Cell Bio 9

Question 1

Biomembranes (2 Functions)

Answer 1

Define what is a cell

Allow for specialized functions to occur in a local manner

Question 2

Biomembrane Components

Answer 2

Lipids
Sterols
Proteins

Question 3

Due to ______, phospholipids spontaneously…

Answer 3

Amphipathicity, phospholipids spontaneously form lipid bilayers in acqueous solutions

Question 4

Properties of Fatty Acids will

Answer 4

Confer properties onto the bilayers

Question 5

Proteins Give…

Answer 5

Many functional characteristics to the membrane

Question 6

Fatty Acids

Answer 6

Building blocks of the components of the membrane (phospholipids; sphingolipids)

Long hydrocarbon tails attached to a polar (variabled) carboxyl head group.

Amphipathic

Often Cx;y, where x= number of carbon molecules, y = number of double bonds

Question 7

No double bonds

Answer 7

Saturated

Question 8

One double bond

Answer 8

Unsaturated

Question 9

More than one double bond

Answer 9

polyunsaturated

Question 10

Melting Point

Answer 10

Increases with the length of the chain and decreases with the number of double bonds.

Length of chain increases the number of interactions.

Double bonds introduce kinks in the chain preventing tight packing of lipids against one another.

Question 11

Membranes need to be

Answer 11

Fluid, not a solid or liquid at any temperature.

Adjust chain length, units of unsaturation and cholesterol such that the membrane cn be the right consistency at a certain temperature.

Question 12

Properties of Biomembranes

Answer 12

Fluid
Closed Compartments
Semi-Permeable
Asymmetric

Question 13

Fluid

Answer 13

Two dimensional fluids:
Rapid lateral diffusion
Rare transeverse (flip-flop) movement between leaflets

Fluidity is composition dependent.

Question 14

Determinants of Fluidity

Answer 14

Fatty Acids length+saturation

Steroids: cholesterol can increase or decrease fludity

Proteins: large and strcutural, they can be tethered to the cytoskeleton, fixing a spot and things have to move aorund them altering fludidity

Temperature: not a mechanism used in living cells

Question 15

FRAP

Answer 15

Fluorescent recovery after photobleaching

Measurement of membrane fluidity.

Flurescently label the plasma membrane proteins and bleach a certain area of the fluorescence and look for recovery of fluorescence.

Measure fluorescence, before and after bleaching.

Determine %recovery

Recovery can only occur if the proteins laterally diffuse in or out of this bleached area.

Level of recovery is related to mobility in the plasma membrane, 50% recovery tells you 50% are mobile and the other half are not.

Question 16

Closed Compartments

Answer 16

Cytosolic Face = internal face

Exoplasmic face = external

Same orientation even in vesicles

Question 17

Semi-permeability

Answer 17

Small, apolar (hydrophobic) molecules can diffuse freely

Large, ions, polar, charged molecules cannot diffuse freely and need to be transported

Question 18

Protein Asymmetry + Membrane Function

Answer 18

Phospholipid composition differes between leaflets

Carbohydrates are found exclusively on the exoplasmic face

Proteins are either embedded in the bilayer in a fixed orientation or associated with only one side.

Pumps are placed in the membrane in a symmetric way so that are always pumping the same way.

Question 19

Membrane Protein Categories

Answer 19

Integral
Lipid-Linked
Peripheral

All asymetric

Question 20

Proteins in the membrane

Answer 20

Give a lot of function to what cell do

Protein within the membrane can carry out biological functions

Question 21

Integral

Answer 21

Several domains of the protein are embedded in the hydrophobic integral part of the membrane.

Cyroplasmic Domain

Transmembrane Domain

Exoplasmic Domain

Question 22

Cytoplasmic Domain

Answer 22

Amino Acids such as Arg and Lysine

(+)vely charged AAs are near the cytosolic side to interact with the polar head groups.

Anchor the protein to the lipid bilayer, the charged amino acids prevent the protein form being pulled into the hydrophobic core

Question 23

Transmembrane Domain

Answer 23

Hydrophobic

Secondary and tertiary structures span the lipid bilayer.

