lipids and proteins

some carbohydrates in form of glycolipids and glycoproteins

(NO free carbohydrate)

• Glycerophospholipids (phosphoglycerides)
• Sphingolipids
• Cholesterol

What are the 2 most common phosphoglycerides in membranes?

What are their charges at physiological pH?

• phosphatidylcholine (lecithin)
• phosphatidylethanolamine (cephalin)
• both are zwitterion (net charge = 0)

Phosphoglyceride characteristics

• amphipathic (nonpolar hydrophobic region w FA's, polar hydrophilic polar head)

• polar head is ionized at physiological pH
• C₁ always has saturated FA
• C₂ can be unsaturated

• phosphatidyl serine
• phosphatidyl inositol

• Has phosphate groups at C-4 and C-5
• important in the membrane for the action of certain hormones because it is the source for signaling proteins

• myristic acid (14C
• palmitic acid (16C)
• stearic acid (18C)

• palmitoleic acid (C16:ln-7)
• oleic acid (C18: ln-9)
• linoleic acid (C18:2n-6)
• linolenic acid (C18:3n-3)
• arachidonic acid (C20:4n-6)

Old ex: 18:2Δ ^9,12

where 18 is # of C, 2 is # of double bonds

9 and 12 are locations of d.b.'s from COOH end

New ex: C18:2n-6

where 6 is the location of the first d.b. from CH₃ end w sequential d.b.'s every 3C until "n"

In what configuration are unsaturated FA's found in the body?

How does this configuration affect melting point for a phosphoglyceride?

• natural usFA's are always cis
• d.b. introduces a kink in the chain that ↓m.p. due to prevention of close packing, well ordered crystals during solidification
• trans usFA can be synthesized and are straight chained

What is the most abundant sphingolipid?

Characteristics? Fxn?

• sphingomyelin
• similar physical characteristics to phosphoglycerides
• sphingomyelinase acts on sphingomyelin in the membrane to release ceramide into the cell as a second messenger

Glycosphingolipids:

Definition?

Two types?

• sugar reside added to terminal OH group of ceramide (spingosine + FA + sugar)
• cerebrosides
• gangliosides

Cerebrosides:

definition

2 types

location in body

• sugar on the ceramide is either glucose or galactose
• (ceramide + glucose/galactose)
• glucocerebroside
• galactocerebroside
• predominantly in brain

Galactocerebrosides with a sulfate group esterified at position 3 on galactose

(ceramide + galactose + esterified sulfate group)

ceramide + oligosaccharides

(oligosaccharide has 1+ residues of sialic acid)

GM1 ganglioside is the receptor for cholera toxin and E.coli heat-labile enterotoxin

Cholesterol

characteristics?

which membranes is it in?

fxn in membrane?

• amphipathic (polar -OH, non-polar ring/chain)
• not in plant or prokaryote membranes
• in animal cell plasma membrane, 1:1 molar ratio with phospholipids
• provides stability to membranes

What are the amphipathic lipids?

How do they interact with water?

Which interaction is most favored?

phosphoglycerides and sphingomyelin

1. form a monolayer film on surface of water

2. form a suspension in water, micelles at ↑ conc

3. form a bilayer as in membranes

bilayer most favored, driven by hydrophobic interactions

• peripheral - bind to external hydrophilic region
• integral - amphipathic, embedded into nonpolar hydrophobic portion of membrane
• lipid-anchored

How would you remove proteins from the membrane?

How does that affect the membrane?

• integral - detergents or organic solvents which disrupt membrane structure
• peripheral - treatment with salt solutions of low/high ionic strength or extremes of pH, cytoskeletal proteins responsible for cell shape are on cytoplasmic surface so membrane structure is maintained

• rotational
• lateral
• NOT movement from one leaflet to the other

Polarized cells do not have random protein distribution, but have apical and basolateral membranes with very different protein composition that are separated by tight junctions

(ex: intestinal, renal tubular)

Fluorescence Recovery After Photobleaching

shows lateral movement of lipids and proteins in the membrane (ex: mouse/human fused cell exper)

What does it mean if anionic phospholipids (PI, PS) are found on the outer leaflet?

How can we see this?

• often in diseased or apoptotic cells anionic phospholipids are transported to the outer leaflet as a signal to scavenger receptors on phagocytes that phagocytosis should be initiated
• PS binds to annexin V which can be labeled to fluoresce (and nml cells will not)

What is the influence of temperature on the fluidity of the membrane?

Which has a bigger impact?

• ↓ FA chain length, ↑ fluidity
• ↑ unsaturation, ↑ fluidity
• saturation has a bigger impact

at low conc ↑ cholesterol, ↑ fluidity

by interfering with close packing of FA's

at high conc ↑ cholesterol, ↓ fluidity

↓ permeability of small soluble molecs

but ↑ enhances membrane stability

Which AA's would be likely to exist in the region of an integral protein that is partially embedded in the bilayer?

What shape would they be in?

hydrophobic AA's (FeW MC's have GAVLI)

Phe, Trp, Met, Cys, Gly, Ala, Val, Leu, Iso

(*not Pro bc it interferes w helix)

α-helical conformation

• β-sheets which curl to form a β-barrel structure
• hydrophobic AA's face outside of the barrel, hydrophilic AA's face inward to form aqueous pore