Membrane Proteins Flashcards
Lipid bilyer vs protines function
Lipid bilayer provides the basic structure of biological membranes,
proteins perform most of the specific tasks (functional properties)
Amount of protein in a biological membrane typically reflects function:
plasma membrane vs myelin
Plasma membrane 50% lipid, 50% protein
Myelin 75% lipid, 25% protein
Membrane proteins show ____ across the bilayer
asymmetry
Unique orientation set at biosynthesis, kept throughout protein’s lifetime.
Membrane protines are synthisised in the
in the ER - made in a cetine way - transported everywhere else - maintain a set orentation
Types of Membrane Proteins
Transmembrane Proteins
Membrane Associated Proteins
Lipid Associated Proteins
Transmembrane Proteins:
Hydrophobic regions pass through the membrane, hydrophilic groups exposed to water.
Includes single-pass TM (1), multipass TM (2), and β barrel (3).
Membrane Associated Proteins location
do not extend into hydrophobic interior of the lipid bilayer; they are bound to either face of the membrane by non covalent interactions with other membrane proteins
Lipid-Associated Proteins are attached by
Proteins attached to bilayer by covalent attachment to lipid anchors
Transmembrane Domains
portion of __
of ___ ameno acids
form ___
Portion of the integral membrane protein (trans-membrane domain) is embedded within the lipid bilayer.
20-30 NON POLAR amio acids
amost always form a alfa helex .
polar peptide bonds are forced into H bonding in non aqueous enviorment (hydrophobic core of lipid bilayer)
non polar side chains stick out facing hydrofillic lipid bilayer
Single-Pass Transmembrane Protein
types __
Polypeptide chain crosses only once
Typed according to topography.
Type 1: N-terminus out, C-terminus in.
Type 2: N-terminus in, C-terminus out.
N - free amino
C - free carboxyl
Multipass Transmembrane Proteins
Polypeptide chain crosses multiple times
α helices occupy specific positions in the folded protein structure that are determined by interactions between the neighboring helices, important for function of many channels and transporters
slight conformational change from newly synthiszed to folded
Hydropathy Plot
Localize potential α helical membrane spanning segments
If energy is required (+) to transfer from non polar to water - means hydrophobic
determines which arease are hydrophobic - most likely a trans membrane domain
- analizes in segements (20-30) of each alfa helisces
β Barrel are more __ than Alda helies because ___
are abundant in ___
More rigid in comparison to multipass proteins with α helices.( α has more flexability - slight conformational changes) because of alighnmens of H bonds in structure
Porins are well studied examples
Abundant in outer membranes of bacteria, mitochondria, and chloroplasts
Would a β Barrel require a long strech of hydrophobic ameno acids (~20) in transmembrane portion of a protine?
No
R sides of alfs - stick out
R side of β - alternate
would be an alternating strech of hydrophobic and fillic
Can a hydropath plot be used for a β Barrel?
No
because the plot looks at/analized the polypeptide in seggments (20-30 of hydropobic) would not work
(B barrel can be ~10) and alternating fobic and fillic
Isolation of Integral Membrane Proteins requires
___
to remain soluble Isolated protein requires ____
disruption of the bilayer by detergents.
detergent.
Many Detergents used for isolation of integral membrane proteins
Amphipathic
non-ionic (Triton) - mild - denaturing
ionic (SDS) - harsh - polypeptide is extended/unfolded - react with hydrophobic core (use for western blot, seperate on MW)
Mild detergents: Purify protein in ___
active form
Steps of mild detergent isolation of inegral membrane protines
add deterngts (mild, if too harsh would not be able to suddy)
Hydrophobic interactions with proteins - soluablizes membrane proteins
Lipid micells (detergents) added
Purfication of protine of intrests
add phospolipids and remove detergent
results in formation of a liposome - just studdy the spcific membrane protine’s functions
Peripheral Membrane Proteins are___
assoacated by __
issolated by __
Membrane proteins non-covalently bound to integral proteins or lipid head groups.
