Chapter 4: 4.6 Membrane Protein Function - Specifics Flashcards
Define:
Bacteriorhodopsin (bR)
A light fueled proton pump found in halobacterial membranes (consist of ~75% of membrane)
What type of membrane protein is bacteriorhodopsin?
A multi-spanning membrane protein
Membrane proteins are hard to…
- Crystallize
- Study
Can membrane proteins be studied with NMR?
No, membrane proteins need to be removed from the membrane with detergents making them too large for NMR
The purity and abundance of bR in the membrane provide an opportunity to study…
The crystal structure of a membrane protein
How many transmembrane alpha-helices surround a chromophore?
7
What surrounds a chromophore?
7 transmembrane alpha-helices
In non-activated bR:
Describe the structure of bR
A retinal molecule is bound to a key Lys residue
True or False:
The retinal in non-activated state is in cis conformation
False, everything is in trans conformation
What is the key Lys residue in bR known as?
Schiff base
List:
The steps of light activation of bR (5)
- Light causes the C-13 to take on a cis conformation
- The conformational change results in proton transfer to a nearby Asp )Asp85) and proton release from Glu194/Glu204 pair
- The Schiff base acquires a proton form Asp96
- Asp96 takes up a proton from the cytosol
- Asp85 re-protonates the Glu194/Glu204 pair resetting the system
What is the light activation process of bR known as?
“Proton hopping”
Is “proton hopping” faster or slower than diffusing?
Faster
State:
Structure of porins
Consist of beta-barrels
Where are porins located?
Located in outer-membranes of cells and organelles
What can pass through porins?
Small hydrophilic molecules pass through
Describe:
The amino acids in strands in the porins
Alternate between polar and non-polar
* Polar amino acids face inwards
* Non-polar amino acids face outwards
What can some bacteria use porins for?
Can use porins to lyse host cells
Describe:
The process of bacteria lysing cells using porins
- 7 identical beta sheet pairs (beta-turn-beta) are added into a host membrane creating a pore
- Cell contents spill out
List:
The two types of diffusion
- Passive Diffusion
- Facilitated Diffusion
Define:
Passive Diffusion
Not facilitated by transporter proteins
What can move through in passive diffusion?
Small, uncharged molecules
* Move WITH their concentration gradient (from high to low concentration)
Define:
Facilitated Diffusion
Integral proteins transport molecules in a thermodynamically favourable direction
What special behaviour does facilitated diffusion exhibit?
- Can be very specific in their cargo
- Exhibit saturation behavior
Define:
Ion Channels
Very selective for their respective ions
True or False:
Ions move at a slow rate through ion channels
False, ion channels move ions at very high rates (10^8 per second)
Describe:
Ion Channels
They are gated, meaning they have an open and closed state
Define:
Potassium Ion Channels
Play a key role in electrical impulse formation in neurons
* Has additional roles
Describe:
Structure of Potassium Ion Channels
Made up of 4 identical subunits (predominantly alpha-helices)
Describe:
Structure of each subunit of a Potassium Ion Channels
Has 5 key amino acids lining the active site (TVGYG)
Describe:
Structure and Purpose of the opening for Potassium Ion Channels
Opening of the small size
* Forces these resides to replace the water that surrounds the ion extracellularly
* Size specific for K+
How is a Potassium Ion Channel opened/closed?
Can be opened/closed via a conformational change in a conserved glycine residue
State:
The steps that Receptors undergo when they act
- Bind or “sense” an extracellular stimuli
- Transmit to secondary messengers
- Cell responds
Define:
GPCR
G-Protein-Coupled Receptors
What can GPCR bind to?
Both natural and synthetic stimuli (ligands)
In GPCR, what are signals transduced by?
GTP binding proteins
Describe:
G-proteins in GPCR
Hertero-trimers consisting of α, β, and γ subunits
In a GPCR:
Describe what happens once a ligand binds to the receptor
The α subunit binds GTP and dissociates from the βγ subunits
In GPCR:
The dissociation of α subunit leads to what?
- Activates an effector enzyme
- Causes a series of enzymatic events in the cell
- Ultimately leads to cell response
True or False:
Signals by GPCRs can be amplified
True
How can signals by GPCRs be amplified?
One hormone-receptor complex can activate more than one G-protein
In GPCRs:
How are GPCRs turned off?
- GTP converted to GDP during activation of the effector enzyme
- Brings the α subunit back into complex with the βγ subunits
