein.

Question 20

What is CAMP( cyclic adenosine monophosphate)

Answer 20

a nucleotide and a second messenger

Question 21

Explain the differenc between AMP and CAMP

Answer 21

AMP vs. cAMP:

AMP has one more oxygen and is not cyclic.

In cAMP, the phosphate forms a bond within the same nucleotide, unlike the phosphodiester bond in DNA/RNA which links two nucleotides.

Question 22

What is the function of Adenylate cyclase

Answer 22

Converts ATP to CAMP

Question 23

What is adenylate cyclase + what does it interact with

Answer 23

• A transmembrane znzyme
  -Interacts with an a subunit of G protein

Question 24

What is the function of phopshodiesterase + when is it active

Answer 24

Breaks down cAMP into AMP
-It is continously active

Question 25

What is Adenylate cyclase regulated by + explain how the work and what they contain

Answer 25

- Gs protein( stimulatory) Contains as subnits activates adenylaye cyclase leads to increased CAMP - Gi protein( Inhibitory) Contains ai subunits Inhibits adenylate cyclase leads to decreased CAMP

Question 26

when GPCRs are coupled to gs proteins when activated what does it lead to

Answer 26

INcrease in CAMP PRODUCTION

Question 27

when GPCRs are coupled to gi proteins when inactivated what does it lead to

Answer 27

Decrease in CAMP

Question 28

What does cholera toxin target

Answer 28

alpha subunit of Gs proteins

Question 29

What does pertussis toxin target

Answer 29

Gi proteins

Question 30

CAMP mainly acts by activating what

Answer 30

Protein Kinase A

Question 31

What is protein kinase A

Answer 31

serine threonine kinase meanng it phosphorylates the OH groups of serine and threonine on target proteins

Question 32

What is inactive Protein kinase A + what does it contain

Answer 32

A tetramer containing: -2 regulatory subunits -2 catalytic subunits

Question 33

Explain how the the catalytic subunits in the protein kinase A is activated

Answer 33

CAMP binds to the regulatory subunits causing a conformational change -This releases the catalytic subunit activating them

Question 34

After the catalytic subunits in Protein Kinase A is activated what can they now do

Answer 34

- Phosphorylate target proteins

Question 35

Why does CAMP have different effects in different cell types in relaion to PKA

Answer 35

- As different cells have different target proteins for PKA

Question 36

In unstimulated cells , what is the function of phosphodiesterase

Answer 36

- Keeps cAMP low and PKA stays inactive

Question 37

In relation to adenylate cyclase, wht happens in stimulated cells

Answer 37

Adenylayte cyclase makes lots of cAMP -This overwhelms phosphodiesterasm raising cAMP and activating PKA

Question 38

How can CAMP cause long term chanegs

Answer 38

By activating gene transcription

Question 39

What do many cAMP responsive genes contain

Answer 39

A CRE( cyclic AMP response element) in their promotor

Question 40

Explain the CREB and CBP mechanism in activating gene transcription

Answer 40

CREB and CBP Mechanism CREB (CRE-binding protein) recognizes and binds the CRE DNA sequence. PKA phosphorylates CREB on a serine residue. Phosphorylated CREB recruits CBP (CREB-binding protein = coactivator). CBP activates transcription of target genes.

Question 41

What are phospholipids

Answer 41

Lipids with : -a hydrophilic head( contains a phosphate grp) -2 hydrophobic fatty acid tails

Question 42

What are phosphlipid connected by

Answer 42

By a glycerol backbone

Question 43

What are glycerophospholipids

Answer 43

A major class of phsopholipids found in cell membranes

Question 44

What are glycerophospholipids made up of

Answer 44

- 1 glycerol 2 FATty caids 1 phosphate group

Question 45

What are phospholipase enzymes

Answer 45

Enzymes that cut phospholipids at specific bonds to produce bioactive signaling

Question 46

Name 2 phospholipase enzyme

Answer 46

-Phospholipase C (plc) -Phospholipase A2

Question 47

wHAT IS the function of phospholipase C + what does it produce + release

Answer 47

- Cuts the bonds betwene the phosphate and glycerol at the 3rd carbon -Produces Diacylglycerol( dag) -Phopshate containing head group -This releases the second messenger inositol triphosphate (IP3)

Question 48

What does phospholipase C play a role in

Answer 48

Signal transduction

Question 49

What is the function of phospholipase A2 + what is the arachidonic acid then used to make+ WHAT are these moleculars involved in

Answer 49

Cuts of arachidonic acid from the 2nd carbon of glycerol -Prostaglandins -Leukotrienes -These molecules are involved in inflammation

Question 50

Some GPCRs signal through what + what is the function of the contents

Answer 50

Gq proteins -Gq proteins activate phospholipase phospholipase C-β (PLCβ).

Question 51

What is the name of the phospholipid cleaved by Phospholipase C

Answer 51

phosphatidylinositol 4,5-bisphosphate (PIP₂).

Question 52

Explain the structure of the phospholipd cleaved by phospholipase C ((PIP₂)

Answer 52

-Inositol head group with 3 phosphates+ glycerol backbone, and 2 fatty acid tails

Question 53

explain which one is hydrophobic/hydropholic : -second messenger IP₃ -Diacylglycerol

Answer 53

IP3= hydrophilic DAG = hydrophobic

Question 54

What are the 3 functiosn of secodn messenger IP3: -What it binds -Opens

Answer 54

- Binds to IP₃ receptors on the endoplasmic reticulum (ER). Opens Ca²⁺ release channels → Ca²⁺ floods into cytosol.