Alpha helix (20-25 AAs in length)

Beta Barrel

Question 24

Exoplasmic Domain

Answer 24

Glycosylated

adding sugars to the charged polar groups adds more polarity preventing the protien orm slipping into the membrane

Question 25

Lipid-linked proteins

Answer 25

Proteins anchored to membrane by lipophilic adducté Do not enter the lipid bilayer, attach to different compenents of the lipid bilayer

Question 26

Lipids linked to the extracellular face

Answer 26

Use phosphatidlinositol to form a GPI anchor. GPI anchor: that protein is linked to the lipid bilayer on the exoplasmic side, can diffuse freely laterally in the membrane Link the protein to existing components within the plasma membrane

Question 27

Lipids linked to the cytosolic face

Answer 27

To acylate a protein to the inner lipid bilayer the protein needs to have an N-terminal glycine such, that the protein is linked to the plasma membrane. Prenylation proteins has a cys residue on the c-terminus

Question 28

Peripheral Proteins

Answer 28

Attached through non-covalent interactions: Ionic interactions, hydrogen bonds protein-protien interactions van der Waals forces cytoskeletal filaments can associate with bilayer through peripheral proteins (adaptors) as can ECM components. Integral membrane proteins like cadherins and integrins can link to the cytoskeleton and they use a variety of peripheral proteins to bind the cytoskeleton.

Question 29

Topogenic Sequences Types

Answer 29

N-terminal (cleaved) signal sequence Stop-transfer/membrane anchor sequence (STA) Signal-anchor-internal (uncleaved) sequence (SA) Hydrophobic C-terminus

Question 30

Topogenic Sequence

Answer 30

Sequence gives rise to specific shapes, amino acids fold into different shapes, as transaltion is occuring, topogenic sequences are recognized. Specific topogenic regions gives rise to a specific shape.

Question 31

Membrane Protein Types

Answer 31

Type 1: Exoplasmic amino terminus Type 2: Exoplasmic carboxy terminus Type 3: Short exoplasmic amino terminus Tail Anchored protein: short exoplasmic carboxy terminus Type 4: transmembrane domains, exoplasmic amino terminus GPI anchored proteins: Exoplasmic amino terminus+ carboxy terminus linked to GPI

Question 32

Tail Anchored Proteins

Answer 32

Requires Get3 recognition of hydrophobic C-terminal tial, membrane embedded Get1 and Get2 + ATP hydrolysis. Steps: - This protein has a carboxy hydrophobic tail that gets recognized by GET3 - GET3 takes it to the ER membrane where GET1 and GET2 are waiting, through ATP hydrolysis this hydrophobic tail region is inserted into the membrane - transmembrane protein without a luminal extracellular domain

Question 33

Type 1 Proteins

Answer 33

Have N terminal signal sequence Have stop transfer membrane anchor (STA): 2 topogenic sequence One transmembrane domain luminal N-terminus Steps: 1- Translation starts off in the cytosol; ribosome and mRNA are guided to the translocon by the SRP 2- Ribosome inserts the peptide into the translocon into the ER lumen 3- N-terminal domain is always luminal 4- The n-terminal signal sequence is cleaved by a peptidase. 5- STA topogenic sequence stops the ribosome for transferring the protein into the ER and anchors it into the membrane, the STA sequence becomes the transmembrane portion of the protein

Question 34

GPI-anchored proteins

Answer 34

Begin as Type-1 proteins with N-terminal in lumen and a C-terminal STA AA sequence near membranes recognized by GPI-transamidase, which cleaves and transfers the luminal portion to an adjacent GPI Without a transmembrane domain, not a cytoplasmic domain and can bind to the cytoskelton, GPI anchored proteins have lateral mobility.

Question 35

Type II protein

Answer 35

Have one internal signal-anchor sequence (SA) Orientation determined by positively charged amino acids (kept in cytosol) II= NH3 in cytosol Ribosome and mRNA are in the cytoplasm, translation starts and the ribosome is moved to the ER membrane. The C-terminal domain get put through the translocon and into the lumen because the positively charged amino acids are adjacent to the N terminal. because of these charged amino acids its difficult to get the amino acid chain into the translocon,

Question 36

Type III Proteins

Answer 36

NH3 is the lumen. Positively charged amino acids are adjacent to the carboxy terminus (+)vely charged amino acids are in the cytosol

Question 37

Type IV Proteins

Answer 37

Orientation of intial helix determined by positively charged amino acids next to signal-anchored (SA) sequence. Have alternating SA sequences and STA sequences Can have even or odd number transmembrane domains type 4 proteins are going to be inserted into the membrane, where the ribosome is constantly translating through the locon and through the cytosol.