Usually associated via electrostatic interactions
Can be isolated without bilayer disruption.
change pH - extremly basic - interfeers with interactions of proteins and bylater membrane.
Lipid-Anchored Protein Types
- Proteins with covalently linked acyl chains (fatty acids) –(cytosolic side only)
- Proteins with covalently linked polyisoprenoid anchors (cytosolic side only).
- Proteins with covalently linked (SUGGAR) glycophosphatidylinositol (GPI) anchors (non cytosolic side only). Phosphatidylinositol (PI) is a phosphoglyceride
Myristic acid (C=14)
Protine with covalently linked acytl chains (fatty acids) -
stable ancher - made with protine itself
palmitic acid (C=16)
Protine with covalently linked acytl chains (fatty acids)
denamic ancher - made in response to signal
Lipid anchors Control
Membrane Localization of Some Signaling Proteins
Lipid Anchors and Intracellular signaling
help to relay extracellular signals into cell interior, are bound to cytosolic half of membrane with attached lipid group (ie fatty acid chain)
Src Kinases:
Fatty acid anchor
During its synthesis, myristoyl acid added (attachment to membrane by single anchor not very strong).
Second lipid (palmitic acid) added, more firm anchor. This lipid added in response to extracellular signal.
GPI-Anchored proteins are invovled in
have been involved in membrane protein transportation, cell adhesion, cell wall synthesis, and cell surface protection
Most transmembrane proteins are
glycosylated (noncytosolic side of membrane)
a layer of oligosaccharide and polysaccharide chains that are attached to the lipids and protein domains facing the outside.
This environment decreases the likelihood that intrachain or interchain disulfide (S–S) bonds will form between cysteines on the cytosolic side of membranes.
Carbohydrate layer on cytosolic side purpose
(glycocalyx)-protects cells against mechanical (proteases) and chemical damage, mediate cell adhesion and cell recognition processes
differnt kinds of suggars are sued as surface markers
Although most of the sugar groups are attached to intrinsic plasma membrane molecules, the carbohydrate layer also contains both glycoproteins and proteoglycans that have been secreted into the extracellular space and then adsorbed onto the cell surface
Selections
carbohydrate binding protines (lectins)
lucose roling - only veins not arteries
immune - search for sites
slow immune to cite of injury when large opening - go through cell wall
Diffusion of Membrane Proteins
membrane proteins can be mobile
FRAP
Floresence Recovery After Fotobleaching
Measure of Lateral Diffusion Rates of Membrane Proteins
how much time to fill areas - if florsence lable - immoble areas - not fill quickly
if mobile - fill quickly
Spectrin cytoskeleton enables
red blood cell to withstand stress on its membrane as its forced through narrow capillaries
it lies below the plasma membrane, criss crosses, holding membrane in place
H3(CH2)7CH=CH(CH2)7COOH (Oleic Acid) has a ____ melting point
compared to CH3(CH2)16COOH (Stearic acid).
LOWER
High fluidity (ie presence of cis double bonds) = low phase transition temperature
Also mentioned in class: It takes less energy to melt unsaturated fats (do not
pack tightly) than saturated fats. Since there are more interactions between
saturated fatty acids, it takes more energy to melt (pull them apart)
A bacterium is suddenly expelled from a warm human intestine into the cold
world outside. Which of the following adjustments might the bacterium make
to maintain the same level of membrane fluidity?
a. produce lipids with hydrocarbon tails that are longer and have fewer double
bonds
b. produce lipids with hydrocarbon tails that are shorter and have more double
bonds
c. decrease the amount of cholesterol in the membrane
d. decrease the amount of glycolipids in the membrane
B
Which of the following is most likely to occur after the lipid bilayer is pierced?
a. The membrane reseals.
b. The membrane collapses.
c.. A tear is formed.
d. The membrane expands.
A
