Bc368 biochemistry of the cell ii
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BC368 Biochemistry of the Cell II. Biological Membranes Chapter 11: Part 1 February 11, 2014. Plasma Membrane.

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BC368 Biochemistry of the Cell II

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Bc368 biochemistry of the cell ii

BC368Biochemistry of the Cell II

Biological Membranes

Chapter 11: Part 1

February 11, 2014


Bc368 biochemistry of the cell ii

Plasma Membrane

“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


Bc368 biochemistry of the cell ii

Plasma Membrane


Bc368 biochemistry of the cell ii

Plasma Membrane

Membrane is composed of:

  • Lipids

  • Phospholipids

  • Sterols

  • B.Proteins

  • Integral

  • Peripheral

  • C. Carbohydrates

  • Glycolipids

  • Glycoproteins


Bc368 biochemistry of the cell ii

Plasma Membrane

  • Variable components in different membrane types


Membrane lipids

Membrane Lipids

  • Amphiphilic lipids

  • Major types: phospholipids, glycolipids, sterols

sphingosine

Glycolipid

glycerophospholipid

sphingolipid


Phospholipids

Phospholipids

  • Two classes: glycerophospholipids (aka phosphoglycerides) and sphingolipids

Fig 10-7


Membrane lipids 1a glycerophospholipids

Membrane Lipids: 1A. Glycerophospholipids

  • Two fatty acids and a polar “head group” on glycerol.

  • Vary in the FA’s and head group.


Membrane lipids 1b sphingolipids

Membrane Lipids: 1B. Sphingolipids

  • Named for the enigmatic Sphinx

  • Common in nerve and brain cell membranes


Membrane lipids 1b sphingolipids1

note amide

linkage

Membrane Lipids: 1B. Sphingolipids

  • Named for the enigmatic Sphinx

  • Sphingosine replaces glycerol, so only 1 FA tail


Membrane lipids 1b sphingolipids2

Membrane Lipids: 1B. Sphingolipids

  • Example: sphingomyelin


Glycolipids

Glycolipids

  • Two classes: glycosphingolipids and galactolipids

Fig 10-7


Membrane lipids 2a glycosphingolipids

Membrane Lipids: 2A. Glycosphingolipids

  • Sphingolipids with carbohydrate head group; common on cell surfaces

  • Examples: cerebrosides and gangliosides


Membrane lipids 2b galactolipids

Membrane Lipids: 2B. Galactolipids

  • Diglycerides with galatose groups

  • Common in plant (thylakoid) membranes


Membrane lipids 3 sterols

Membrane Lipids: 3. Sterols

  • Cholesterol and cholesterol-like compounds


Bc368 biochemistry of the cell ii

Lipid Components of Membranes

  • Lipid composition varies across different membranes.

Fig 11-2


Turnover of membrane lipids

Turnover of Membrane Lipids

Fig 10-16


Defects in membrane turnover

Defects in Membrane Turnover

Deposits of gangliosides in Tay Sachs brain


Bc368 biochemistry of the cell ii

Lipid Aggregates

  • Lipids spontaneously aggregate in water as a result of the Hydrophobic Effect.


Bc368 biochemistry of the cell ii

Lipid 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.

Fig 11-4


Bc368 biochemistry of the cell ii

Lipid Aggregates

Fig 11-4

  • Double-tailed lipids form bilayers, the basis of cell membranes.

  • Bilayers can form vesicles enclosing an aqueous cavity (liposomes).

Fig 11-4


Bc368 biochemistry of the cell ii

Lipid Components of Membranes

  • Different types of membranes have characteristic lipid compositions.


Bc368 biochemistry of the cell ii

Lipid Components of Membranes

  • Lipid composition varies across the two leaflets of the same membrane.


Bc368 biochemistry of the cell ii

Membrane Proteins

  • Integral proteins (includes lipid-linked): need detergents to remove

  • Peripheralproteins: removed by salt, pH changes

  • Amphitropicproteins: sometimes attached, sometimes not


Bc368 biochemistry of the cell ii

Single Transmembrane Segment Proteins

  • Usually alpha-helical, ~20-25 residues, mostly nonpolar.

  • Example: glycophorin of the erythrocyte.

Fig 11-8


Bc368 biochemistry of the cell ii

Multiple Transmembrane Segment Proteins

  • 7 alpha-helix motif is very common.

  • Example: bacteriorhodopsin

Fig 11-10


Bc368 biochemistry of the cell ii

Beta Barrel Transmembrane Proteins

  • Multiple transmembrane segments form β sheets that line a cylinder.

  • Example: porins.


Bc368 biochemistry of the cell ii

Lipid-Linked Membrane Proteins

  • Attached lipid provides a hydrophobic anchor.

Fig. 11-14

  • An important lipid anchor is GPI (glycosylated phosphatidylinositol.


Bc368 biochemistry of the cell ii

Membrane Carbohydrates

  • On exoplasmic face only


Bc368 biochemistry of the cell ii

Membrane Carbohydrates

  • On exoplasmic face only

  • An example is the blood group antigens


Bc368 biochemistry of the cell ii

Membrane Dynamics

  • At its transition temperature (TM), the bilayer goes from an ordered crystalline state to an a disordered fluid one.

Fig 11-16


Bc368 biochemistry of the cell ii

Membrane Dynamics

  • Phospholipids in a bilayer have free lateral diffusion.

Fig 11-17


Bc368 biochemistry of the cell ii

Membrane Dynamics

  • Phospholipids in a bilayer have restricted movement between the two faces.

Fig 11-17


Bc368 biochemistry of the cell ii

Membrane Dynamics

  • Flippases, floppases, and scramblases catalyze movement between the two faces.


Bc368 biochemistry of the cell ii

Fluid Mosaic


Bc368 biochemistry of the cell ii

Fluorescent 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.


Bc368 biochemistry of the cell ii

Fluorescent Recovery After Photobleaching

FRAP Movie


Bc368 biochemistry of the cell ii

Protein Mobility in the Membrane

  • Some membrane proteins have restricted movement.

  • May be anchored to internal structures (e.g., glycophorin is tethered to spectrin).

Fig. 11-20


Bc368 biochemistry of the cell ii

Protein 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.

Fig. 11-21


Bc368 biochemistry of the cell ii

Protein 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.

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.


Bc368 biochemistry of the cell ii

Membrane Permeability

  • Membranes are selectively permeable.

  • Permeable to nonpolars and small polar molecules.

  • Impermeable to ions and large polar molecules.


Bc368 biochemistry of the cell ii

Membrane Permeability

  • What actually gets across a membrane depends on several factors:

  • Solubility in the nonpolar lipid environment

  • The concentration gradient

  • Whether a protein transporter exists


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