Chapter 4: 4.6 Membrane Protein Function - Specifics

Question 1

Define:

Bacteriorhodopsin (bR)

Answer 1

A light fueled proton pump found in halobacterial membranes (consist of ~75% of membrane)

Question 2

What type of membrane protein is bacteriorhodopsin?

Answer 2

A multi-spanning membrane protein

Question 3

Membrane proteins are hard to…

Answer 3

1. Crystallize
2. Study

Question 4

Can membrane proteins be studied with NMR?

Answer 4

No, membrane proteins need to be removed from the membrane with detergents making them too large for NMR

Question 5

The purity and abundance of bR in the membrane provide an opportunity to study…

Answer 5

The crystal structure of a membrane protein

Question 6

How many transmembrane alpha-helices surround a chromophore?

Answer 6

7

Question 7

What surrounds a chromophore?

Answer 7

7 transmembrane alpha-helices

Question 8

In non-activated bR:

Describe the structure of bR

Answer 8

A retinal molecule is bound to a key Lys residue

Question 9

True or False:

The retinal in non-activated state is in cis conformation

Answer 9

False, everything is in trans conformation

Question 10

What is the key Lys residue in bR known as?

Answer 10

Schiff base

Question 11

List:

The steps of light activation of bR (5)

Answer 11

1. Light causes the C-13 to take on a cis conformation
2. The conformational change results in proton transfer to a nearby Asp )Asp85) and proton release from Glu194/Glu204 pair
3. The Schiff base acquires a proton form Asp96
4. Asp96 takes up a proton from the cytosol
5. Asp85 re-protonates the Glu194/Glu204 pair resetting the system

Question 12

What is the light activation process of bR known as?

Answer 12

“Proton hopping”

Question 13

Is “proton hopping” faster or slower than diffusing?

Answer 13

Faster

Question 14

State:

Structure of porins

Answer 14

Consist of beta-barrels

Question 15

Where are porins located?

Answer 15

Located in outer-membranes of cells and organelles

Question 16

What can pass through porins?

Answer 16

Small hydrophilic molecules pass through

Question 17

Describe:

The amino acids in strands in the porins

Answer 17

Alternate between polar and non-polar
* Polar amino acids face inwards
* Non-polar amino acids face outwards

Question 18

What can some bacteria use porins for?

Answer 18

Can use porins to lyse host cells

Question 19

Describe:

The process of bacteria lysing cells using porins

Answer 19

1. 7 identical beta sheet pairs (beta-turn-beta) are added into a host membrane creating a pore
2. Cell contents spill out

Question 20

List:

The two types of diffusion

Answer 20

1. Passive Diffusion
2. Facilitated Diffusion

Question 21

Define:

Passive Diffusion

Answer 21

Not facilitated by transporter proteins

Question 22

What can move through in passive diffusion?

Answer 22

Small, uncharged molecules
* Move WITH their concentration gradient (from high to low concentration)

Question 23

Define:

Facilitated Diffusion

Answer 23

Integral proteins transport molecules in a thermodynamically favourable direction

Question 24

What special behaviour does facilitated diffusion exhibit?

Answer 24

• Can be very specific in their cargo
• Exhibit saturation behavior

Question 25

Define:

Ion Channels

Answer 25

Very selective for their respective ions

Question 26

True or False:

Ions move at a slow rate through ion channels

Answer 26

False, ion channels move ions at very high rates (10^8 per second)

Question 27

Describe:

Ion Channels

Answer 27

They are gated, meaning they have an open and closed state

Question 28

Define:

Potassium Ion Channels

Answer 28

Play a key role in electrical impulse formation in neurons
* Has additional roles

Question 29

Describe:

Structure of Potassium Ion Channels

Answer 29

Made up of 4 identical subunits (predominantly alpha-helices)

Question 30

Describe:

Structure of each subunit of a Potassium Ion Channels

Answer 30

Has 5 key amino acids lining the active site (TVGYG)

Question 31

Describe:

Structure and Purpose of the opening for Potassium Ion Channels

Answer 31

Opening of the small size
* Forces these resides to replace the water that surrounds the ion extracellularly
* Size specific for K+

Question 32

How is a Potassium Ion Channel opened/closed?

Answer 32

Can be opened/closed via a conformational change in a conserved glycine residue

Question 33

State:

The steps that Receptors undergo when they act

Answer 33

1. Bind or “sense” an extracellular stimuli
2. Transmit to secondary messengers
3. Cell responds

Question 34

Define:

GPCR

Answer 34

G-Protein-Coupled Receptors

Question 35

What can GPCR bind to?

Answer 35

Both natural and synthetic stimuli (ligands)

Question 36

In GPCR, what are signals transduced by?

Answer 36

GTP binding proteins

Question 37

Describe:

G-proteins in GPCR

Answer 37

Hertero-trimers consisting of α, β, and γ subunits

Question 38

In a GPCR:

Describe what happens once a ligand binds to the receptor

Answer 38

The α subunit binds GTP and dissociates from the βγ subunits

Question 39

In GPCR:

The dissociation of α subunit leads to what?

Answer 39

• Activates an effector enzyme
• Causes a series of enzymatic events in the cell
• Ultimately leads to cell response

Question 40

True or False:

Signals by GPCRs can be amplified

Answer 40

True

Question 41

How can signals by GPCRs be amplified?

Answer 41

One hormone-receptor complex can activate more than one G-protein

Question 42

In GPCRs:

How are GPCRs turned off?

Answer 42

1. GTP converted to GDP during activation of the effector enzyme
2. Brings the α subunit back into complex with the βγ subunits