W4.4_Properties of Lipids

Question 1

Explain the importance of lipids in cells. What is the major property of fatty acids?

Answer 1

• Lipids: ≈2% cell composition, contain highly hydrophobic regions that X interact appreciably with H2O
• Fatty acids: lipophilic/hydrophobic hydrocarbon tail + carboxylic acid head group (ionised of around pKa≈4, carboxylate form is very hydrophilic)
• Lipophilic + hydrophilic = amphipathic (act as surfactants that reduce surface tension of water)

Question 2

Describe the different level of saturation in lipids. What does unsaturation indicate in lipids? How are fatty acids used in biological system?

Answer 2

• Level of saturation: saturated -> mono-unsaturated -> poly-unsaturated
• Unsaturation: lower flexibility of lipids, change properties on itself and membranes that incorporate them
• Fatty acids: simplest form of lipids -> biological system use them directly to incorporate/transform into more complex molecules

Question 3

What is membrane lipid? Explain the different positions in a membrane lipid. State the purpose of very hydrophilic zwitterionic head groups in membrane lipids.

Answer 3

• Membrane lipids: most common form as glycerophospholipids (based on glycerol)
• sn3 position: highly charged hydrophilic phosphatidic acid head group
• sn and sn2 positions: fatty acid chains linked to glycerol by ester bonds
• Phosphate can be modified into ethanolamine or choline
• Very hydrophilic zwitterionic head groups (high pKa of cation and low pKa of anion -> high water solubility -> X react with ionic compounds to maintain structural integrity)

Question 4

What are the other membrane lipids? Explain why lipids are usually organised in a self-assembling way.

Answer 4

• Other membrane lipids: sphingolipids, cholesterol
• ∵ Strong interactions between water and ionised head groups
• ∵ Very poor aqueous solubility of lipid regions
• ∴ Self-assembly of organised structures (can also form monolayers at water-air interface)

Question 5

State the different organised structures of lipids in the order of increased ratio of length of lipid tails to head group.

Answer 5

• Spherical micelle
• Cylindrical micelle
• Liposome/bilayer vesicles
• Bilayer sheet/ lamellar phases
• Inverted micelles/ microemulsions

Question 6

Describe different examples of lipids as signalling molecules.

Answer 6

• ex. Sphingosine-1-phosphate (S1P) regulates vascular/immune systems, targeted by S1P receptor modulators
• ex. Arachidonic acid -COX enzyme-> PGH2 (key receptor of immune system), where COX inhibitors (NSAIDs like ibuprofen) prevent formation of PGH2 to reduce inflammatory signalling
• ex. Cholesterol converted into aldosterone (control salt secretion in kidneys), cortisol (immunosuppressive), hormones (testosterone and estradiol)

Question 7

What is the main purpose of lipids in drug formulation? State some examples with brief explanation.

Answer 7

• Organise around to act as emulsifiers
• Used in creams and suspensions (but not always as they stabilise emulsions)
• Utilise natural/unnatural surfactants (ex. SDS, as known as SLS)
• Used in vaccines: vesicles/liposomes create a sufficiently stable system to deliver mRNA into cells