Question 55

What is the function of the diacylglycerol + where does it remain

Answer 55

Activates protein kinase C , which phosphorylates target proteins -Remains in the membrane

Question 56

What is the role of calcium in signaling

Answer 56

- Ca2+ acts as a second messenger -The rise in cystolic ca2+ activates ca2+ binding proteins

Question 57

What is calmodulin + structure

Answer 57

Major ca2+ sensor inside cells - Single polypeptide with 4 high affinity ca2+ binding sites

Question 58

When calmodulin is bound to ca2+ what does it undergo

Answer 58

It undergoes allosteric activation

Question 59

What is Calmodulin-dependent kinases + activated by what

Answer 59

A family of protein kinases activated indireclty by calmodulin

Question 60

What do calmodulin dependent kinases phosphorylate

Answer 60

They phosphorylate target proteins

Question 61

What is phospholipase C activated by

Answer 61

- Gq protein a subunit -Tyrosine kinase receptors

Question 62

What does bipolar disorder involve

Answer 62

Extreme mood swings between mania and depression

Question 63

What are lithium salts used for

Answer 63

To stabilise mood in bipolar patients

Question 64

Lithium affects signalig by what>?

Answer 64

- By distrupting phosphoatidylinositol pathways - involvig PLC and PCK -Inhibiting GSK-3 (Glycogen synthase kinase 3)

Question 65

When cells are exposed to high levels of ligand for long time what happens

Answer 65

They adapt/ desensitize

Question 66

Name the 3 steps in GPCR deseneitization

Answer 66

-Receptor sequestration -Receptor down regulation -Receptor inactivation

Question 67

Explain what happens in the 3 steps of GPCR desensitization

Answer 67

Receptor Sequestration GPCRs are temporarily internalized into the cell (cannot access extracellular ligands). Receptor Down-Regulation Internalized GPCRs are destroyed in lysosomes. Receptor Inactivation GPCRs are modified so they no longer interact with G-proteins.

Question 68

What is the function of GPCR kinases(GRKs)

Answer 68

- They phosphorylate activated GPCRS

Question 69

Once the the activated GPCRs are phosphyorylated by GPCR kianses what happens ? -arrestin + promotes

Answer 69

-Arrestin binds to the receptor -Then blocks GPCR G protein interaction -Promotes internalisation of the receptor

Question 70

Name the 3 different types of G proteins

Answer 70

Gs, Gq, GI

Question 71

What is Nitroc oxide

Answer 71

Small hydrophobic gas

Question 72

What is nitric oxide synthesized by

Answer 72

Endothelial cells from L-arginine via NO synthase

Question 73

Wht is NO synthase activated by

Answer 73

Increase intracellular ca2+

Question 74

Explain the half life of NO

Answer 74

NO has a short half life

Question 75

Explain the mechanism of NO action

Answer 75

- NO enters smooth muscle cells and activates soluble guanylate cyclase in the cytosol. Guanylate cyclase converts GTP → cGMP. cGMP activates protein kinase G (PKG). PKG reduces Ca²⁺ levels in smooth muscle cells → leads to: Muscle relaxation Vasodilation (widening of blood vessels) Increased blood flow Phosphodiesterase breaks down cGMP → GMP to terminate the signal (just like cAMP → AMP).

Question 76

What is Adenosine diphosphate ribose (ADP-ribose)

Answer 76

-ADP + ribose attached to it via phosphate

Question 77

What is ADP ribosylation+ function

Answer 77

Adding ADP- ribose to a target protein -It modifes the proteins activity - either activate or inhibit target protein

Question 78

What is ADP ribosylation is used by

Answer 78

Bacterial toxins

Question 79

What is cholera caused by + how is it transmitted

Answer 79

Vibrio cholerae Transmitted via contaminated water or food

Question 81

What are symptoms of cholera

Answer 81

Severe watery diarrhea, rapid dehydration, can be fatal

Question 82

What is Pertussis caused by

Answer 82

Bordetella pertussis

Question 83

What are symptoms of pertussis

Answer 83

Whooping couch , lasts weeks

Question 84

Explain the mechanism of cholera toxin from entery

Answer 84

CT enters intestinal cells (enterocytes) and reaches the cytosol. It ADP-ribosylates Gas (Gs alpha subunit). This inhibits GTPase activity of Gαs→ it can’t convert GTP → GDP. Gαs remains permanently active → constantly activates adenylate cyclase. This leads to excess cAMP production → excess activation of PKA. PKA phosphorylates CFTR channels (chloride channels). CFTR opens, causing massive efflux of Cl⁻, Na⁺, K⁺, HCO₃⁻, and water into the intestinal lumen.

Question 85

Explain the mechaism of how Pertussis toxin works

Answer 85

ADP-ribosylates Gα_i (Gi alpha subunit). Gαi is inactivated (opposite of cholera toxin). Normally, Gαi inhibits adenylate cyclase. Inactivated Gαi → can’t inhibit adenylate cyclase → so enzyme becomes overactive. Excess cAMP is produced. Excess PKA activity occurs → varied downstream effects depending on the target cell.