B.Sc. Biochemistry II Cellular Biochemistry Unit 1 Basics of Cell
BC368 Biochemistry of the Cell II
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Transcript of BC368 Biochemistry of the Cell II
BC368Biochemistry of the Cell II
BC368Biochemistry of the Cell II
Biological Membranes Chapter 11: Part 1February 10, 2015
“Possibly the decisive step [in the origin of life] was the formation of the first cell, in which chain molecules were enclosed by a semi-permeable membrane which kept them together but let their food in.” J. B. S. Haldane,
1954
Plasma MembranePlasma Membrane
Plasma MembranePlasma Membrane
Membrane is composed of:
A. Lipids Phospholipids Sterols
B. Proteins Integral Peripheral
C. Carbohydrates Glycolipids Glycoproteins
Plasma MembranePlasma Membrane
Plasma MembranePlasma Membrane
Variable components in different membrane types
Membrane LipidsMembrane Lipids
Amphiphilic lipids
Major types: phospholipids, glycolipids, sterols
Glycolipid
sphingosine
glycerophospholipid sphingophospholipid
PhospholipidsPhospholipids
Two classes: glycerophospholipids (aka phosphoglycerides) and sphingophospholipids Fig 10-7
PhospholipidsPhospholipids
Two classes: glycerophospholipids (aka phosphoglycerides) and sphingophospholipids
Two fatty acids; phosphate and polar “head group” on glycerol.
Vary in the FA’s and head group.
Membrane Lipids: 1A. GlycerophospholipidsMembrane Lipids: 1A. Glycerophospholipids
Membrane Lipids: 1B. SphingophospholipidsMembrane Lipids: 1B. Sphingophospholipids
Named for the enigmatic Sphinx
Common in nerve and brain cell membranes
Membrane Lipids: 1B. SphingophospholipidsMembrane Lipids: 1B. Sphingophospholipids
Named for the enigmatic Sphinx
note amidelinkage
Sphingosine replaces glycerol, so only 1 FA tail
Example: sphingomyelin
Membrane Lipids: 1B. SphingophospholipidsMembrane Lipids: 1B. Sphingophospholipids
Head group = phosphocholine or phosphoethanolamine
GlycolipidsGlycolipids
Two classes: glycosphingolipids and galactolipids
Fig 10-7
Membrane Lipids: 2A. GlycosphingolipidsMembrane Lipids: 2A. Glycosphingolipids
Sphingolipids with carbohydrate head group; common on cell surfaces
Examples: cerebrosides and gangliosides
Glucose or galactose
SugarSugar
Ganglioside
Membrane Lipids: 2B. GalactolipidsMembrane Lipids: 2B. Galactolipids
Diglycerides with galatose groups
Common in plant (thylakoid) membranes
Membrane Lipids: 3. SterolsMembrane Lipids: 3. Sterols
Cholesterol and cholesterol-like compounds
Lipid Components of MembranesLipid Components of Membranes
Lipid composition varies across different membranes.
Fig 11-2
Lipid composition varies across the two leaflets of the same membrane.
Lipid Components of MembranesLipid Components of Membranes
Turnover of Membrane LipidsTurnover of Membrane Lipids
Fig 10-16
Defects in Membrane TurnoverDefects in Membrane Turnover
Deposits of gangliosides in Tay Sachs brain
Lipids spontaneously aggregate in water as a result of the Hydrophobic Effect.
Lipid AggregatesLipid Aggregates
Amphiphilic lipids form structures that solvate their head groups and keep their hydrophobic tails away from water.
Above the critical micelle concentration, single-tailed lipids form micelles.
Lipid AggregatesLipid Aggregates
Fig 11-4
Double-tailed lipids form bilayers, the basis of cell membranes.
Lipid AggregatesLipid Aggregates
Bilayers can form vesicles enclosing an aqueous cavity (liposomes). Fig 11-4
Fig 11-4
Membrane ProteinsMembrane Proteins
Integral proteins (includes lipid-linked): need detergents to remove
Peripheral proteins: removed by salt, pH changes
Amphitropic proteins: sometimes attached, sometimes not
Usually alpha-helical, ~20-25 residues, mostly nonpolar.
Example: glycophorin of the erythrocyte.
Single Transmembrane Segment ProteinsSingle Transmembrane Segment Proteins
Fig 11-8
7 alpha-helix motif is very common.
Example: bacteriorhodopsin
Multiple Transmembrane Segment ProteinsMultiple Transmembrane Segment Proteins
Fig 11-10
Multiple transmembrane segments form β sheets that line a cylinder.
Example: porins.
Beta Barrel Transmembrane ProteinsBeta Barrel Transmembrane Proteins
Fig. 11-14
Attached lipid provides a hydrophobic anchor.
Lipid-Linked Membrane ProteinsLipid-Linked Membrane Proteins
An important lipid anchor is GPI (glycosylated phosphatidylinositol.
On exoplasmic face only
Membrane CarbohydratesMembrane Carbohydrates
An example is the blood group antigens
On exoplasmic face only
Membrane CarbohydratesMembrane Carbohydrates
glycosphingolipids
At its transition temperature (TM), the bilayer goes from an ordered crystalline state to an a disordered fluid one.
Membrane DynamicsMembrane Dynamics
Fig 11-16
Phospholipids in a bilayer have free lateral diffusion.
Membrane DynamicsMembrane Dynamics
Fig 11-17
Phospholipids in a bilayer have restricted movement between the two faces.
Membrane DynamicsMembrane Dynamics
Fig 11-17
Flippases, floppases, and scramblases catalyze movement between the two faces.
Membrane DynamicsMembrane Dynamics
Fluid Mosaic
Fluorescent Recovery After PhotobleachingFluorescent Recovery After Photobleaching
Fluorescent tag is attached to a membrane component (lipid, protein, or carbohydrate).
Fluorescence is bleached with a laser.
Recovery is monitored over time.
Fluorescent Recovery After PhotobleachingFluorescent Recovery After Photobleaching
FRAP Movie
Fig. 11-20
Some membrane proteins have restricted movement.
May be anchored to internal structures (e.g., glycophorin is tethered to spectrin).
Protein Mobility in the MembraneProtein Mobility in the Membrane
Fig. 11-21
Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.
Thicker and less fluid than neighboring domains.
Protein Mobility in the MembraneProtein Mobility in the Membrane
Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.
Thicker and less fluid than neighboring domains.
Protein Mobility in the MembraneProtein Mobility in the Membrane
Lipid Rafts
Nature Reviews Molecular Cell Biology 4, 414-418 (May 2003)
Nature Reviews Molecular Cell Biology 4, 414-418 (May 2003)
Domains of gel/fluid lipid segregation in a model membrane vesicle, which is a mixture of fluid dilaurylphosphatidylcholine phospholipids with short, disordered chains and gel dipalmitoylphosphatidylcholine phospholipids with long, ordered chains. A red fluorescent lipid analogue (DiIC18) partitions into the more ordered lipids, whereas a green fluorescent lipid analogue (BODIY PC) partitions into domains of more fluid lipids. These domains in a model membrane are much larger than the domains of cell membranes.