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BIOCHEM 405 > Lecture 13 > Flashcards

Lecture 13 Flashcards

1
Q

Biological Membranes

A

• Must maintain H+ and ion gradients
• Must be flexible - adopts various shapes and is self-sealing
• Must accommodate expansions and contractions - cell growth, cell movement, vesicle formation and fusion
• Must accommodate proteins of various types and shapes.
• Extrinsic, Trans-membrane, and Integral membrane proteins and still maintain a barrier
• Must be adaptable to varying cellular conditions and functions
(eg. pH, temperature, ionic strength, etc)
• Must be fluid to allow for diffusion (both Lipids and Proteins)
- but it must also allow for complex organization and architectures
• Must allow for transmission of signals and cellular communication

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2
Q

fluid mosaic model

A

proteins are embedded in a phospholipid bilayer and are free to move in the plane of the membrane

  • The Lipid Bilayer – a thin two-dimensional fluid
  • Membrane is assembled by non-covalent interactions
  • Membrane is Asymmetric
  • Lipids, Peripheral and Integral Proteins, Glycosylation

In general, Fatty Acyl chains tend to be extended but can adopt multiple conformations

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3
Q

peripheral membrane proteins

A

associated w 1 side of the bilayer and can be separated from the membrane w/o disruptig the bilayer.

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4
Q

intergral membrane proteins

A

deeply embedded int he bilayer and can only be extracted under conditions that distrupt membrane structure.- many extend through bilayer

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5
Q

transition temperature

A

depends on lipid comp

lipids w longer saturated tails tend to increase the transition temperature

those w more cis double bonds a/o shorter tails will reduce the transition temperature

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6
Q

translocon

A

facilitates the insertion of integral membrane proteins into the membrane bilayer

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7
Q

membrane rafts

A

rich in cholesterol, shingolipids, and GPI- linked proteins

the bilayer is thicker in the raft domains than in the surrounding membrane

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8
Q

Fluorescence Recovery After

Photobleaching (FRAP)

A
  • Lipids labeled with fluorescent molecules
  • Bleach fluorescence in a defined area with a laser
  • Monitor recovery of fluorescence with the bleached area
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9
Q

Movement within phospholipid membranes is Temperature dependent

A

Lipid composition also influences membrane properties

lower temp= less gel like
higher temp= more fluild, disordered, liquid-like

s shaped graph

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10
Q

Movement within phospholipid membranes is dependent on Temperature and Lipid Composition

A

Changes in membrane fluidity mirror properties of
component fatty acids

Shorter chains, Unsaturated
Chains - ^ fluidity v Tm
Longer chains, Saturated
Chains - v fluidity ^ Tm

biological importantance?

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11
Q

Bacteria Adjust Membrane Fatty Acid Composition with Temperature

A

Cholesterol “plasticizes” phospholipid membranes

• Cholesterol has a rigid, bulky structure
• Found only in Eukaryotic Membranes
• Impacts membrane fluidity – less variation with temperature
1. Hinders phospholipid movement at higher temperatures
2. Hinders packing and solidification at low temperatures

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12
Q

Cholesterol plays a role in membrane organization

A

Cholesterol can help packing of Glycosphingolipids

  • Cholesterol doesn’t readily form a bilayer
  • Tends to associate with longer, saturated fatty-acyl tails of other lipids
  • Sphingolipids tend to have longer hydrophobic tails
  • Can also help in packing between large glycolipids
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13
Q

Lipid Rafts

A

Cholesterol and sphingolipids combine to form a microdomain in the plasma membrane

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14
Q

Lipid rafts and shingolipids

A

tend to have longer, saturated chains

- larger head groups

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15
Q

Lipid rafts and GlycoSphingolipids

A

cluster in the outer leaflet of the plasma membrane

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16
Q

Enriched in cholesterol and sphingolipids

A
  • more ordered, thicker membrane
    • Specifically modified proteins preferentially associate with lipid rafts
    • One principle of membrane organization
    • Platforms for cellular signaling, membrane budding
    • Viruses (influenza, measles, Ebola, HIV) appear to localize to lipid rafts
17
Q

Lipid composition varies with Eukaryotic membrane

A

Different membranes have different functions

18
Q

Membrane Bilayers are Asymmetric

A

Distribution of Phospholipids in the Membrane is not at Equilibrium

Cerebrosides and gangliosides are generally found in the outer leaflet

Presence of PS in the outer leaflet can be a signal for cell death

19
Q

Protein Transporters in the membrane maintain Asymmetry

Fluid Mosaic Model –Transverse Diffusion is very slow

A

Protein groove accommodates a phospholipid head group.

