Biochemistry I Fall Term, 2000

Lecture 25: Lipids

Assigned reading in Campbell: Chapter 6.1-6.3

Take the Review Quiz on Lecture 24 concepts.

Summary:

Fatty Acids:

1. Lipids are better energy sources than sugars because they are less oxidized than carbohydrates.
2. Synthesized in:
   • Adipose (fat) tissue
   • Mammary gland
   • Liver
3. Nomenclature for saturated fatty acids (Campbell, Table 6.1):
   N = 12 Lauric acid
   N = 14 Myristic acid
   N = 16 Palmitic acid
   N = 18 Stearic acid
   N = 20 Arachidic acid
   
4. Unsaturated fatty acids have one or more cis double bonds e.g. oleic acid = cis 9,10 stearic acid. The cis bond conformation causes a sharp bend in the carbon(acyl) chain. This lowers the melting temperature because it reduces the amount of contact between acyl chains.

Physical Properties

1. Fatty acids form micelles, aggregates of fatty acids with a polar (charged) surface and a hydrophobic, waterless, interior.
2. The non-polar interior can dissolve hydrophobic compounds (oily 'dirt')
3. Fatty acids and modified fatty acids (e.g. SDS) are the principal components of soap, shampoo, and other detergents.

Triglycerides

1. Three fatty acids + glycerol condense to give triglyceride.
2. If all three fatty acids are the same, the triglyceride is called triacylglycerol (e.g. olive oil = trioleic glycerol)
3. Otherwise they are named by the type of fatty acid at position (e.g. 1-palmitoyl-).
4. Principal storage form of fatty acids is adipose tissue.
5. Principal form of fat in milk.
6. Used for energy storage, but the response to energy needs is slower than the release of glucose from glycogen.
7. Olestra^® is a non-digestible fat: glycerol with many OH's such that enzymes cannot hydrolyze the ester bonds. (Despite heavy promotion, sales of this synthetic food additive have been a big disappointment to Monsanto.)
8. Margarine: Hydrogenated oil; the saturated double bonds result in an increase in melting temperature.

Phospholipids (Campbell, Fig. 6.5)
(Campbell uses "phosphoacylycerol"; no one else does!)

1. Two fatty acids plus phosphate attached to glycerol.
2. Various head groups are attached to the phosphate, giving a diverse set of lipids.

Head group	Name of Phospholipid
-H	phosphatidic acid
ethanolamine	phosphatidylethanolamine
choline (trimethyl ethanolamine)	phosphatidylcholine (lecithin)
Serine	phosphatidylserine
Glycerol	phosphatidylglycerol (cardiolipin)



3. The nomenclature indicates the type of fatty acid attached to the 1 and 2 positions of glycerol as well as the type of head group on the phosphate:
4. sn-1-palmitoyl-2-palmitoyl-3-phosphatidylcholine. (also known as dipalmitoyl phosphatidyl choline, abbr. DPPC.) The common name is lecithin, the major lipid in egg yolk.
5. Phospholipids forms bilayers (two opposing layers of phospholipids ) instead of micelles because the cross section of the head group is roughly equal to the cross section of the 2 fatty acid chains.
6. The bilayers form closed, water filled, vesicles with a 60-100 Å wall.

Properties of Lipid Bilayers

Permeability properties of bilayers are due in part to the asymmetry of the outer and inner layers.
Compounds that dissolve more readily in oil have higher permeability

Phase transition.

• Lipid bilayers undergo a phase transition at T_m. (DPPC T_m=42°C)
• Below T_m the lipids exist as a solid-like gel; the acyl chains are packed tightly against each other.
• Above T_m the lipids are in a liquid-like liquid crystal phase. The acyl chains are disordered. Thus the phase transition is called the gel to liquid-crystalline transition.
  Also above T_m rapid lateral diffusion of lipids (and proteins) occurs in the plane of the membrane.

Other Phospholipids:

Sphingomyelin & cerebroside

• Found in peripheral nerve (sphingomyelin) and central nerve (cerbroside) cell membranes.
• In cerbrosides, the phosphate group is replaced by b-1 linkage to galactose.

• Blood group antigens are cerbrosides with multiple sugars attached. The number and type of sugars attached define the antigen. Individuals have different antigens depending on what enzyme they have to add sugar to the core sugar:

  People with O type blood lack the enzyme entirely and do not add anything to the core sugar.

  People with A type blood add N-acetyl galactose to the core sugar.

  People with B type blood have an altered from of the enzyme that links galactose to the core sugar.
  O antigen: 'core' sugars
  A antigen: 'core' sugars + N-acetyl galactose
  B antigen: 'core' sugars + galactose

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