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Lipids
Unlike the other classes of biomolecules we have studied so far, lipids are defined by their solubility. How are amino acids and carbohydrates defined?
I. Storage Lipids (Combustion reactions? Energy yield?)
A. Fatty acids: Hydrocarbon derivatives
- Nomenclature a) # of C atoms b) # of double bonds ( cis- & unsaturation comments) c) position of double bonds ()) d) example, arachidonic acid: 20:4()5,8,11,14^ )
- Trans- fatty acids a) Occurrence in nature? b) Formation c) Health related outcomes re. consumption
- Physical properties (Table 10-1) a) Solubility in H 2 O? b) Melting point (intermolecular attractive forces?), biological significance (Fig. 10-2) i) chain length ii) cis- double bonds
B. Triacylglycerols: Fatty acid esters of glycerol
- Name is apt.
- Simple triacylglycerols have 3 of same fatty acid.
- Non-simple ones are common in nature and us. a) Naming requires positional information b) Stereochemical considerations?
- Triacylglycerols are quite hydrophobic
C. Triacylglycerols store energy & provide insulation.
- Energy considerations a) Oxidation state? Ave C atom? b) Compare with carbohydrates i) oxidation state ii) hydration state
- “... people with 15 to 20 kg of triacylglycerols ... could meet their energy needs for months...” p. 347
- Insulation: seals, walruses, penguins, blubber
- Buoyancy modulation in sperm whales (Box 10-1) a) Solid spermaceti oil (m.p. ~31°C) is more dense than the liquid state of the oil. b) Regulation of spermaceti organ temperature?
D. Many foods contain triacylglycerols
- See Fig. 10-5 re. saturation levels
- Rancidity associated with oxidation of double bonds
E. Waxes: Energy stores and water repellants (duck’s back?)
- Esters of long chain fatty acids (C (^) 14 to 36 ) and long
- Cholesterol: a) important membrane component b) steroid hormone synthesis precursor
- Bile acids emulsify fats (more readily metabolized).
III. Lipids as Signals, Cofactors, and Pigments
(for the 1st two: minor amounts, major importance)
A. Phosphatidylinositols and sphingosine derivatives act as intracellular signals. More in Chapter 12.
B. Eicosanoids carry messages to nearby cells.
- Contrast endocrine vs. paracrine hormone function
- Types: (See synthetic pathway, Fig. 10-18) a) prostaglandins (many functions) b) thromboxanes (reduction in blood flow at clots) c) leukotrienes (act on lung, other muscle types)
C. Steroid hormones carry messages between tissues. Fig. 10-19.
D. Plants use phosphatidylinositols, steroids, and eicosanoid-like compounds in signaling.
E. Vitamins A and D are hormone precursors.
- Vitamin D prevents rickets, see Fig. 10-
Comment re. skin melanin levels and how close to the equator a given human population has pre-historically lived?
- Vitamin A and vision, see Fig. 10-
F. Vitamins E and K & the lipid quinones are oxidation- reduction cofactors. Fig. 10-22.
G. Dolichols are required for some carbohydrate synthesis steps.
IV. Working with Lipids
A. Lipid extraction requires organic solvents
B. Adsorption chromatography separates lipids of different polarity.
C. Gas-liquid chromatography resolves mixtures of volatile lipid derivatives.
D. Specific hydrolysis aids determination of lipid structure.
E. Mass spectrometry reveals complete lipid structure.