Acyl tails stay in the hydrophobic part of the membrane bilayer

Maintaining Asymmetry requires energy (ATP)

20
Q

How do you move a polar head group across the hydrophobic core of the membrane?

A

Must overcome a significant energy barrier!

21
Q

Membranes can be crowded with proteins

A
  • ~30% of the genome encodes membrane proteins
  • Protein content of membranes ranges from 18-76% (by mass)
  • Lipids have to interact with a diverse array of membrane proteins.
  • Fill holes in protein-lipid interfaces (self-sealing)
  • Are there certain characteristics that define membrane proteins?
22
Q

Integral Membrane Proteins (ppt)

A

• Extensive contacts with
hydrophobic regions of the
lipid bilayer
• Require strong detergents for release from membranes
• Most span the bilayer
• Have only ONE orientation in the membrane.

Non-polar side chains contact lipids.

  • Satisfy polar interactions within hydrophobic bilayer
  • Membrane-Spanning Proteins: Hydrophobic “belt” ~26-30 wide
23
Q

Peripheral Membrane Proteins (ppt)

A 
• Interact with exposed
surfaces of integral
membrane proteins or
phospholipid head groups
• Can be dissociated by high
salt or change in pH
24
Q

Properties of membrane-spanning α-Helices

A

23 residue transmembrane
segment

Glycophorin A (131 Amino Acid Residues)- SEPEITLIIFGVMAGVIGTILLISYGIRRLIKK
- need at least 20 residues ~30Å for a helix to span the membrane

Not just the properties of ONE residue,
but also surrounding residues

25
Q

How do membrane phospholipids interact with integral membrane proteins?

A

Integral Membrane Proteins – intimately associated with membrane lipids
- Require detergents to remove protein from the membrane

26
Q

Annular lipids – (next

to the protein)

A

Acyl chains adapt to knobs and grooves of hydrophobic protein surface

Network of salt-bridges and hydrogen bonds with lipid polar head groups

Provides smooth surface for bulk lipids

Self-Sealing

27
Q

(Aquaporin:

A

allows the rapid movement of water across a cell membrane)

28
Q

Fatty Acylation

A

Myristate: linked to protein N-terminus

Palmitate: usually a thioester linkage

is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4’-phosphopantetheine moiety.

29
Q

Prenylation -anchor type

A

Farnesyl (C15) or
GeranylGeranyl (C20)
Linked to Cys (CaaX motif) near protein C-terminus

is the addition of hydrophobic molecules to a protein or chemical compound. It is usually assumed that prenyl groups (3-methyl-but-2-en-1-yl) facilitate attachment to cell membranes, similar to lipid anchors like the GPI anchor, though direct evidence is missing.

30
Q

Phosphoethanolamine

A

(linked to protein C-terminus)

31
Q

Glycosyl-Phosphatidyl- Inositol (GPI)-Linked

A

a glycolipid that can be attached to the C-terminus of a protein during posttranslational modification. … The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane

32
Q

Lipid modification can affect protein localization within a membrane

A

GPI-Anchored
Proteins - Outer leaflet of Plasma Membrane

Palmitoyl-dependent Raft association of Integral Membrane Proteins

Prenylated Proteins -
• associate with non-raft regions of the membrane
• Inner leaflet

33
Q

Membrane associated proteins can influence membrane shape and function

A

Caveolin forms dimers, is palmitoylated, and binds
cholesterol

Promotes membrane curvature : Caveola
Enriched in cholesterol, shingolipids, and GPI-anchored proteins

Caveolin associates with lipid rafts and promotes membrane curvature

34
Q

Membrane proteins can give the membrane shape

A

Local Structure - Caveola

Caveola tend to be enriched in sphingolipids and cholesterol- Specific GPI anchored proteins are often found in Caveola

Caveola tend to be enriched in sphingolipids and cholesterol- Specific GPIanchored proteins are often found in Caveola

35
Q

Membrane cytoskeletal proteins give the cell its shape.

A

• Biconcave disk
• Increases surface area
allows rapid diffusion of O2 across the membrane to Hb
• Shape is maintained by a network of proteins (cytoskeleton)
• The phospholipid membrane can “flow” over the skeleton.
Can restrict diffusion of membrane proteins
• The erythrocyte can be deformed and then recover its shape (Squeeze through small capillaries)

36
Q

Summary

A

Membrane proteins can be loosely divided into Integral and Peripheral Membrane Proteins

Membrane-spanning proteins have to “hide” polar backbone atoms α-helical and β-barrel membrane spanning proteins

Lipid modified proteins can also associate with membranes

Lipid rafts and Lipid modifications help organize proteins in a membrane

Cytoskeleton also gives the membrane shape and function

BIOCHEM 405 (25 decks)

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