lecture 3 Flashcards
Types of membrane proteins in the membrane (how they sit in the membrane)
- Goes through membrane
2. Goes through membrane multiple types
3. Circular, goes through membrane
4. Only goes into 1 side
5. Attached to surface
6. Attached to something thats attached to surface
7. Attached to ANOTHER protein (e.g. 1)
Attached to ANOTHER protein on other side
describe integral proteins
- Embedded within membrane, amphiphilic
- 1,2,3 above
- Need agents to disrupt hydrophobic interactions to isolate protein (e.g. Detergents, organic solvents)
- E.g. Glycophorin (protein found on blood cell surface)= (type 1), G protein-coupled receptors, helical bundle (type 2), Maltoporin, porin (found in gram neg. Bacteria), Fatty acid transporter (type 3)
compare a-helix, helical bundle, and beta barrel
refer to image on onenote
type 1
-recognition + receptors
- Single transmembrane domain
type 2
-helical bundle
- enzymes, transporters, receptors
- Multiple transmembrane domains
-usually goes through membrane 7 times
Forms a-helix + B-sheets
Membrane protein type 1 + 2
type 3
- transporter (channel proteins) + receptor
- B-sheets is zig-zag, runs in rows, parallel
Most structured as 18 strands
Note: membrane spanning domain (contains polar NH and C=O bonds), is the part of the protein thats inside the membrane, is hydrophobic, hence why its inside, this also enables interactions with the hydrophobic core of the membrane
The amino acids from the NH form H-bonds with each other, making a-helixes + B-sheets
This further shields amide bonds
describe peripheral proteins
- Globular proteins
- 7,8 above
- Soluble with mild detergent or high salt conc.
- Easily released by icnreasing pH (leaves membrane intact)
- Not strongly bound to membrane
- May form ionic interactions + H bonds with polar lipid head groups/othe proteins
- May only interact with inner membrane using a hydrophobic loop or a amphipathic a-helix (when protein only enters half of membrane), while most of protein floats outside
- Cell-cell recognition + adhesion
refer to image on onenote
describe lipid anchored proteins
- 5,6, above
- Covalently linked to lipids in membrane
- Transient (short lived)- anchors can be reversibly attached to membrane- for signalling
- Modulate activity of protein
- Can also be linked integral membrane proteins as well
○ 4 types of anchors
§ Amide-linked myristoyl
§ Thioester linked fatty acyl
§ Thioster-linked prenyl
§ Glycosyl phosphatidylinositol
describe hydropathy plots
- Represents if protein has a transmembrane domain (and therefore has a hydrophobic part)
- Plots 19-25 amino acids, and if the hydropathy score is above > 0 for each protein, indicates the hydrophobic regions
- refer to image on onenote
how much can proteins move in the membrane?
move up to 10microns/min, some are anchored and move slower
how much can lipids move in the membrane?
several microns sec. (1 sec for bacterial, couple of minutes for animal cells)
how long does it take for the membrane to flip?
Takes several days for half of the lipids in a bilayer to flip from 1 side to other- lipases enzyme used to flip
What can/cannot be transported across the membrane
- most water soluble compounds cannot diffuse across due to hydrophobic core of membrane
- Small solutes (CO2, O2, nitrogen, alcohol can diffuse)
- So hydrophobic (O2, CO2, N2 etc.) molecules easily travel through
- Small uncharged polar molecules (H2O, urea, glycerol), some can travel through
- Large uncharged polar molecules (glucose, sucrose), uses help to travel across
- Ions (Mg+, H+, Na2+) CANNOT
- High to low permeability (H2O, urea, glycerol, glucose, Cl-, K+, Na+)
describe membrane transport proteins
- Is the integral membrane proteins
- Specific movement
- Using a channel or carrier proteins
- Transport proteins make up 15-30% of all membrane proteins
- there are 2 types, carriers + channels
describe carrier proteins (facilitated + active transport)
- Proteins that bind to solute
- Changes conformation, carries through, then releases on inside
- Then returns to original structure
- Can be saturated
- Affected by temp. (does work in temp. Below membrane transition temp.)
- Requires energy + moves against conc. Gradient
- 2 types
- Uniporter- moves 1 molecule
- Co-transporter- moves 2 molecules
- Symporter- moves in SAME direction
- Antiporter- moves in DIFFERENT directions
describe channel proteins (facilitated transport)
- Forms a pore
- Doesnt interact with solute at ALL
- Only small and/or polar solutes
- Selective
- Faster then carrier proteins
- Used for signalling
- Can be gated
- Ligand
- Ionotropic receptor
- Responds to chemical messages (neurotransmitters)
- Depolarisation for excitation
- Hyperpolarisation for inhibition
- E.g. Nicotinic acetylcholine receptors
- When open, sodium enters, potassium leaves
- voltage
- Moves WITH conc. Gradient
- E.g. K+ pump
- Selectivity filter- made from loops of type 2 proteins
- Has 4 primary binding sites, but can only fill 2 at a time
- K+ important for cell process, regulation of cell volume, formation of neurons, secretion of hormones
- Movement of the protein opens/closes the channel
- Ligand
describe passive diffusion
- Simplest
- No help needed, follows conc. Gradient
- Electrochemical potential = conc. Gradient + electrical potential difference
describe facilitated diffusion
- Faster then passive diffusion
- Requires energy, but follows energy gradient
- Selective, uses channels/pores
- E.g. Glucose facilitated transport
- Found in each blood cell
- Glucose is too large to diffuse passively
- Has 12 transmembrane domains
- 3 a-helixes, form a channel, and 2 a-helixes form a hydrophobic pocket next to the glucose binding domain
- Acts like a normal carrier protein
- Forms a trimer (made of 3 proteins bound together)
