Blake_Biochem_16_Membranes I

Question 1

What kind of assemblies do biological membranes form?

Answer 1

non-covalent

Question 2

Are membranes symmetrical?

Answer 2

no

Question 3

What does it mean that membranes have a fluid structure?

Answer 3

lipids and protein moleculed diffuse rapidly in the plane of the membrane, but they donot rotate accross the membrane.

Question 4

What is the polarity of the membrane?

Answer 4

inside is negative

Question 5

What direction does a phospholipid face when floating on the surface of water?

Answer 5

polar heads are in contact with water

non-polar hydrophobic tails project into the air.

Question 6

What structures can lipids form in aqueous solution?

Answer 6

Micille (tails in, together in a sphere)

Lipid Bilayer (includses liposome)

Question 7

Micelle (3)

Answer 7

limited structure

less than 20 nm

formed when soaps and detergents are added to water

Question 8

Why are lipid bylayers more thermodynamically stable than micelles?

Answer 8

• Fatty acid chains are too bulky to fit in a micelle. There is tighter packing possible in a bilayer model.
• Bimolecular sheets can have diameters as large as 1mm (10⁶ nm)a

Question 9

Formation of lipid bilayers is a self-assembly process. What are the major driving forces for their assembly?

Answer 9

HYDROPHOBIC INTERACTION

Van der Waals btwn hydrocarbon tails

Electrostatic and H-bonds

[same as proteins]

Question 10

Lipid bilayers are held together predominantly by____

Answer 10

hydrophobic interactions

Question 11

Hydrophobic interactions have 3 significant consequences:

Answer 11

• lipid bilayers have an inherent tendency to be extensive
• lipid bilayers tend to close on themselves so there are no edges with exposed hydrocarbon chains, and so they form a compartment
• lipid bilayers are self-sealing because holes are energetically unfavorable

Question 12

Lipid bilayers are self-sealing because___

Answer 12

a hole is energetically unfavorable

Question 13

Lipid bilayers have a very ________ permeability for ions and most polar molecules.

Answer 13

LOW

Question 14

Which has a higher degree of permeability?

Na⁺

^H₂0

Answer 14

H₂O

Question 15

How does water transvers the membrane?

Answer 15

Aquaporins

and small size and lack of complete charge

Question 16

How are membrane proteins classified?

Answer 16

By their degree of dissociability

Interal or Peripheral

Question 17

Integral Membrane Proteins

(3)

Answer 17

• Interact extensively with the hydrocarbon tails of membrane lipids
• Can be released only by agents that compete for these nonpolar interactions, such as organic solvents and detergents
• Span the lipid bilayer

Question 18

How are protein components visuallized in a laboratory setting?

Answer 18

SDS-polyacrylamide gel electrophoresis

Question 19

Peripheral membrane proteins

(3)

Answer 19

1. bound to membranes primarily by electrostatic and hydrogen bond interactions with the head groups of lipids
2. Dissociated from membranes by adding salt or pH changes
3. Bound to the surfaces of integral membrane proteins
   
   • either on the cytostolic or extracellular side
   • anchored to the lipid bilayer by a covalently attached hydrophobic chain (fatty acid)

Question 20

How are peripheral membrane proteins bound to the membrane?

Answer 20

1. by electrostatic and hydrogen bonds with head groups
2. to integral proteins
3. linked by hydrophobic chains

Question 21

Which type of secondary structure of protein more readilly crosses the lipid bylayer?

Answer 21

alpha-helices

Question 22

What is the most comon motif in membrane proteins?

Answer 22

alpha-helices

Question 23

Bacteriorhodopsin

Answer 23

• A light-driven proton pump, converts the energy of light into transmembrane proton gradient that is used to synthesize ATP
• Built almost entirely of alpha-helices (with mostly non-polar amino acids) arranged almost perpendicularly to the bilayer plane

Question 24

(COX)1

Answer 24

Cyclooxygenase

[or (PGHS)1 (prostaglandin H₂ synthase)]

1. an integral protein that binds to the luminal leaflet of the ER
2. A homodimer that consists of primarily alpha-helices
3. does NOT span the membrane

Question 25

How are lipid-linked proteins associated?

Answer 25

covalently

Question 26

Lipids anchor lipid-linked proteins to the _________ and mediate ____-____ interactions

Answer 26

membrane

protein-protein

Question 27

Three kinds of Lipid-Linked protein modification

Answer 27

1. Palmitoylation of cysteine residues by a thioester bond
2. Farnesylation of cysteine residues at the C-terminus
3. Glycosylphosphatidylinositol-link to the carboxyl terminus (GPI)

Question 28

Palmitoylation

Answer 28

Lipid linking to a cysteine residue by a thioester bond

Question 29

Farnesylation

Answer 29

Lipid-linking of a cysteine residue at the C-terminus

Covalent attachement of a farnesyl (C₁₅) unit to the C-terminal tetrapeptide CAAX in which Cys is followed by 2 aliphatic residues

Question 30

Glycosylphosphatidylinostitol-link

Answer 30

lipid-linking to the carboxyl terminus

Question 31

Steps in Farnesylation

Answer 31

1. Covalent attachement of a farnesyl (C₁₅) unit to the C-terminal tetrapeptide CAAX in which Cys is folowed by 2 aliphatic residues
2. After the farnesyl group is appended to the protein in thioether linkage with the Cys residue, the AAX tripeptide is hydrolytically cleaved away

Question 32

What is the function of Farnesyl-linked proteins

Answer 32

Anchoring the protein to the membrane and facilitating protein-protein interactions

Question 33

What is the function of GPI-linked protein?

Answer 33

• Anchors the proteins to the outer leaflet of the plasma membrane
• Many cell-surface hydrolytic enzyme and adhesions are tethered to cells by a GPI unit.

Question 34

What are the parts of a mitochondrion?

Answer 34

Two membranes: internal and external

Two compartments: Intermembrane and Matrix

Question 35

What is the significant feature of the mitochondrial inner membrane?

Answer 35

Internal ridges (cristae)

Question 36

What function takes place in the intermembrane space of mitochondria?

Answer 36

Oxidative phosphorylation

Question 37

What important functions take place in the mitochondrial matrix?

Answer 37

TCA cycle

Fatty acid oxidation

Question 38

What is the permiability of the different mitochondrial membranes?

Answer 38

Outer membrane is very permeable to most small molecules and ions

The inner membrane is impermeable to nearly all ions and polar molecules

Question 39

How do molecules and ions enter the inner and outer membranes of the mitochondria?

Answer 39

Inner membrane: Transporters shuttle metabolites such as ATP, Pyruvate, and citrate

Outer membrane: Mitochondrial porins (30-35kDa pore-forming proteins) known as Voltage-Dependent Anion Channel (VDAC)